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Scheduling with regard to crop evapotranspiration (ETc)

1. 12 50 Irrigation User s Manual Table 12 13 Estimated design crop factors for agronomic crops in the winter rainfall area June 1990 Agronomic crops Mealies Wheat Soya beans Potatoes Potatoes Potatoes Potatoes Planting date End of growing season um im a 31 0 6 Feb 0 4 0 7 0 7 Months of the year Nov es oss oss os 0 3 Lp perle 0 5 0 65 0 4 Irrigation scheduling 12 51 Table 12 14 Estimated design crop factors for vegetables in the winter rainfall area June 1990 Portion of growing season Beans 0 25 0 3 0 5 0 5 0 55 Brassicas 058 Cuowbits 0 04 Pea 025 0 Onions 0 25 0 3 0 5 0 5 Tomatoes 0 25 0 3 0 5 0 55 0 55 Vegetable crops 12 92 Irrigation User s Manual Table 12 15 Penman Monteith Crop Factors Perennials Assumptions 1 Weekly irrigations 2 Cover Mature orchards vineyards 7596 Young orchards vineyards 4096 Other crops 100 3 Wetted area Orchards vineyards 5096 Other crops 100 Crop Crop options Climatic region Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Almonds Mature middle season areas 0 90 0 87 0 64 0 37 0 24 0 24 0 24 0 24 0 56
2. per day for a 7 day working week 80 38 per hour for a 10 hour working day Gross irrigation requirement n 281 35 0 8 351 7 Irrigation scheduling 12 25 6 Scheduling techniques There is a variety of ways in which irrigation scheduling can be applied i e with which to determine when to irrigate The decision of which irrigation scheduling technique should be used depends on the producer s choice The factors influencing the system choice as discussed in Chapter 2 Guidelines for irrigation system choices also influence which scheduling technique the producer will use Scheduling aids will only be to the producer s benefit if at least two readings can be taken per irrigation cycle It is recommended that the readings be obtained from the different scheduling aids and plotted on graphs to identify tendencies in water consumption and possible problems with the irrigation management in time The most practical method is to follow a program calculated by means of the historic evapotranspiration and adapted by soil water measuring at strategic points in an irrigation block The calculation of evapotranspiration can be simplified by the use of the models as suggested in Section 7 Continuous soil water measuring is recommended for especially open hydroponics systems where irrigation 1s applied daily The amount of water to be applied depends on how empty the SWR is at that specific stage The techniques can be
3. sample is then accepted as a percentage of the oven dry mass An example is discussed in detail in Chapter 4 Soil of this manual Benefits i It is very accurate at least for the relevant soil sample on which the determining is done ii Itis an objective method and the personal judgement of the person doing the determining 15 not applicable un Itis a cheap method and a large number of samples be handled simultaneously iv The salt content of the soil does not influence it Disadvantages 1 It requires laboratory equipment e g a balance scale and drying oven i e the determining cannot be done in situ 1 It is time consuming requires 18 hours of drying time plus additional cooling time and answer can only be obtained at least a day after the sample was taken method is destructive in the sense that a sample is physically removed from the soil and a hole remains iv Since it is destructive it is impossible to do a follow up determination on the same spot Measurements done over time therefore include an element of inaccuracy because of the spatial variation present in any landscape when moving from one position to another v In order to convert the gravimetrical reading to a volumetric reading it is also necessary to determine the bulk density Many soils become dense during sample taking which gives a false image of the bulk density 12 28 Irrigation User s Manual Figure 12
4. be taken repeatedly on the same spot without disturbing the soil iii many measuring points as required be used without a significant cost increase i e one instrument can be used on many different spots iv Measurements can be done at any depth except the uppermost 150 mm to obtain a continued profile of soil water with depth v Soil water can be determined over the entire soil water spectrum vi Soil water is measured over a large soil volume and the soil water values are given directly in volume units which simplifies irrigation calculations Disadvantages 1 Health and safety risk Prolonged use can hold a radiation danger with accompanying problems Because of the radioactive source it contains the instrument is subject to certain regulations 1 large volume of soil is sampled and this can cause problems when non uniform soil water profiles are determined e g where there is a sharp transition between a wet and a dry soil layer or where there are sharp texture differences in depth e g a duplex soil iii It cannot be used summarily near the soil surface A special shield is required for this but is not very effective iv Neutron water measurements are influenced by soil density and soil type therefore many calibrations are necessary v The cost of the instrument is high R35 000 to R60 000 at 2001 prices vi Readings can also not be taken automatically on a continuous basis 6 5 Tensiome
5. Irrigation scheduling 12 43 8 Scheduling calculations Example 12 4 Fixed cycle length of 1 day will be followed by the producer irrigation requirement NIR 178 Emitter spacing L x Le 4 1 Application efficiency na 85 Emitter delivery qe 4 h Solution o 10 NIR Gross irrigation requirement per day 311 10x178 ni m day per ha 31x0 85 m 67 55 m day per Flow rate of system per qe x number of emitters 2 ae TUM m h per ha 1000 4 1 10 per ha Standing time t GIR Q 67 55 h 10 6 hours 45 minutes Example 12 5 A micro sprayer system is used to irrigate a block of table grapes The effective root depth is 750 mm The soil water capacity of the soil is 65 mm m The allowable water depletion during the specific growth phase is 70 The emitter delivery is 32 the nozzle spacing vine spacing is 1 8 m in the row the row spacing is 3 5 m and the wetted strip width is 3 m Crop factor is 0 6 Accept effective rainfall is equal to 0 and application efficiency is 80 The following must be determined a How much evaporation must occur from the A pan before it is irrigated again b How much water must be applied and c How long must irrigation be done Solution a Readily available water in root zone From Equation 12 5 RAW soil water capacity mm m x effective root depth m x allowa
6. W J Uys F P J Van der Merwe and P D Viljoen 1996 Irrigation Design Manual ARC Institute for Agricultural Engineering RSA Hoffman J E 2002 Klasnota s opgestel vir die grond en waterbestuur kursus Department of Soil science University Stellenbosch RSA Jordaan H 2000 n Ondersoek na sagteware vir besproeiingskedulering ARC Institute for Agricultural Engineering RSA Jordaan H 2001 Grondwatersensors ARC Institute for Agricultural Engineering RSA Koegelenberg F H 2002 Norms for the design of irrigation systems ARC Institute for Agricultural Engineering RSA Piaget J 1991 Tensiometers Beskrywing voorbereiding opstelling en gebruik Elsenburg Agricultural Development Institute for the Winter Rainfall Area RSA Scheepers I J Piaget W A G Kotze and P A Myburgh 1991 Riglyne vir Besproeiingskedulering van Permanente Gewasse in die Winterre ngebied Elsenburg Agricultural Development Institute RSA Van Zyl J L 1981 Waterbehoefte en besproeiing In Reds Burger J and Deist J Viticulture in South Africa ARC Infruitec Nietvoorbij Stellenbosch RSA 12 46 Irrigation Usser s Manual Table 12 9 Estimated design crop factors for perennial crops in the summer rainfall areas June 1996 Perennial crops Citrus Table grapes Deciduous fruit Wine grapes Pastures Pastures Alfalfa Avocado Coffee Litchi Macadamia Pecan nuts Bananas Tea Mangos M
7. soil and crop data it gives a complete description of the soil plant atmosphere continuum The model contains sufficient data and equations to simulate the growth of plants mathematically SWB is based on the improved general crop version of NEWSWB The program was developed by the University of Pretoria s Department of Plant Production and Soil Science and Dr N Benade of NB Systems The Water Research Commision University of Pretoria Chamber of Mines Agricultural Research Council s Institute for Vegetable and Ornamental Plants Potatoes South Africa and Langebaan Foods funded the program SWB is mainly used for actual betimes irrigation scheduling Researchers commercial farmers irrigation officers and consultants are the main users of the program 7 4 VINET 1 1 VINET 1 1 Estimated Vineyard Evapo transpiration for Irrigation System Design and Scheduling was designed to aid the producer in the decision making process on when how much and for how long irrigation must be applied In the past decision making was made difficult because of the variation between vineyards because there were differences between foliage soil and climatic factors Dr PA Myburgh and Mr C Beukes both from the ARC Infruitec Nietvoorbij developed VINET 1 1 The research was partly funded by Dried Fruit Technical Services Deciduous Fruit Producers Trust and Winetech VINET 1 1 1s currently used by commercial farmers consultants engineers and small farmers
8. 0 93 0 80 Green peppers Spring Summer 110 All areas 0 38 0 72 1 09 0 77 Ground nuts Spring 150 All areas 0 39 0 61 1 04 1 14 0 79 Hubbard squash Spring 140 All areas 0 53 0 71 0 87 0 87 0 72 Hubbard squash Summer 130 All areas 0 55 0 74 0 89 0 83 0 69 Lentils Spring 170 All areas 0 42 0 79 1 09 1 09 0 98 0 58 Lettuce Spring 75 All areas 0 44 0 81 0 92 Lettuce Autumn 100 All areas 0 49 0 74 0 95 0 93 Spring medium Maize grower 135 areas 0 39 0 75 1 14 1 14 0 93 Summer short Maize grower 120 areas 0 36 0 79 1 14 1 03 Oats Winter 140 All areas 0 64 0 82 1 12 1 11 0 66 Onions Autumn transplant 160 All areas 0 58 0 60 0 76 0 86 0 82 0 67 Paprika 130 areas 0 51 0 76 0 98 1 00 1 00 Actually a bi annual Parsley Spring 270 All areas 0 49 1 13 1 15 1 15 1 15 1 15 1 15 1 15 0 73 plant Pastures Autumn 170 All areas 0 45 0 77 0 80 0 80 0 80 0 69 Peas Autumn Winter 110 All areas 0 49 0 82 1 09 1 07 Potatoes Spring Summer 120 areas 0 29 0 89 1 09 1 01 Pumpkin Spring Summer 125 areas 0 38 0 67 0 88 0 79 0 59 Pumpkin Autumn Winter 150 All areas 0 39 0 63 0 86 0 87 0 70 Radishes Spring Summer 45 All areas 0 72 0 86 Ryegrass Autumn 250 areas 0 61 0 97 1 00 1 00 1 00 1 00 1 00 1 00 0 74 Sorghum Spring 170 areas 0 45 0 77 0 80 0 80 0 80 0 69 Soybeans Spring short grower 120 All areas 0 43 1 04 1 14 1 04 Spring medium Soybeans grower 130 areas 0 44 0 97 1 14 1 14 0 83 Soybeans
9. 1 11 1 10 0 65 Brocolli 80 All areas 0 63 0 86 0 98 Brussels sprouts Autumn 120 All areas 0 64 0 87 1 07 1 01 Butternut 110 areas 0 52 0 89 0 98 0 82 Cabbage Early Spring 75 All areas 0 58 0 93 0 99 Canola Spring 120 areas 0 39 0 66 1 10 0 90 Carrots Spring Summer 100 areas 0 44 0 67 0 94 0 98 Carrots Autumn Winter 107 All areas 0 48 0 67 0 92 1 00 Cauliflower Main Spring 100 areas 0 55 0 82 0 99 0 99 Cauliflower Main Summer 90 All areas 0 53 0 84 0 99 0 99 Cauliflower Main Autumn 120 All areas 0 59 0 80 0 98 1 00 Cauliflower Main Winter 100 All areas 0 73 0 89 0 99 0 99 Cereals Grazing Autumn 190 areas 0 48 0 60 0 88 0 90 0 90 0 80 0 50 Eastern Cape Chicory Spring 195 warm 0 43 0 51 0 76 0 90 0 90 0 90 0 90 Chillies 110 areas 0 39 0 73 1 09 0 75 Coriander Spring 90 All areas 0 43 0 96 0 93 Spring medium Cotton grower 160 Warm areas 0 40 0 63 1 01 1 09 1 07 0 93 Cow peas Spring 100 areas 0 45 0 93 1 06 0 81 Cucumbers 105 All areas 0 45 0 93 1 09 0 75 Cucurbits Spring Summer 130 All areas 0 54 0 75 0 93 0 93 0 72 Irrigation scheduling 12 55 Tablel2 17 continue Climatic k Months from plant Crop Plant Crop option Days region Mnth Mnth Mnth Mnth Mnth Mnth Mnth Mnth Mnth ek 1 2 3 4 5 6 7 8 9 en Cucurbits Autumn Winter 140 All areas 0 68 0 86 0 99
10. 13 Apparatus required for the gravimetric method 6 3 Determining the soil s electric resistance The electric resistance of a volume of soil depends among others on the soil water content If the electric resistance of a soil is determined such a reading can after calibration be converted to soil water content An example of a measuring instrument which is based on this principle is the gypsum block Figure 12 14 The instrument consists of a porous gypsum block in which two electrodes are placed which are connected to two electric cables When the block is buried in the soll the water in the gypsum block will equilibrate with the soil water Water will move through the pores of the block until the matrix potential soil water tension inside and outside the block is the same The resistance that the electric current experiences in flowing between the two electrodes can then be determined by means of an ordinary resistance bridge The resistance 15 equal to the prevailing soil water tension but the resistance can also be calibrated against the soil water content gravimetric or volumetric Soil water tension can also be directly determined while the absolute soil water content can be read indirectly from a soil water characteristic The two electrodes can also be placed in porous nylon or fibreglass blocks Irrigation scheduling 12 29 Figure 12 14 Resistance block Benefits The gypsum blocks are relatively cheap and the resistan
11. 45 0 37 0 29 0 24 0 24 0 24 0 24 0 28 0 48 0 55 0 55 Salt bush areas 0 31 0 31 0 31 0 31 0 29 0 26 0 26 0 26 0 29 0 31 0 31 0 31 Strawberries areas 0 75 0 51 0 38 0 38 0 38 0 38 0 38 0 38 0 50 0 74 0 86 0 86 Sugar Winter harvest KwaZulu Natal Lowveld 1 15 1 15 1 15 1 15 1 12 0 68 0 39 0 51 0 77 1 02 1 15 1 15 Sugar Spring harvest KwaZulu Natal Lowveld 1 15 1 15 1 15 1 15 1 15 1 15 1 15 1 15 1 12 0 67 0 44 0 88 Tea Lowveld 0 68 0 68 0 68 0 70 0 72 0 75 0 76 0 76 0 76 0 75 0 72 0 70 Walnuts Mature areas 0 90 0 90 0 58 0 23 0 23 0 23 0 23 0 23 0 23 0 56 0 90 0 90 Walnuts Young areas 0 53 0 53 0 38 0 23 0 23 0 23 0 23 0 23 0 23 0 37 0 53 0 53 12 54 Irrigation User s Manual Table 12 17 Penman Monteith Crop Factors Annuals Assumptions 1 Weekly irrigations 2 Wetted area 100 Climatic k Months from plant Crop Plant Crop option Days region Mnth Mnth Mnth Mnth Mnth Mnth Mnth Mnth Mnth R 1 2 3 4 5 6 7 8 9 eid Babala Spring Summer 140 areas 0 38 0 65 1 05 1 02 0 59 Barley Winter 140 All areas 0 64 0 82 1 12 1 11 0 66 Beans Dry Spring 100 areas 0 45 1 02 1 14 0 87 Beans Green Spring Summer 90 All areas 0 45 0 91 1 03 Beetroot 90 areas 0 46 0 94 0 98 Brinjals Spring 140 areas 0 39 0 66
12. Crop Factor Planting Details Crop type Maize late plant 7 Option Med variety Geographical Region N Cape Karoo ha Planting Date 15 Month December Irrigation scheduling Cover at Full Growth 100 x Leaf Area Index Jan 0 39 Dec 0 22 12 What to Do Enter or Edit own Data Crop Factors vs Months Reference E vaporation Apr 1 01 97 Jol xi x Frequency of Wetting Initial Rest of Season days Wetted Area Water Req Back to Map Ref Print Window Edit Crop Factor What to Do Crop Factors Figure 12 9 Crop coefficients screen SAPWAT 3 5 Calculation of the nett irrigation requirement 12 19 The nett irrigation requirement NIR is the amount of water required by the crop to comply with the crop evapotranspiration in a specific growth phase and during a specific period It can be calculated as follows where tr Mm gt ge 4 Soil NIR NIR f E R xL xL 12 5 Nett Irrigation Requirement period crop factors for direct use with A pan evaporation fraction crop spacing m row spacing m A pan evaporation mm period In this section the factors influencing the size of the soil water reservoir are dealt with briefly For further information Chapter 4 Soil of this manual can be consulted The definitions relating
13. delay measurement is also based on the measurement of the dielectrical constant of soil or materials The frequency delay measurement is also called the radio frequency RF capacitance technique Capacitance technique is usually referred to as it measures the soil s capacitance Two three or four electrode probes are also inserted in the soil The probes are collectively connected to a test pin and the probes of some types Delta T probe can screw in if it has to be replaced The soil acts as dielectricum by completing the capacitance circuit which forms part of the feedback circuit loop of the high frequency transistor oscillator High frequency radio waves of between 100 and 150 Mhz are also pulsed through the capacitance circuit A natural resonant frequency is thereby established which is dependent on the capacitance of the soil The soil s capacitance is related to the dielectricum constant which is created by the geometry of the electrical field around the electrodes A number of commercial apparatus using this technique are available namely Netafim Flori SDEC s HMS 9 000 Delta T s ThetaProbe and the Aquaterr probe Irrigation scheduling 12 37 3 4 BSP EN thread 112mm 3mm dia rods 4 off 60 Figure 12 20 Example of a capacitance test probe Theta Probe These capacitance probes are installed in the same way as TDR wave conductors in the side of a profile hole Some manufacturers arrange the electrodes around
14. divided broadly into four groups namely a monitoring of the soil water content b monitoring of the matrix potential scheduling regarding the set up of a water balance sheet by means of the calculation of the crop evapotranspiration and d monitoring of the plant s reaction The first three methods are discussed in this manual Most of the methods that measure the plant s water status are only used in tests and contains slow readings and processing with expensive apparatus At this stage it 1s therefore not recommended to producers on a commercial basis Techniques that determine the water status of plants include infrared thermometers pressure chambers dendrograph that reads the expansion of the plant stem and equipment that measures certain physiological processes 6 1 Feel method Determining soil water by means of feeling and observation 1s one of the oldest methods used for determining the soil water status This is a simple method but practice and experience is necessary to determine this exactly Soil samples taken at different depths the root zone is collected with a soll auger after which it is thoroughly studied and feeled The soil is classed on the basis of the observation and the soil water can be determined with the aid of tables The soil water can be determined within 10 1596 accuracy with a little practice Table 12 7 shows the relation between soll water and soil appearance This method of scheduling 1s no
15. influence the readings adversely The higher the humus in the soil the higher the registered water content v The sensors can also not be installed near the soil surface A portion of the heat pulse escape above the surface and a lower reading is then obtained 12 40 Irrigation User s Manual Table 12 8 shows a comparison between the above apparatus and methods Table 12 8 Comparative review between different scheduling techniques Jordaan 2001 Apparatus or Measured Reaction Disadvantages Benefits method parameter time Feel Appearance and 10 15 accurate Cheap feeling 1 2 min Easy Gravimetrical Mass of water Destructive test content Vo dry 24 hours Time consuming Very accurate mass vs wet Automatic control Independent of salt mass impossible content of soil Dry mass density must be known Electrical Soil water Individual calibration resistance potential by 2 3 Varying calibration Relatively cheap method means of hours with time Non destructive test electrical Limited lifespan resistance Less accurate reading Tensiometer Soil water Limited reading area of potential soil 2 3 0 8 bar water tension hours Needs a retention curve to convert reading to volumetric water Relatively cheap content Non destructive test Hysteresis effect reading Requires good maintenance Neutron water Volume water High cost meter content o water 1 2 min Dependent dry mass
16. irrigation design shows the same calculations in mm This chapter contains information regarding scheduling techniques obtained from Class notes and Water management course of the University of Stellenbosch as edited by Dr J E Hoffman The information describes scheduling techniques applied in practice namely a the monitoring of the soil water content and or soil water stress or b by calculating evapotranspiration by means of monitoring the climate Information regarding the climate crops and evapotranspiration is obtainable from the Irrigation Design Manual of the ARC Institute for Agricultural Engineering 12 2 Irrigation User s manual 2 Climate Climate is influenced by the following 2 1 Rainfall Rainfall is the depth of rain measured in a correctly set up rain gauge for a specific period 2 1 1 Rainfall parameters The rainfall for an area is often characterised by the average of the total annual rainfall measured over a long period Usually the results of at least 30 years are used to determine an acceptable average As extreme values strongly influence the average a tendency to express rainfall for an area in terms of a median value has arisen The median is the middle value obtained when yearly values are arranged in ascending order The average annual rainfall is used the planning of dam capacities and also in irrigation water balance calculations The shorter the period for which averages are determined the less
17. iv The use of the apparatus is not a health hazard v Continuous data logging of the soil water content at different depths is possible The probes can be connected to any type of data logger Disadvantages 1 Probes must be placed in the soil very carefully and good contact must be ensured over the entire length of the soil Vacuum along the rods can result in incorrect readings 1 Installation procedures of access tubes are critical Problems are experienced with vacuum around the tube in stony ground A paste must then be made of the soil to ensure good contact uni hand test probe must fit tightly into the access tube Vacuum around the probe give inaccurate readings iv Systems that work at low frequencies lt 20 MHz are influenced by the soil s salt content Frequencies of 100 MHz are therefor normally used v Capacitance probes or combination rods cost between R500 and R12 000 depending on the length Irrigation scheduling 12 39 6 8 Heat pulse measurements Heat pulse sensors or Phene cells are made of porous blocks in which two electrodes are implanted These blocks are connected to an instrument that determines the water content of the soil Heat pulse sensors are also made of stainless steel rods of 15 cm long and 10 mm in diameter The temperature the sensors is read before and after a small heat pulse The size of the heat pulse transmitted by the soil is proportional to the water content that forms in t
18. must be available on demand at all times If water 1s only available to a producer e g every four weeks 1f he has a turn to use water every four weeks and has no storage dam he cannot schedule correctly A distinction must also be made between intensive irrigation and supplementary irrigation In the latter case the soil water supply is only supplemented during occasional drought periods in humid climates or the crop is only supplied with water during critical growth stages Strictly spoken scheduling does not apply in supplementary irrigation The same amount of water used for irrigation at all times with the same cycle length is also seen as an incorrect concept of scheduling according to the above definition The water 1s applied regularly but obviously the wrong amounts of water at incorrect times If irrigation scheduling is applied incorrectly it can lead to over or under irrigation Under irrigation mainly damages the size and quality of the harvest while over irrigation damages the root system that can cause the crop to die Although plant roots do not grow in dry soil it remains healthy and undamaged within limits until sufficient water is available again On the other hand over irrigation reduces the air content of the soil which promotes the contamination by anaerobic pathogens such as Phytophthora Even the absence of the pathogens a low oxygen level in the soil can damage the roots It 1s therefor important that the basic
19. on an oscilloscope The time taken for the signal to be reflected time delay is measured accurately by the cable tester With the length of the cable and wave conductors known the reproductive speed of the transversal electromagnetic wave can be calculated The dielectric constant is inversely related to the reproductive speed of the electromagnetic wave i e a faster reproductive speed delivers a lower dielectrical constant and therefore a lower soil water content A higher dielectric constant will therefore be an indication of a higher water content in the soil Wave conductors inserted into the soil consist of two or three parallel stainless steel probes arranged approximately 50 mm apart The wave conductors are usually inserted vertically horizontally or at an angle of 45 into the soil Some manufacturers use a wave conductor with three probes A screened off parallel connector cable conducts the electromagnetic volumetric wave between the wave conductors and the cable tester The TDR instrument measure the average volumetric water content over the length of the wave conductors The sphere of influence of an instrument around the wave conductors measuring point has a diameter of approximately 1 5 times the spacing of the parallel probes Irrigation scheduling 12 35 P3Z P3 P3S 34 tg w Red diem eler 35mm 181 160 35 35 Magus wo amargemen Rod dime 2mm Figure 12 19 Presenta
20. plants also functioning as a solvent in plants in which gases minerals and organic solutions are transported from one part to another Water plays an important role within the plant with photosynthesis and many hydrological processes as well as maintaining turgor which is imperative to cell enlargement and produces plant growth More than 90 of the water absorbed by a plant s roots is released in the atmosphere as water vapour The process 1s known as transpiration and is defined as the loss in water to the atmosphere from a growing plant which 1s regulated by physical and physiological processes in the plant Why do plants then lose such large quantities of water through transpiration The answer lies in the composition of a leaf The main function of a leaf is photosynthesis which manufactures food for the entire plant the required energy being obtained from sunlight The plant must therefore expose the largest possible transpiration area to sunlight Sunlight 1s however only one of the requirements of photosynthesis as the chlorophyll also requires carbon dioxide Carbon dioxide 15 normally readily available in the air surrounding the plant but before it can reach the plant cells by 12 12 Irrigation User s Manual diffusion it must be dissolved Carbon dioxide in the air must come into contact with a moist cell surface because the cell walls cannot readily absorb it in a gaseous form Evaporation occurs wherever water 1s exposed to air
21. reliable the results are 2 1 2 Effective rainfall rain that falls is not available to plant roots because of interception evaporation run off and seepage Rainfall figures are often used as if all rainfall reaches the soil surface This is not the case as dense foliage needs more water to wet the leaf cover to such an extent that water will move between the leaves to wet the soil The part of rainfall which remains on branches and leaves of plants is known as intercepted water which is considered only as water being lost by evaporation and not water which may eventually trickle down the stalk A relatively large quantity of water is therefore subtracted from the measured rainfall as evaporation losses and depends mainly on the following e Density of foliage Trees and shrubs intercept more water for the same area than strawberries or onions e Leaf area Leafy crops intercept more water than stalky crops e g potatoes as opposed to wheat e Rainfall duration A short shower will have a higher percentage of interception than a long shower e Rainfall intensity High intensity showers have a lower percentage of interception and vice versa for the same time Statistics obtained from South African weather experts indicate that interception by plants makes up no more than 10 15 of annual rainfall With forests however the loss may be as high as 25 of total annual rainfall There are varied opinions on the hydrological signifi
22. s water consumption varies during the season and with a crop increase the water requirement also increases to a point Plant water absorption is also higher with a high groundwater level 3 4 1 Daily and seasonal evapotranspiration Evapotranspiration 1s at its highest during the middle of the day and lowest during the night Seasonal evapotranspiration is used to determine the amount of water required for irrigation during one season EENSEEEEZTTER ols ette Ad ONE SRE REE dus Rate eRe x Cla ee ELLE LI TL sS LETT Tm LLL LE LL LIE LT 1 E dde DAYS AFTER PLANTING EVAPOTRANSPIRATION Figure 12 6 Typical seasonal evapotranspiration 3 4 2 Peak periods of evapotranspiration The peak period is the plant s growth stage with the highest average evapotranspiration An irrigation system must be designed to supply sufficient water during the plants peak period of evapotranspiration This peak consumption period usually occurs when plants start to produce their crops 12 16 Irrigation User s Manual 3 4 3 Factors influencing evapotranspiration The rate of groundwater withdrawal by the evapotranspiration process is mainly determined by climate groundwater storage irrigation practice soil texture tilling practice type of natural plant or crop being cultivated salinity of soil or irrigation water Of the above climate is the factor which has the highest influence
23. shallower tensiometer will indicate when irrigated should be started while the deeper placed tensiometer 18 used to indicate whether irrigation is done correctly 1 e that the matrix potential at that depth will not rise substantially higher than approximately 5 kPa field capacity For shallow rooted crops only one tensiometer is necessary Note that with this method only the matrix potential is monitored To calculate the soil water content and consequentially the irrigation amount the soil water characteristics curve of the specific soil must be known Irrigation scheduling 12 33 30cmDepth 100 cm Depth 4 Tensometer reading kPa Date Changes in the tensiometer readings under the influence of soil water comsumption and irrigation a did not wet the soil deep enough and the deeper tensiometer s readings kept rising Irrigation b was slightly too heavy and the tensiometer indicated a water saturated condition for too long readings smaller than 19 kPa Irrigations c and d was given at the right time and the correct amount of water was applied Figure 12 18 An example of the changes in the tensiometer reading at two depths as a function of water consumption and irrigation van Zyl 1981 Benefits i ii iti iv The tensiometer readings are a direct indication of the amount of energy that a plant must apply to take up water from the soil After installation the soil water content is determined on t
24. store the data If the soil is wetted again as during a rain shower or by irrigation and the soil water tension in the soil reduces to below that of the tensiometer the tensiometer will indicate a zero reading when the soil is fully saturated The fluctuations in the tensiometer reading therefore indicates how the soil water tension reduces during irrigation or rises during drying A graph showing such fluctuations during an irrigation season is shown in Figure 12 18 Pressure meter Plug SOIL Tube Water Porous point head Figure 12 17 Schematic presentation of a tensiometer with a vacuum meter The installation and maintenance of tensiometers require considerable knowledge and attention New tensiometers must first be examined for possible leakages all the air must be removed and it must then be set at a zero reading Directives in this regard are supplied by Piaget 1991 The mercury manometer type tensiometer is more sensitive and accurate than the vacuum meter type especially in the low water tension area It is however more breakable than the vacuum meter type Mercury manometer tensiometers are therefore mostly used as research instruments while the vacuum meter type is better suited for practical application by producers The effective and rational use of tensiometers in irrigation agriculture requires that they should be placed correctly At least two tensiometers are required for deep rooted crops The
25. the matric potential in the root zone must not drop lower than 50 kPa Each time the matric potential drop to this value irrigation water must be applied Both terms are usually used interchangeable and refers to the amount of water to be maintained the soil For further information see Chapter 4 Soil of this manual A typical example of the relation between the allowable water depletion and the water tension is shown in Table 12 5 Table 12 5 Typical example of the relation between the allowable water depletion and the water tension for table grapes Scheepers et al 1991 Allowable water depletion Maximum tensionmeter reading Month within root zone kPa SWC mm m SWC mm m 50 250 50 250 Winter 70 100 50 70 August 70 100 50 70 September 50 70 30 50 October 50 70 30 50 November 50 70 30 50 December 50 70 30 50 January 50 70 30 50 February 50 70 30 50 March 50 70 30 50 April 70 100 50 70 Please note Similar information for other crops can be obtained by contacting your local crop specialists The soil water capacity SWC in this case is considered between a soil water tension 10 kPa and 100 kPa From the above table it seems clearly that a smaller water extraction and smaller maximum tensiometer reading for soils with a low 50 mm m SWC must be maintained than for a high SWC It reduces the risk that if an area experiences a heat wave crop growt
26. water potential When roots absorb water the soil water content reduces at that point causing an increase in the soil water tension binding the remaining water The soil water tension will be at its highest when the soil moisture content has dropped close to wilting point in other words the attracting or suction force of the water in the soil 1s at its highest However rapid root growth during active growth periods maintains sufficient water contact even though the soil water profile is decreasing as a whole and no capillary addition occurs In any case root growth is more rapid than capillary water movement during active root growth It may eventually happen that the roots are unable to supply water to the plant fast enough this 1s called temporary wilting and if it continues too long the plant may wilt permanently 3 2 1 Root systems The nature of the root system which a plant can develop under optimal soil and climatic conditions is predetermined by its genetic code see Table 4 3 and Figure 4 3 Therefore each plant species has its own characteristic natural root growth pattern Some plants have a tap root which tends to penetrate rapidly deep into the subsurface layers while others have slowly growing roots which develop a shallow primary system with many lateral secondary and tertiary roots The natural lateral root distribution of trees 1s normally as wide as the drip area of the tree Irrigation scheduling 12 13 Table 12 3 Na
27. 0 90 0 90 0 90 Almonds Young middle season areas 0 53 0 51 0 41 0 29 0 23 0 23 0 23 0 23 0 37 0 53 0 53 0 53 1 Mature middle season areas 0 98 0 93 0 69 0 42 0 28 0 28 0 28 0 28 0 28 0 50 0 95 0 98 Apples Young middle season areas 0 57 0 55 0 43 0 30 0 23 0 23 0 23 0 23 0 23 0 34 0 55 0 57 Apricots Mature middle season areas 0 71 0 59 0 47 0 35 0 28 0 28 0 28 0 28 0 56 0 89 0 90 0 83 Apricots Young middle season areas 0 44 0 39 0 33 0 25 0 24 0 24 0 24 0 24 0 37 0 53 0 54 0 50 Asparagus areas 0 96 0 89 0 56 0 38 0 38 0 38 0 38 0 38 0 38 0 66 0 96 0 96 Avocado Mature Lowveld 0 98 0 88 0 68 0 58 0 58 0 58 0 58 0 58 0 58 0 68 0 88 0 98 Avocado Young Lowveld 0 57 0 51 0 37 0 31 0 3 0 31 0 31 0 31 0 31 0 37 0 50 0 57 Bananas Ratoon Lowveld 0 90 0 90 0 84 0 70 0 56 0 49 0 49 0 49 0 49 0 49 0 69 0 90 Brambles Winter rain Highveld 1 11 1 03 0 86 0 69 0 61 0 61 0 61 0 61 0 79 1 08 1 11 1 11 Cherries Mature middle season Highveld 0 74 0 60 0 46 0 32 0 25 0 25 0 25 0 25 0 32 0 73 0 90 0 87 Cherries Young middle season Highveld 0 46 0 40 0 34 0 28 0 24 0 24 0 24 0 24 0 27 0 46 0 53 0 52 Citrus Mature average production areas 0 63 0 63 0 63 0 62 0 61 0 61 0 61 0 62 0 63 0 63 0 63 0 63 Citrus Young average production areas 0 38 0 38 0 38 0 39 0 39 0 39 0 39 0 38 0 38 0 38 0 38 0 38 Coffee Mature Lowveld 0 98 0 88 0 68 0 58 0 58 0 58 0 58 0 58 0 58 0 68 0 88 0 98 Coffee Young Lowveld 0 57 0 51 0 37 0 31 0 3 0 31 0 31 0 31 0 3
28. 1 0 37 0 50 0 57 Cut flowers areas 1 15 1 15 1 15 0 90 0 62 0 62 0 62 0 62 0 88 1 15 1 15 1 15 Date palm Mature Karoo North Cape 0 98 0 98 0 98 0 98 0 79 0 59 0 59 0 68 0 88 0 98 0 98 0 98 Date palm Young Karoo North Cape 0 57 0 57 0 57 0 57 0 45 0 32 0 32 0 38 0 51 0 57 0 57 0 57 Fescue pasture areas 0 80 0 80 0 80 0 80 0 76 0 72 0 72 0 74 0 78 0 80 0 80 0 80 Grapes Table middle season areas 0 66 0 57 0 45 0 37 0 26 0 26 0 26 0 26 0 34 0 63 0 67 0 67 Grapes Wine middle season areas 0 55 0 55 0 49 0 34 0 26 0 26 0 26 0 26 0 29 0 50 0 55 0 55 Guavas Mature Winter rainfall 0 90 0 90 0 90 0 65 0 38 0 38 0 38 0 38 0 38 0 41 0 67 0 87 Guavas Young Winter rainfall 0 53 0 53 0 53 0 40 0 25 0 25 0 25 0 25 0 25 0 27 0 39 0 51 Litchi Mature Lowveld 0 83 0 78 0 73 0 68 0 65 0 72 0 84 0 91 0 91 0 91 0 91 0 88 Litchi Young Lowveld 0 50 0 47 0 44 0 42 0 40 0 44 0 50 0 54 0 54 0 54 0 54 0 52 Lucerne Semi dormant Areas with frost 0 86 0 86 0 78 0 62 0 47 0 38 0 38 0 46 0 62 0 78 0 86 0 86 Lucerne Semi dormant Areas without frost 0 86 0 86 0 86 0 77 0 61 0 52 0 52 0 57 0 69 0 80 0 86 0 86 Macadamia Mature All areas 0 90 0 79 0 57 0 35 0 24 0 24 0 24 0 24 0 56 0 90 0 90 0 90 Macadamia Young All areas 0 53 0 48 0 38 0 28 0 23 0 23 0 23 0 23 0 37 0 53 0 53 0 53 Mangoes Mature Lowveld 0 72 0 67 0 62 0 56 0 51 0 53 0 53 0 78 0 83 0 83 0 82 0 78 Mangoes Young Lowveld 0 45 0 43 0 41 0 39 0 37 0 37 0 37 0 48 0 50 0 50 0 50 0 48 Pastures Summer amp Winter All are
29. 4 6 2 Penman Monteith methods short grass reference 12 18 3 5 Deternination of the nett irrigation requirements iii 12 18 A SOM ae AE ee 12 19 5 ItriBation Sy Stel veren annen teen 12 23 6 Scheduling techniques e lets 12 25 Os Feel method ctt 6 2 Gravimetrical soil water determination i 12 27 6 3 Determining of the soil s electric resistance 12 28 6 4 Neutron watermeter 6 5 Tensiometry 6 6 Pulse delay measuring Time domain reflectometry 12 34 6 7 Frequency delay measuring 6 8 Heat pulse measuring 6 9 Scheduling with regard to crop evapotranspiration ET annen 12 41 12 42 8 Sscheduling calcu lation e ertet ente estet ete deett ete debe bs Qi Reterencesca oe eet ute Maat a nie ih rel a APPENDIX A Cropfactors and coefficients 12 45 Irrigation scheduling 12 1 1 Introduction The main purpose of irrigation is to supplement the soil water reserve order to operate optimal crop production in regions where it would not otherwise be possible However to apply irrigation efficiently it is essential to measure the requirements of the crop and then applying the correct volume of water in the correct place and at the correct time This organised way on which water 15 supplemented is called irrigation scheduling In order to schedule correctly water
30. 5 5 0 7 0 6 95 Area A Limpopo Eastern Transvaal Lowveld and Northern Natal Area Loskop Rust De Winter and Barberton AreaC Vaalharts Karoo Eastern Cape and Transvaal middle veld 12 48 Irrigation User s Manual Table 12 11 Estimated design crop factors for vegetables in the summer rainfall areas June 1996 Vegetable crops Beans Brassicas Cucurbits Peas Onions Tomatoes 0 20 0 e s Ajo 20 40 40 60 0 4 0 4 Portion of growing season 0 7 60 80 0 a 2 2 6 lt 80 100 0 7 0 7 0 7 0 6 0 7 0 7 Irrigation scheduling 12 49 Table 12 12 Estimated design crop factors for perennial crops in the winter rainfall area June 1990 Months of the year Perennial crops Description HE Sie oan Table grapes o4 Deciduous fruit Late 04 04s Medium o4 045 os Barly o4 045 os E Sub intensive 02 025 025 Intensive Late 04 Barly o3 04 os Pasture Mixed 055 055 0 55 Pasture Kikuyu 055 oss 055 Alfalfa Frost areas 0 55 0 55 0 55 Guavas Prune Aug 0 4 0 5 0 4 0 4 0 3 0 3 0 3 0 2 0 2 0 2 0 3 0 4 Citrus Wine grapes
31. CONTENTS Irrigation scheduling 1 Introduction mn der tt tte odes s teta 12 1 12 1 2 1 Rainfall 12 1 2 1 1 Rainfall parameters 2 1 2 Effective rainfall 2 2 Evaporation 2 2 1 Evaporation measurement 2 2 1 1 Erection en 12 3 2 2 1 2 Calibrating the scale 12 3 2 2 1 3 Reading the scale 12 4 2 2 1 4 Maintenance i eee ettet deis 12 6 2 3 ettet ettet 2 4 Sunshine duration 2 5 Temperature dst E M Ne 2 60 Water VADOUE Saba Ne a eeens 2 7 Wild the A A S ACLOPS ecl b 3 1 The role played by water in plants 3 2 Plant TOOLS etm 3 2 RooESystems eo sott 3 2 2 Factors influencing root development 3 3 Groundwater withdrawal patterns s 3 4 BvapotransplratlOtnx eost ete ere END bles ad 3 4 1 Daily and seasonal evapotranspiration 3 4 2 Peak periods of evapotranspiration s 3 4 3 Factors influencing evapotranspiration 3 4 4 Effect of groundwater levels on crop growth and harvest 212 16 3 4 5 CriticalperlOds 3 4 6 Determination of crop evapotranspiration 3 4 6 1 A pan evaporation with crop factors 3
32. Spring long grower 140 areas 0 43 0 87 1 14 1 14 0 99 Sweet melon Spring Summer 105 All areas 0 64 0 95 0 96 0 83 12 56 Irrigation User s Manual Table 12 17 continue Climatic k Months from plant Crop Plant Crop option Days radi Mnth Mnth Mnth Mnth Mnth Mnth Mnth Mnth Mnth gion Remarks 1 2 3 4 5 6 7 8 9 Sweet melon Autumn Winter 120 All areas 0 66 0 91 1 00 0 90 Spinach Autumn 190 All areas 0 39 0 51 0 77 0 98 0 99 0 98 0 95 Squash Spring 105 All areas 0 53 0 79 0 89 0 82 Sugarbeet Spring 240 All areas 0 42 0 60 0 96 1 15 1 15 1 15 1 15 1 15 Sunflower Spring Summer 100 All areas 0 40 0 92 1 14 0 86 Sweet potatoes Spring 150 areas 0 42 0 82 1 09 1 09 1 03 Sweetcorn Spring Summer 90 All areas 0 53 0 82 0 99 Tobacco Spring Summer 120 Warmer areas 0 41 0 84 1 09 0 92 Tomatoes Processing 100 All areas 0 67 1 06 1 02 0 85 Tomatoes Table 160 All areas 0 58 0 77 0 97 1 09 1 09 1 09 Watermelon Early 85 All areas 0 65 0 98 0 97 Watermelon Late 100 areas 0 64 0 96 0 98 0 95 Wheat Medium Winter 125 All areas 0 57 0 79 1 12 1 06 1 02 Short Autumn Wheat Spring 80 All areas 0 57 1 06 1 04
33. The evaporation of water from the leaf surface has a two fold function namely the absorption of carbon dioxide as described above and plant cooling Plants have developed a number of special methods to limit evaporation which in turn limits carbon dioxide absorption Photosynthesis and loss of water through transpiration are therefore firmly bound in the life of green plants 3 2 Plant roots Plant activity is normally proportional to soil water availability and therefore also the rate of water absorption by the root system While small quantities of water may be absorbed by external plant components under certain conditions the root system is usually the organ of absorption for virtually all the water required by upper plant sections Therefore the root system depth of each crop determines the soil water reservoir size or the total available water in normal and deep soils The water withdrawal pattern 1s then determined by the lateral distribution and specific properties of the roots Absorption of water and food substances takes place through the root hairs Older root sections suberise and transmit less water and dissolve food substances Plant roots make contact with water in two ways capillary movement of water to the roots and or e root growth from dry to moist areas Osmosis is the process by which root hairs absorb water it is the movement of water through a selectively permeable membrane from a higher water potential to a lower
34. WAT can also be used for calculating similar data by means of the Penman Monteith method Example 12 3 A pan evaporation E information is at the producer s disposal for setting up a water balance for a specific orchard Suppose that the readily available water in the root area is 30 mm The crop factor f during the period is 0 76 Use the above particulars to determine the crop evaporation transpiration and the water consumption of the crop with the water balance method Accept that the water reservoir is full at the beginning Solution Cumulative water Soil water reservoir dud E consumption mm content mm 1 4 3 3 27 2 9 7 10 20 3 9 7 17 13 4 6 23 7 3 7 5 28 2 6 8 6 6 24 7 8 6 12 18 8 9 7 19 11 9 7 5 24 6 10 7 5 29 1 After days 5 10 the cumulative water consumption will exceed the readily available water in the root area and the deficit 28 mm on day 5 and 29mm on day 10 must be supplemented with irrigation The influence on rainfall on the evaporation transpiration calculation is shown in Example 12 1 7 Scheduling models There are also other proven models that can be used for irrigation scheduling few of the models are discussed briefly more information is available from the report Ondersoek na sagteware vir besproeiingskedulering Jordaan 2000 report only available in Afrikaans 7 2 BEWAB BEWAB Besproeiingswa
35. a cylindrical test probe rod at different distances The test probe is then lowered into a uPVC access tube into the soil Soil water contents are then determined at the different depths according to the electrode spacing The depths vary in increments of 100 to 200 mm which can be specified by the user at some types and installed during manufacturing With other types such as the ADCON C probe and Sentek s EnviroSCAN the user can change the spacing The Troxler Sentry 200 AP apparatus and the DIVINER 2 000 Sentek uses an access tube similar to that of a neutron water meter to determine the soil water content at different depths The test probe of this type of apparatus fits tightly into the access tube and takes readings while it is lowered into the soil A natural resonant frequency or frequency movement between the radiated and received reflected frequency is measured by the test probe The DIVINER apparatus measures soil water content in volumetric units at pre programmed depths while it is lowered into the soil The data 1s then shown and stored on a data logger The data can also be downloaded onto a computer and various analyses can be executed thereon The access tube must be manufactured from a schedule 40 uPVC material The size and wall thickness of a tube ensures a tight fit of the test probe in the tube It ensures that the electromagnetic signal 1s radiated effectively Installation of the access tube must be such that the tube fit
36. al pointer at see Figure 12 2 15 just submerged The right hand side of the scale 1s then adjusted with the nuts at B until the water surface at G gives a scale reading of 138 and locked position Then the pan is filled until the scale reading is approximately 50 this reading being recorded in column 4 see Table 12 1 This process 1s repeated whenever the pan is cleaned or moved Evaporation pan readings may be recorded on a form see Table 12 1 which makes provision for the date column 1 rainfall column 2 water level before regulation column 3 water level after regulation column 4 and evaporation column 5 Irrigation scheduling 12 5 2 2 1 3 Reading the scale As with all weather readings the evaporation is measured at 08 00 The position where the scale cuts the water surface 15 wetted by finger before reading the scale The reading at the contact point is then taken Note that the white lines on the scale indicate EVEN values e g 70 72 74 76 78 while the blue lines indicate UNEVEN values e g 71 73 75 77 79 The value to be recorded from the enlarged part of Figure 12 2 is 73 Regularly make sure that the connecting hole between the pan and stilling basin is not blocked The evaporation for a specific day 1s determined by subtracting the reading for that day from the value in column 4 from the previous day dividing the difference by two and adding it to the rainfall for the day The unit is millimetre Evap
37. ally occur during the latter part of the growing season This is during the period of flowering to fruit ripening Irrigation scheduling 12 17 Table 12 4 The critical periods of crops Blossoming until fruit growth Bulb formation until harvesting Blossoming until fruit growth Vegetative growth stage 3 4 6 Determination of crop evapotranspiration Determination of crop evapotranspiration 1s the first step in project planning and the design of irrigation systems Various methods are used worldwide to determine crop evapotranspiration but only those used in South Africa will be treated in this section 3 4 6 1 A pan evaporation with crop factors This method assumes that for a given period crop evapotranspiration ET is directly proportional to the A pan evaporation E The standard method currently in use in South Africa 1s based on the average monthly A pan evaporation and crop factors The place and vicinity where the A pan 15 erected 1s important in obtaining a true reflection of the evaporation Different crops have different crop factors which vary during the growth season and should be adapted to suit the area as shown in Appendix A Figure 12 7 Direct determination of crop evapotranspiration by using A pan evaporation and crop factors 12 18 Irrigation User s Manual The equation to determine crop evapotranspiration ET directly from A pan evaporation is as follows f E 12 3 where ET crop ev
38. apotranspiration mm period f crop factor for direct use with A pan evaporation fraction evaporation mm period 3 4 6 2 Penman Monteith method short grass reference The alternative method for the calculation of crop evapotranspiration depends on the use of climate data from weather stations and the amended Penman Monteith equation Crop evapotranspiration can be calculated by means of the following equation ET k ET 12 4 where ET crop evapotranspiration mm period crop coefficient fraction ET reference evapotranspiration mm period Figure 12 8 The calculation of crop evapotranspiration with the aid of a weather station The computer program SAPWAT uses crop coefficients for the estimation of crop evapotranspiration for all crops irrigated in South Africa The program is available on the Internet on http sapwat org za please note no www address and can be used to examine different irrigation approaches Training 1s however essential The reference evapotranspiration 1s shown in SAPWAT as average daily values per month for the different weather stations Figure 12 9 1s an example of the screen on SAPWAT where the reference evapotranspiration 1s shown for a specific weather station Kakamas The bottom curve on the graph is the Penman Monteith reference evapotranspiration ET and the top curve is the A pan evaporation values at the same station ik Sapwat File Window E
39. as 0 85 0 85 0 85 0 85 0 85 0 85 0 86 0 86 0 86 0 86 0 86 0 85 Pastures Summer perennial All areas 0 81 0 81 0 76 0 66 0 55 0 50 0 50 0 50 0 55 0 65 0 76 0 81 Pawpaws Mature Lowveld 0 94 0 94 0 94 0 67 0 39 0 39 0 39 0 48 0 66 0 85 0 94 0 94 Irrigation scheduling 12 53 Table 12 16 continue Crop Crop options Climatic region Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Pawpaws Young Lowveld 0 55 0 55 0 55 0 41 0 26 0 26 0 26 0 31 0 40 0 50 0 55 0 55 Mature middle season areas 0 80 0 66 0 51 0 36 0 29 0 29 0 29 0 31 0 69 0 94 0 94 0 93 Peaches Young middle season areas 0 49 0 42 0 35 0 28 0 24 0 24 0 24 0 26 0 43 0 55 0 55 0 54 Pears Mature middle season areas 0 97 0 82 0 60 0 39 0 28 0 28 0 28 0 28 0 31 0 62 0 94 0 98 Pears Young middle season areas 0 56 0 49 0 39 0 29 0 23 0 23 0 23 0 23 0 25 0 40 0 55 0 57 Pecan nuts Mature areas 0 82 0 66 0 50 0 34 0 25 0 25 0 25 0 33 0 82 0 90 0 90 0 90 Pecan nuts Young areas 0 50 0 43 0 36 0 29 0 25 0 25 0 25 0 28 0 49 0 53 0 53 0 53 Pistachio Mature areas 0 82 0 80 0 61 0 39 0 28 0 28 0 28 0 28 0 54 0 82 0 82 0 82 Pistachio Young areas 0 49 0 48 0 39 0 29 0 24 0 24 0 24 0 24 0 36 0 49 0 49 0 49 Plums Mature middle season areas 0 89 0 72 0 55 0 38 0 29 0 29 0 29 0 29 0 37 0 80 0 94 0 94 Plums Young middle season areas 0 53 0
40. ble depletion dec 65 x 0 75 x 0 7 mm 34 13 mm Maximum soil water reservoir available per vine From Equation 12 6 SWR RAW xvine spacing m x wetted strip width 34 13 1 8 37 184 12 44 Irrigation User s Manual Nett irrigation requirement per vine From Equation 12 4 NIR A pan evaporation mm x crop factor effective rainfall mm xvine spacing m x row spacing m evaporation x 0 6 x 1 8 x 3 5 3 78 x A pan evaporation mm The SWR must be equal to the NIR therefore the amount that must evaporate from the A pan mm SWR 3 78 184 3 78 mm 48 75 mm b Amount of water to be applied SWR 6 wetted strip width m x vine spacing m x application efficiency dec 184 3 x 1 8 x0 8 mm 43 mm c Standing time From Equation 12 8 ts SWR 6 emitter delivery h x application efficiency dec 184 32 x0 8 h 7 hours 15 minutes Irrigation scheduling 12 45 9 References 10 Bennie A T P M J Coetzee van Antwerpen and D van Rensburg 1988 n Waterbalansmodel vir besproeiing gebaseer profielvoorsieningstempo gewaswaterbehoeftes WRC Report nr 144 1 88 Crosby C T 1996 SAPWAT 1 0 A computer program for estimating irrigation requirements in Southern Africa WRC Report No 379 1 96 Heyns P J J H Burger E P J Kleynhans F H Koegelenberg M T Lategan D J Mulder H S Smal C M Stimie
41. c Crop water relationships and climate 4 9 Table 12 2 continued Dry bulb Difference between wet and dry bulb thermometer reading 1 32 os 86 so e 57 sa a6 37 32 29 24 20 16 2 s a 93 86 80 e 2 se s as a 36 31 27 2 7 w 93 86 79 73 67 ot s5 so a s 34 30 25 a v n 5 93 86 9 2 6o oo sa do se 5 28 19 i5 n 7 3 93 5 79 hz os 50 sa ae a2 37 o o s 1 27 5 92 85 78 m ee se sr 4o do 34 29 mm t 9 sj 5 o2 s4 77 57 so a as 33 a7 2 Pam 2 se 77 7 ar ts sj 92 4 6 e oo 55 ae 36 2 8 3 o 93 76 68 ot sa 47 oo 34 zs 12 16 a__ 91 83 75 67 oo 46 39 26 120 ia 8s 3 Pe ae 9 oi 82 74 65 se 50 a3 3s 29 9 EN EN EN ENEN 6 1 6 8 4 97 2 15
42. cance of interception but not in a plant physiological sense Regarding plant physiologically intercepted rain 15 not a loss as the water on the leaves help to cool the plant thereby saving an equal amount of groundwater In view hereof interception losses should be seen as an alternative and not as additional to transpiration Irrigation scheduling 12 3 The monthly effective rainfall is determined as follows using long term average rainfall figures 20 R Raver 12 1 2 where effective rainfall mm month Raver long term average monthly rainfall mm month Daily effective rainfall may be determined amongst others by the following equation R 1 5E 12 2 where R effective rainfall mm day R rainfall mm day E A panevaporation mm day Effective rainfall is zero 1f a negative value 1s obtained SAPWAT Crosby 1996 may also be consulted for determination of effective rainfall 2 2 Evaporation Water evaporation rate depends on humidity temperature radiation air movement and attitude above sea level 2 2 1 Evaporation measurement Evaporation 1s measured by exposing free water to the atmosphere and then determining the loss through evaporation Therefore evaporation 15 not really a weather parameter but may be seen as a combination of different influences The South African Weather Bureau and the Department of Agriculture make use of the American Class A evaporation
43. ce can be determined with any commercially available resistance bridge The blocks function over the entire soil water spectrum i e from dry to wet but the accuracy and sensitivity 1s better in the dry area than in the wet area of the spectrum The sensitivity of nylon blocks in the wet area is better than that of the gypsum blocks After installation of the blocks the soil water content can be determined on the same spot every time The blocks can also be buried at any soil depth The apparatus can be connected to an automatic register Disadvantages Each block must be calibrated individually since even small differences in the dimensions of the blocks will cause a change in the resistance reading The calibration curve also changes with time especially in the case of the gypsum blocks where the use of the blocks lead to situations where high accuracy is not a requirement Soil characteristics other than the water content also influence the resistance reading It is especially dissolvable salts in the soil solution that plays a role here because the more dissolvable salts there is in the soil the better the soil will lead an electric current The changes in resistance reading are therefore not necessarily only because of a change in the soll water content Special set ups current circuits are necessary when more than one block at different depths in the same place is connected to a data register An example of a calibration curve i
44. culated as follows Irrigation scheduling 12 21 RAW SWC ERD lt 12 6 100 where RAW readily available water in root zone mm SWC soil water capacity of the soil mm m ERD effective root depth m allowable water depletion in the root zone 96 e Soil water tension or matrix potential kPa the suction power that the roots of a crop must exercise to extract the soil water The term soil water level and soil water tension are both used to refer to a certain condition which must be created and maintained in the soil by means of organised irrigation The term soil water level refers to the lowest soil water content in the root horizon or in a certain layer of the design root depth in the soil profile to which the available water AW is allowed to drop before irrigation must be applied It is shown as a percentage of the AW that should remain the soil layer after a portion thereof has been consumed With a e g 15 soil water level is meant that 85 of the AW can be consumed but that 15 of the AW must remain in the soil to prevent the entire soil profile from drying out to the PWP The term soil water tension is used exactly as the soil water level i e it refers to a certain water content to be maintained in the soil It is however also sometimes used to refer to the minimum matric potential that must not be exceeded in the root zone of plants When talking about a 50 kPa soil water tension it means that
45. e Plants however do not react to radiation in the same way as the human eye It is therefore incorrect to compare light as observed by eye to photosynthesis The unit of light intensity is the lux Ix 1 W m 680 Ix Radiation measurement is usually done by measuring the temperature relationship which occurs when the sun shines on a blackened surface Radiometers must be handled with great care 2 4 Sunshine duration In reality sunshine duration when the sun shines should be considered as the period between which the sun just begins to appear above the horizon until it just disappears below the horizon Weather experts consider sunshine duration as the period of full sunshine According to this definition full sunshine occurs when the sun burns a visible mark on special paper through a glass sphere of specific dimensions There are also other so called sunshine meters so full sunshine will depend on which one is used Electronic sunshine meters are available but not widely used due to high costs In South Africa the sunshine duration is measured with the Campbell Stokes sunshine meter Irrigation scheduling 12 7 2 5 Temperature The measuring unit for temperature is Kelvin K In South Africa air temperature is indicated in degrees Celsius It is however acceptable to use C as the unit for temperature C 273 In measuring temperature special care must be taken to eliminate the effect of sun radiation or
46. e irrigation system must be operated in such a way that the producer s available water resources can be utilized to its maximum The question often arises when to irrigate According to the design norms it is recommended that the micro irrigation system is designed for 144 per week s irrigation Night irrigation is however more efficient than day irrigation as the evapotranspiration is lower during the night Another advantage 1s that the energy cost outside peak times 1s lower There are unfortunately disadvantages to night time irrigation such as blockages leakages that can occur because there 1s no supervision Open hydroponics systems OHS require water application daily during the active transpiration period of the crop The OHS approach is discussed in Chapter 8 Micro irrigation systems It must always be kept in mind that with conventional irrigation systems a continuous strip in die crop row is irrigated With the OHS approach pots are irrigated which leads to a smaller soil water reservoir The theoretical cycle length and maximum standing time is calculated as follows and adapted according to the producer s requirements SWR os m 12 8 NIR where te theoretical cycle length days SWR soil water reservoir available per tree plant nett irrigation requirements per tree plant day Most producers prefer to follow a fixed irrigation cycle length and adapting the standing time according to the season If the
47. e temperature at which air will be saturated with water vapour provided that it is cooled down at a constant pressure Dew point is not dependant on temperature like relative humidity and it is also measured in K or C 12 8 Irrigation User s Manual Table 12 2 Percentage relative humidity only applicable for heights from the sea level to 450 m above sea level and wind speeds 1 5 m s Dry bulb Difference between wet and dry bulb thermometer reading 11 2 4 5 6 7 2 a6 v so Pos s9 sa so 75 v ee o ss se si ae a s s 9 Los pss 79 s o 66 ss se so 47 a oo 37 34 os josje se 79 74 v0 66 e sy ss so as 43 39 36 ai sb 79 a vo 6s ot ss Pao as ao so s x 29 s 79 less eo se s2 as as ar se as m 28 5 7 64 oo se 2 as a ar 37 34 Pm 28 s9 s3 7s 73 os o 59 55 si a7 as ao 36 33 30 27 ss s3 hm 3 os os 9 ss si a7 as s 36 32 29 a v os os se se so ao a se as st 2 25 T 40 s Pas 4 4 e 28 25 m se eem je he so si s ss mo 2 ss si 46 ao 38 34 26 oo oo 6 5 s m e
48. h could be inhibited 12 22 Irrigation User s Manual Figure 12 11 indicates the relation of soil water tension and percentage extraction of the total available water 10 Percentage extraction of the total available water 76 100 Clay 170 mm m 8 Loam 120 mm m 4 Sandy loam 90mm m Fine sandy loam 70 mm m Loamy sand 60 mm m Soil water tension kPa 1000 Figure 12 11 Relation between soil water tension and the total available water Itis clear from the above figure that for clay soils the water extraction at a 50 kPa soil water tension is only 15 while for loamy sand the extraction is already 72 of the TAW With the above information available the soil water reservoir per tree plant can be calculated GWR RAW L B 12 7 where SWR soll water reservoir per tree plant RAW readily available water in the root zone mm L crop spacing m W wetted strip width of the emitter m The management of the crop water reservoir is extremely important to make effective irrigation possible More irrigation than what the soil water reservoir can hold must not be applied as it can lead to wastage of water and nutrients Cash crops can be planted when the soil water reservoir is full and in this way less water have to be irrigated over the season Irrigation scheduling 12 23 S Irrigation system Th
49. h results show that the dielectrical constant is independent of the gross density of the soil v Continuous readings and data storage with the aid of data loggers are possible 12 36 Irrigation User s Manual Disadvantages 1 wave conductors should be installed very carefully to ensure contact along the entire length of the probes Vacuum along the probes cause faulty readings The probes must also remain parallel else the wave conductors do not function correctly 11 Wave conductors cannot readily be used in stony soils and special precautionary measures must be taken The access tubes of the probe are installed with a paste of the same soil iii Cable test apparatus is essential for analyses of the wave patterns Problems with the apparatus have not been completely solved 1 Soil brackishness of the gross electric conductivity of the soil influences the attenuation of the electromagnetic pulse in the soil The higher the salt content the lower the accuracy of the the soil Research is currently being done to find suitable 1solating materials for the probes to make it suitable for taking readings in brackish soils v TDR equipment is very expensive because of the high cost of the cable testing apparatus TDR soil water meters cost between R48 000 and R90 000 Individual sets of wave conductors cost between R160 and R840 depending on the length 6 7 Frequency delay measurement by means of Capacitance Frequency
50. he block In the case of the stainless steel rod the size of the reflected heat pulse 1s an indication of the water content of the sensor This means that a wetter soil or medium will warm slower than a dry one The increase in temperature or cooling 15 read with an accurate temperature sensor in the sensor It is calibrated at soil water content for the specific soil or sensor Benefits 1 The heat pulse sensors blocks are relatively cheap and can be read with a variety of commercial resistance meters 1 The sensors work over the entire soil water spectrum from wet to dry but the accuracy is better in the dryer portion of the spectrum iii soil water content can continuously be read on the same spot for different depths The sensors can be buried at any depth iv Both temperature and soil water content can be determined by the apparatus v It can be connected to a data logger to store data Disadvantages 1 Sensors have a high power requirement if readings must be taken very regularly 1 Each block must be calibrated individually small difference in measurement and depth influence the readings 11 The heat pulse sensor must have good contact with the soil which is not always possible Poor contact result in incorrect readings iv The thermic conduction or conductivity readings are also influenced by other soil characteristics except by the water content thereof Organic materials and humus content can
51. he same spot each time Changes in the soil matrix potential can continuously be captured with a data logger The soil water characteristics can be determined in situ in the soil with the aid of the apparatus Disadvantages i Gi iii iv The apparatus is quite fragile a single crack in the porous head is enough to render the tensiometer useless The tensiometer functions only in a relatively wet area of the soil water spectrum between 0 and 80 kPa If the tension rises above 80 kPa the meniscuses in the pores of the ceramic head breaks and air invades the tensiometer freely The fact that the tensiometer will only indicate soil water contents which are bound with tensions lower than 80 kPa is not a substantial problem on coarsely structured soils At such soils most of the plant s available water is in any case bound in the tension area 0 to 80 kPa In finer textured soils however a significant amount of the plant s available soil water can fall outside the mentioned tension area The apparatus must be services regularly It must be filled with de aerated water weekly and a vacuum must be created again This is done with a hand vacuum pump A time delay of up to ten minutes occurs in registering of the change in matric potential readings It is detrimental for continuous data capturing 12 34 Irrigation User s Manual 6 6 Pulse delay measurements by means of Time Domain Reflectometry TDR This method as well a
52. nce It is imperative that the pan regularly be cleaned once per month The following procedure is followed e Record the scale reading e Invert the pan and remove all accretions silt duckweed etc Ensure that the connecting hole between the pan and stilling basin is open Rinse the whole pan thoroughly e Inspect the pan especially the base and seams for possible leakages and rust spots When rusting becomes severe the A pan must be painted with aluminium bituminous paint e Always clean the openings between the upper beams to ensure good ventilation Re erect the pan level as close as possible to the previous position on the frame with water and calibrate the scale as indicted in Section 2 2 1 2 e Fill the pan to the starting reading prior to cleaning 2 3 Radiation All daily seasonal and cyclic climate changes can be traced back to the energy that reaches the earth in the form of electromagnetic radiation from the sun Radiation intensity is expressed in terms of watt per square metre W m The intensity relates to the rate at which energy is received At times it may be necessary to indicate a quantity of energy The unit of energy is the joule J An intensity of 1 W m is equal to 1 J s m The intensity of full summer sunlight is in the order of 1 000 W m It is often referred to as light intensity when radiation relates to photosynthesis Light is the radiation to which the human eye is sensitiv
53. om the root zone under given soil and climatic conditions and unlimited evapotranspiration ET still taking place The combination of allowable water depletion and the soil water capacity of the soil SWC 15 approached in practice according to two different methods o On the one hand and probably the popular method laboratory determined soil water capacity between 10 kPa and 100 kPa SWC 00 and a determined adapted set of values of a for the specific crop is used o On the other hand the soil water capacity between 10 kPa and 1 500 kPa SWC 1500 is used with a different set of values for a with reference to the same crop Note that although both methods give almost the same results care must be taken with the values of a and SWC so that applicable values be used for the specific calculation Minimum soil water level 100 the minimum percentage of the available water AW that must always be retained in the root zone for unimpeded evapotranspiration ET and crop growth under given climatic conditions This value indicates the replenishment point of the soil water reservoir Readily available water RAW mm the depth of water available to a certain crop in the effective root depth and unlimited evapotranspiration ET and allows crop growth under certain climatic and soil conditions The term also takes into consideration the allowable water depletion The readily available water in the root zone can be cal
54. on evapotranspiration The use of mulch also has an effect on evapotranspiration The mulch may consist of organic material such as hay or a permanent green crop such as clover In the case of dry material the mulch can aid in limiting evaporation from the soil and thereby reduce the evapotranspiration The irrigation requirement is therefore reduced There is however a danger that the organic material may prevent the crop from absorbing the chemicals applied to the surface In the case of green mulch the evapotranspiration will increase as a result of the water utilised by the mulch crop Provision must therefore be made for the additional irrigation water required during system planning 3 4 4 Effect of groundwater levels on crop growth and harvests Most plants absorb water easily at a high minimum water level low soil water depletion level provided that sufficient air is present in the soil As the groundwater level reduces the plant must use more energy to withdraw water from the soil Crop damage may occur if the groundwater level drops too low and plants may wilt temporarily or even permanently if the condition continues too long 3 4 5 Critical periods Most plants have a critical period during the growth season when a high groundwater level must be maintained for optimal production If enough water 1s available for germination as well as the development of a sufficient growth density with yearly crops the critical period will gener
55. onal irrigation system to meet these requirements Provision for the additional required water must therefore be made during planning as well as for as the costs it may implicate 2 6 Water vapour The relationship between the different gases comprising the atmosphere is exceptionally constant the only exception being water vapour which can change from one minute to the next The atmosphere is saturated when the maximum water vapour is present When saturated air is cooled excess water vapour will condense in the form of clouds mist dew or frost Alternatively the air will become unsaturated with an increase in temperature Such air can become very dry without losing any water vapour depending on the temperature increase Relative humidity is usually measured with a hygrograph placed in a Stevenson sun shield the measuring element being human hair which stretches as 1t becomes moist Water vapour can also be measured with two thermometers the bulb of one being covered with a moist cloth The dryer it becomes the quicker water evaporates from the moist cloth cooling the bulb down like a canvas water bag The relative moisture can be obtained from tables see Table 12 2 by making use of the temperature difference between the dry and wet bulb thermometers The cloth on the wet bulb thermometer should 1deally be moistened with distilled water A measure of the actual amount of moisture present in the air is the dew point The dew point is th
56. ong period as measured with ordinary anemometers is of little value The average speed and direction is usually required for a shorter period like an hour In many cases instant peak values are also important There are various types of wind meters using different techniques to continually measure and register wind speed and velocity Wind meters are erected so that air movement at a height of 2 m or 10 m is measured It is important to keep the area of exposure free from obstructions as wind measurement is strongly influenced by local conditions The recognised unit for wind speed is metres per second m s although it 1s often indicated km hour 1 m s 3 6 km hour Seafarers use knots one knot being equal to 0 514 m s With certain instruments air movement is measured over long periods of 6 hours or 24 hours normally referred to as wind run Wind run is measured in kilometres and is equivalent to the distance travelled by an air particle during the particular period at the average wind speed Wind direction is given as the compass direction that the wind is blowing from 3 Crops Crop water requirements depend on the relationship between water absorption and transpiration 3 1 The role played by water in plants More than half of all living fibre as well as more than 90 of all plant fibre consists of water therefore water is by far the largest plant component Water influences the metabolism physiological activity and growth of
57. onths of the year Description ai Mar Ap os os os Late 05 050 020 Medium Early 0 Sub intensive 2 EE T Intensive Rye grass Kikuyu Frost areas 5 2 5 5 gt gt oe A l o os os os os os os os o7 o7 o7 o7 0 65 0 65 0 65 0 65 0 65 0 65 0 65 0 65 0 6 0 6 0 7 0 7 0 8 0 7 0 65 May Aug 0 55 0 70 EEN ct 0 2 0 3 0 7 0 60 0 8 0 7 065 0 0 65 Nov 0 45 0 3 0 0 30 0 30 0 40 0 25 0 4 7 8 0 70 0 80 0 65 0 65 0 75 0 65 0 65 0 0 7 0 65 0 5 0 4 0 40 0 40 0 50 0 25 0 50 0 7 0 7 0 65 0 65 0 75 0 65 0 65 0 8 0 7 0 65 Irrigation scheduling 12 47 Table 12 10 Estimated design crop factors for agronomic crops in the summer rainfall areas June 1996 Agronomic Planting End of growth Months of the year crops date season gs loef m os ow 171 Tum T os os Pow pepe Area C 1 Nov 15 Apr 0 7 0 8 0 6 0 5 0 2 0 35 HN 05 07 08 08 0 3 0 6 0 75 9
58. oration pan readings are divided by two to allow for the enlarged scale used in the Class A Evaporation Pan NB When the water level is higher than 50 mm due to rain water is removed until approximately the 50 level and the value recorded in column 4 When the water level is lower than 100 due to evaporation water is added up to approximately 50 and the value recorded in column 4 Example 12 1 See Table 4 1 Reading procedure for Class A Evaporation Pan e Dayl Start with a reading of 50 and record it in column 4 Day2 Read the water level record the value e g 72 in column 3 as well as column 4 determine the A pan evaporation for day 1 as indicated and record it in column 5 e Day3 The same as for day 2 Dayd4 Read the water level record the value e g 112 in column 3 and regulate to 50 by adding water Water level after regulating In this case it is 48 which is recorded in column 4 the A pan evaporation is determined as indicated and recorded in column 5 Day 5 The same as for day 2 and 3 Day Record the rainfall for day 5 in column 2 Record the water level reading in column 3 and 4 determine the evaporation as indicated and record in column 5 Table 12 1 example of recording A pan evaporation reading Date Rainfall Water level Water level Evaporation Calculations before after mm regulating regulating ES Water added to A pan 12 6 Irrigation User s manual 2 2 1 4 Maintena
59. pan to measure evaporation The pan is circular 1 22 m in diameter and 250 mm deep The water surface height is read off from a scale located diagonally in the water As with a rain gauge the evaporation pan must be placed away from obstructions and in such a way that sun and wind can move around it freely Only direct rain must fall in the pan but it must also not be screened off The pan must never be in the shadow The Symonds pan is not recommended for measuring evaporation for irrigation 2 2 1 1 Erection The evaporation pan rests on a framework approximately 200 mm high which 15 placed level on the ground The openings between the lower beams of the framework may be filled up but those between the upper beams must be kept clean to promote ventilation and ease leak detection Grass and weeds around the pan must be kept short 12 4 Irrigation User s manual VECES 1220 NES MERANTI BEAMS GROUND LEVEL A PAN Figure 12 1 Erection of a Class A Evaporation pan 2 2 1 2 Calibrating the scale A stilling basin which is connected to the main pan by a hole is provided around the scale When water 1s added to the pan it takes about a minute for the water in the stilling basin to reach the same level therefore wait a few minutes before taking a reading Figure 12 2 The Class A Evaporation Pan scale The scale is calibrated as follows Once the pan has been positioned on the framework it is filled with water until the met
60. per volume density and salt content Non destructive Calibration needed for different soils Radio active danger Heat pulse Volume water Fast wear in alkaline reading content by means 2 3 soils of heat hours Readings affected by Wide working distribution salts in soil range Calibration needed for different soils Time domain Volume water Readings affected by reflectometry content by means 1 2 min salts in soil Non destructive test of a high Calibration needed for frequency pulse different soils Frequency Volume water Readings affected by domain content by means 1 2 min salts in soil Non destructive test reflectometry of retardation on Calibration needed for oscillator different soils Capacitance Volume water Readings affected by Non destructive test content by means 1 2 min salts in soil obtain profile of capacity Calibration needed for different soils of soil water content Irrigation scheduling 12 41 6 9 Scheduling with crop evapotranspiration ET and a water balance sheet As discussed the can be calculated by two methods namely A pan evaporation and crop factors and the Penman Monteith method The water balance method comprises the daily ET calculation and gives an indication of when to irrigate Scheduling models were developed for executing these calculations automatically Section 7 The example discussed here uses the A pan method The computer program SAP
61. principles of irrigation scheduling should always be maintained Various factors influence the amount of water applied 1 e the standing time e effective root depth of the crop and the critical periods during the growing season of the crop when water stress must be avoided Section 3 e size of the soil water reservoir which is dependent on the soil water capacity of the specific soil and the allowable water depletion from the soil before irrigation Section 4 e The irrigation system s gross application rate on the wetted area and the application efficiency of the water application Section 5 The question when to irrigate i e the cycle length is influenced by the following Climatic factors such as rainfall humidity etc Section 2 The evapotranspiration requirements of the crop Section 3 The soil factors as mentioned above Section 4 The wetted strip width of the irrigation system i e the percentage wetting Section 5 The influence of the climate and the crop on irrigation scheduling 15 discussed fully in this chapter while the other factors e g the irrigation system Chapter 2 Choice of system and the soil Chapter 4 Soil is discussed briefly in this chapter since they are already discussed fully in the chapters mentioned Examples of the calculation of standing time and cycle length are shown in this manual and depend on the litre water per crop principle Chapter 3 Planning and evaluation of an
62. radiation from the earth s surface Temperature is measured with suitable thermometers in a Stevenson shield The Stevenson shield 15 used to house measuring instruments and is designed to shield as many of the sun s rays and reflections from the earth s surface as possible from the instruments while allowing unrestricted air flow over them The purpose is to measure air temperature without the influence of radiation The Stevenson shield 15 erected on a metal frame so that the instruments therein are 1 2 m above the ground The shield must open southwards so that the sun does not shine on the instruments when readings are taken A Stevenson shield usually contains a maximum minimum and standard or dry bulb thermometer The maximum and minimum thermometers are mostly only read twice per day and indicate the highest and lowest temperatures respectively since the previous readings The standard thermometer indicates the temperature at the time of reading At times a continuous record of the temperatures is required which can be measured with a thermograph Thermograph readings must always be corrected by comparing them to the maximum and minimum thermometers Both extremely high and low temperatures can have an adverse effect on a crop Cooling as well as frost combating can be executed by application of water during critical periods and it will have an influence on the total water requirement of the crop It may also be necessary to install an additi
63. rk forms Dark in colour Will form a ball a weak ball or none a weak ball forms a ball small lumps can be at all 67 mm m slightly brittle pressed flat without 42 mm m 83 mm m crumbling 92 100 mm m 35 40 Dry does not form Slightly discoloured Slightly dark forms Slightly dark forms a ball by water will not a weak ball a weak ball lumps 50 58 mm m form a ball 100 108 mm m crumble 75 83 mm m 117 125 mm m Smaller than 20 Very dry loose Dry loose flows Light colour Hard baked flows through through fingers powdery dry cracked light in fingers 108 133 mm m 133 167 mm m colour 67 83 mm m 150 208 mm m NOTE The values in brackets indicate the estimated soil water shortage mm m to field capacity for uniform soils Squeezing the soil hard your hand forms a ball Rolling the soil between the thumb and index finger forms a sausage Irrigation scheduling 12 27 6 2 Gravimetrical soil water determination For many years the gravimetric method was the standard technique according to which the soil water content was determined The method contains the following steps i 1 iii iv removal of a soil sample from its field condition weighing the sample in its wet condition drying the sample at 105 C for about 18 hours and determining the oven dry mass of the sample The mass difference between the wet and dry
64. s 1 15 90 52 44 36 27 20 12 5 12 10 Irrigation User s Manual Table 12 2 continued Dry bulb ra 4 KAES es oo oo oo oo oo oo oo A gt N N oo oo oo 1 A AA A N N A N gt ON N oo Difference between wet dry bulb thermometer reading Irrigation scheduling 12 11 2 7 Wind Wind originates due to differences in air pressure Air moves from an area of high pressure to one of low pressure These pressure differences may occur locally or could originate in pressure systems spanning thousands of kilometres Wind measurement consists of two components namely direction and speed At most weather stations only speed is measured by means of an anemometer which has three circular cups mounted on a vertical axis The speed with which the cups rotate is proportional to the wind speed The axis about which the cups rotate is connected to a mechanical counter which is read at a fixed time always 08 00 The distance that the wind has moved during a period of time 1s measured by determining the difference between two consecutive readings The distance usually describes the wind run and may be interpreted as the average wind speed for the particular period For most purposes the average wind speed over a l
65. s shown in Figure 12 15 12 30 Irrigation User s Manual RESISTANCE OHM 1000 1 0 02 5 10 15 20 25 30 GRAVIMETRICAL SOIL WATER Figure 12 15 Calibration curve that indicates the relation between electrical resistance of a gypsum block and soil water content 6 4 Neutron water meter Since the neutron watermeter measures the absolute amount of water in volumetric units in the soll this technique will also directly give an indication of the amount of water in mm to be irrigated The method of soil water measuring by means of neutron dispersion dates back to the 1950 s It has since been accepted as an effective and reliable technique used increasingly in South Africa The neutron water meter consists of two main components namely 1 a probe containing a source of high energy and fast moving neutrons as well as a sensor which is sensitive to slow moving neutrons and ii a micro processor that can register the flow of slow moving neutrons in the soil A schematic representation of the most important components that the instrument consists of 15 given in Figure 12 16 Micro processor Soil surface Detector Detector cannot read fast moving neutrons Detector only read Sl slow moving neutrons Source of neutrons n from hydrogen E slow neutrons fast neutrons Figure 12 16 Schematic representation of the working components of a neutron water meter Irrigation scheduling 12 31 The
66. s the frequency delay method is vested 1n the principle of measuring of the dielectrical constant of materials The dielectrical constant of a material is the measuring of the capacity electrical permissiveness of a non conductive material to conduct high frequency electromagnetic waves or pulses The dielectrical constant of a dry soil varies between two and five while that of water is 80 at frequencies of between 30 MHz and 1 GHz Research results have shown that the measuring of a soil water medium s dielectrical constant reflects an accurate measurement of the soil s water content Relative small differences in a soil s water content results in large differences in the electromagnetic characteristics of the soil water medium The soil water content of a soil can therefore be determined solely by determining the dielectrical constant of a soil The time domain reflectometry technology for soil water content determination is vested in cable testers such as the Tektronix 1 502B This equipment was originally used for testing the breaks and joints in subsurface cables Various manufacturers therefore use the apparatus to conduct a high frequency transversal electromagnetic wave next to a cable which is connected to parallel conductive probe The parallel conduction probes two or three are inserted into the soil and serve as wave conductors The wave conductors reflects the transversal electromagnetic wave back to the cable tester where it 15 reflected
67. s tightly into the soil In stony soils a paste is made from the soil The access tube 1s then installed in the paste in the soil so that no vacuum exists around the access tube The apparatus must be calibrated for the different soils for each depth The calibration can also be calibrated against a calibrated neutron water meter The change of gross density with depth also 12 38 Irrigation User s Manual requires calibration at every depth where the soil water content is to be determined The sphere of influence of measurements in the absence of vacuum is not influenced by soil water content and is approximately 100 mm vertical and 250 mm horizontal in diameter The apparatus is very accurate if it is correctly installed and calibrated Figure 12 21 An example of a probe type capacitance sensor used exactly as a neutron watermeter DIVINER 2 000 from Sentek Benefits 1 Readings be taken easily and quickly soil water content be determined simultaneously at different depths A few milliseconds are necessary for the DIVINER apparatus to take the readings while the probe 1s sinking into the soil The apparatus takes sixteen readings at different depths in less than two minutes 1 The capacitance technique 1 very accurate 1f it 1s correctly installed and calibrated iii Accurate readings can be taken near the surface Readings can be taken as shallow as 100 mm from the surface in increments of 100 to 200 mm
68. soil water content at a specific depth of a soil is measured by dropping the probe in a pipe which was installed in the soil beforehand to a desired depth and of which the bottom is closed A neutron count is then taken for a minimum of 16 seconds but preferably 32 seconds It is however important to note that the shorter the count time the accuracy and dependability of the answer obtained reduces The effective volume of soil of which the soil water content is measured is determined by the radioactive strength of the neutron source and the degree of wetness of the soil The effective volume sampled by the neutron water meter is therefore greater in dry than in wet soll For the generally used sources such as Ra Be and Am Be the radius of the spherical volume of soil in which the soil water content is measured varies typically between 0 1 m for a wet and 0 25 m for a dry soil The practical implications hereof are strictly speaking that a neutron water meter cannot be used to determine soil water near the soil surface approximately 0 15 to 0 20 m A neutron water meter can be calibrated in the laboratory or in the field to determine the relation between the neutron counts and the volumetric soil water content For calibration of the neutron water meter and other soil water meters a specialist in this field must be consulted Benefits 1 Measuring is very fast minute per reading or even less 1s normally sufficient 1 Measurements
69. soil water reservoir is such that 6 hours of irrigation per day 1s needed in the peak month but only 3 hours per day irrigation may be necessary outside peak time 12 9 L n Where t the maximum standing time of irrigation hours Le emitter spacing m qe emitter delivery L crop spacing m nt application efficiency decimal The norm for the application efficiency of the different systems is as follows Table 12 6 Application efficiency for the different systems Koegelenberg 2002 Type of system Application efficiency Drip systems 90 Micro sprayer systems 80 Permanent sprinkler systems 75 Moving systems 80 Portable quick couple sprinkler systems 70 Travelling guns and other portable sprinkler systems 65 Flood irrigation with pipe supply system 80 Flood irrigation with earth channel supply systems 60 12 24 Irrigation User s Manual The efficiency of any irrigation system depends mainly on how the system is operated and maintained The following factors negatively influence among others the efficiency of the system e great pressure difference 220 through the system the use of different types of emitters low infiltration that leads to runoff too high applications poor soil preparation poor management blockages and leakages It is therefore important that the irrigation system should be evaluated on a continuo
70. t plants will tend to develop a shallow root system to adapt to the shallow wetted depth 3 3 Groundwater withdrawal patterns Q 0 D Q e lt 2 2 AX 9 S 20 2 0 A 16 972 2 O 40 H H lt 30 KC 5 a lamer pee eene Elo 15 2 c eoo edo olo cm eum onm Figure 12 5 Typical plant water withdrawal pattern Irrigation scheduling 12 15 With most plants the active roots are mainly concentrated in the upper part of the root zone see Figure 12 5 Therefore the most rapid water withdrawal occurs in the area of highest root concentration under favourable temperature and aerating conditions The reduction of groundwater also takes place more rapidly in the upper soil layer as groundwater evaporates directly from it With the reduction in available water in that part of the root zone the soll water tension binding the remaining water increases Plants then withdraw water at a deeper level where it is bound with less energy 3 4 Evapotranspiration The combined loss of water from a given surface during a specific time by evaporation from the ground surface and through plant transpiration 15 known as evapotranspiration Only a small part of the water which a plant absorbs from the soil 1s taken up by the plant cells By far the largest part of the water is released through the stoma to the atmosphere by transpiration plant
71. t recommended 12 26 Irrigation User s Manual 4 LS Figure 12 12 Water determination by means of feel and observation Table 12 7 Feel method relation between soil water and soil appearance Jordaan 2001 Total available Coarse sand Slightly coarse Medium loam Fine silt loam water loam sand sand loam fine clay loam sand loam 10096 Field Leaves a wet Appears very dark Appears very dark Appears very dark capacity pattern on the hand leaves a wet pattern leaves a wet pattern Leaves slight wet when squeezed 0 mm m on the hand when squeezed forms a short sausage 0 mm m on the hand when squeezed forms a sausage of about 25 mm 0 mm m pattern on the hand when squeezed forms a sausage of about 50 mm 0 mm m 70 80 Appears wet forms Dark in colour Dark in colour Dark in colour a weak ball forms a hard ball forms a plastic ball forms a plastic ball 17 25 mm m 25 33 mm m gets a smooth finish easily forms a when rubbed forms sausage with a a sausage of 10mm smooth finish 33 50 mm m 42 58 mm m 60 65 Appears slightly Dark in colour Dark in colour Dark in colour wet forms weak forms a good ball forms a hard ball forms a hard ball brittle ball 50 mm m forms a weak forms a sausage of 33 mm m sausage 5 10 mm gets 67 mm m smooth finish when rubbed 75 mm m 50 Appears dry forms Slightly da
72. ter Bestuursprogram Irrigation water management program is a water balance model that uses research data to make irrigation recommendations The water consumption of the crop is estimated according to day to day irrigation requirement curves which were fixed by historical readings BEWAB was developed by Prof A T P Bennie as a result of the Water Research Commission s report n Waterbalansmodel vir besproeiing gebaseer op profielwatervoorsieningstempo en gewasbehoeftes Bennie et al 1988 12 42 Irrigation User s Manual 7 2 Donkerhoek Data irrigation scheduling program The Donderhoek Data Irrigation Scheduling Program makes use of up to date weather data to make an irrigation recommendation on a daily basis so that optimal irrigation can be done The program contains a function that can control the opening and closing of valves in the field The program was developed by Donkerhoek Data Pty Ltd and Mr T du Preez The total development of the program was funded by Donkerhoek Data Pty Ltd The program is used mainly by commercial farmers and consultants in the Western Cape and along the Orange river The program contains an option for calculating a water budget for a season or part thereof Historical data can be used therefor 7 3 SWB SWB Soil Water Balance is an irrigation scheduling model that uses current climatic data to simulate the salt balance and soil water balance of generic crops With sufficient weather
73. tion of TDR wave conductors with three probes The wave conductors are permanently installed on the side of a profile hole with conductors which lie on top of the soil surface Care must be taken to disturb the soil as little as possible It 1s the only method to obtain readings at different depths in one position with the aid of the TDR Horizontally installed wave conductors give a depth specific reading while wave conductors installed at an angle of 45 give an integrated larger volume reading both in the horizontal and vertical directions Hand TDR meters consisting of a wave conductor probe can be used like a neutron water meter to determine the water content with the aid of access tubes in the top 45 to 60 cm of soil The following TDR equipment is currently available 1n no specific order of preference Aquaflex SE 200 soil water meter Campbell Scientific s CS615 L hand feel pin wave conductor Hydrosense from Degacon Trime from IMKO Tektronix Gro point amp Moisture point from ESI Environmental Sensors Benefits 1 Measurements are determined fast Soil water content be determined at different depths simultaneously Readings are taken within one minute 1 measuring technique is very accurate if the apparatus is properly installed and calibrated iii Accurate and dependable readings can be taken near the soil surface Measurements as shallow as 100 mm to a depth of 5 m is possible iv Researc
74. to irrigation scheduling are the following e Field capacity FC mm m the depth of water per metre soil depth after all free water has drained from the saturated soil under gravity A soil water tension of approximately 10 kPa 15 considered as field capacity This value is considered the full point of the soil water reservoir e Permanent wilting point PWP mm m the depth of water per metre soil depth when the majority of plants of a crop will permanently wilt A soil water tension of approximately 1 500 kPa is considered as PWP At this point the water reservoir is empty Soil water capacity SWC mm m the total depth of water per metre soil depth between the FC and the PWP for a specific crop It is important to know exactly between which soil water tension 12 20 Irrigation User s Manual limit the soil water capacity 1s determined because it influences the size of the soil water reservoir Total available water TAW mm it is the total depth of water available to the crop within the effective root depth between FC and PWP Figure 12 10 shows a generalisation of the total available water for different soil texture classes 100 75 50 25 mm 0 Figure 12 10 Generalised available soil water content as a function of texture The soil water content are given as mm water per 300 mm soil Percentage allowable water depletion 96 the maximum percentage of the available water AW that may be extracted fr
75. try Tensiometry is an indirect way of determining the water content of a soil A Tensiometer indicates the matrix potential soil water tension which is then converted to absolute soil water content by means of a soil water characteristic A tensiometer consists of a porous point usually made from ceramic The porous head 1s connected to a mercury manometer or a vacuum meter by means of a long water filled tube Figure 12 17 When the tensiometer is placed in the soil in such a way that there 1s close contact between the soil and the porous head water will move through the pores of the ceramic head The water movement is caused by a difference in the soil water tension in and outside the porous head If the suction tension of the soil 1s higher than in the pores of the tensiometer water will 12 32 Irrigation User s Manual move from the tensiometer to the soil Since the tensiometer and the manometer or vacuum meter forms an airtight closed system a vacuum will be created in the tensiometer or the air pressure in the tensiometer which will in time come into equilibrium with the air pressure in the soil which is lower than that of the atmospheric pressure i e it is negative in relation to atmospheric pressure These pressure differences or vacuum amounts are then registered on the manometer or vacuum meter Electronic tensiometers that can register automatically have an electronic vacuum sensor and are then connected to data loggers to
76. tural root depth Almonds 0 po Avocados 900 7 50 Beams 60 HH Grenpas 400 204 Lwen 9 0 Macadamia 902 608 Mango 0 6066 2 02 n 480 ENTM 900 450 450 y Tomatos 0 1 0 500 450 450 os 12 15 09 CS 12 13 21 14 27 APPLE TREE 15 YEARS Figure 12 3 Natural root depth and distribution 12 14 Irrigation User s Manual 3 2 2 Factors influencing root development Root penetration is seriously deterred by dense subsurface ground layers and the root depth in many South African soils is limited to the upper 250 mm of the profile by a plough sole Roots cannot penetrate a hard layer unless cracks occur and find it difficult or impossible to grow from one to another soil layer where the texture differs drastically A factor limiting root growth may also be a shortage of plant food substances or an underground chemical imbalance Root growth is limited by a high water profile EFFECTIVE SOIL DEPTH RESTRICTIVE LAYER Figure 12 4 Root development due to physical and chemical soil limitations Most roots will occur in the wetted area when soil is only partially irrigated e g with drip irrigation Root depths will also be influenced by irrigation practices e g with a too short cycle length and standing time mos
77. us basis to ensure even water application through the system If it is not done it can lead to over irrigation to provide for portions of the system receiving too little water Modern irrigation systems are not always more efficient because problems can occur without the correct management Thus the efficiency of flood irrigation systems can be improved by techniques such as laser levelling while modern irrigation system such as drip irrigation can be insufficient 1f incorrectly applied and managed The importance of the use of applicable scheduling techniques is therefore of the utmost importance Example 12 2 A producer wants to irrigate 35 hectare of pears at Wolseley with a micro irrigation system He wants to irrigate only 5 out of 7 days and 10 hours per day From the long term climatic statistics of the Winter Rainfall Region the following Peak irrigation month January Crop factor 0 55 Average monthly A pan evaporation 323 mm Average monthly rainfall figure 12mm Use the equations as described in Chapter 3 of this manual Determine the gross irrigation requirement GIR if the application efficiency is 80 Solution Effective rainfall Rainfall mm month 20 2 12 20 2 Evapotranspiration zi Monthly A pan evaporation mm month x crop factor 323 x 0 55 mm month 178 mm month Nett irrigation requirement Evapotranspiration mm month effective rainfall mm month 178 mm month 1 780 m month 57 42

Scheduling with regard to crop evapotranspiration (ETc)

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