Controlling Irrigation with Tensiometers and Time Domain
Contents
1. A greenhouse experiment was conducted in Gainesville Florida to evaluate tensiometer and TDT as moisture sensors for automatic control of a drip irrigation system in container production of ornamental plants Experiment I TDR controlled vs timer controlled irrigation This experiment consisted of two irrigation zones treatments The treatment controlled by CR10 Datalogger and a TDR probe CS616 Campbell Scientific Logan Utah included a zone of 15 containers The other treatment control consisted of 9 containers with the irrigation on a preset schedule timer Figure 1 The irrigation zone for each sensor included a 24 V solenoid valve The water was supplied through a 0 75 in PVC line and spaghetti drip tubes with one 2 GPH emitter and two dribble rings per drip tube to have a more uniform water distribution in the container Six pressure transducer tensiometers were used to monitor water potential of the substrate Two sizes of tensiometers were used a mini tensiometer 3 in in length and 1 in in diameter and a more typical tensiometer 6 in long with 0 5 in diameter All tensiometers were equipped with low tension ceramic cups and were produced by Irrometer Company Riverside California The ceramic cup of low tension instruments has different porosity and is more sensitive to changes in soil moisture conditions when the substrate is close to the saturation point Low tension instruments are recommended for use between 0 a2. However this reduction in instrument accuracy is not critical since typically under production conditions an irrigation system operates in the rage from 15 to 50 VSWC where the calibration equation is more precise y 2 0854 50556x 2 6444 R 0 9731 0 20 0 10 0 00 0 600 0 700 0 800 0 900 1 000 1 100 1 200 1 300 Voltage Figure 6 Calibration curve developed using TDR readings voltage and Weights A calibration equation for the segment from 15 to 40 VSWC is presented below 0 t 3 044 6 098 t 2 706 t This equation provides a better response for most application using a CS616 TDR probe as a soil water content measuring device However it 1s important to emphasize that this equation is developed for the 6 in rods In figure 7 the calibration curve for the typical operation range is presented The values out of the range 15 to 40 have been removed Operation Range y 2 70612 60977x 3 0497 R 0 984 0 700 0 600 0 900 1 000 1 100 Yoltage Figure 7 Calibration curve from 15 to 40 VSWC TDR vs Tensiometer Tensiometers are common sensors to monitor water status matrix potential in the soil They work well in soils or growing substrate with less than 70 porosity since permanent contact between the ceramic cup and substrate is needed to obtain accurate readings Tensiometers also require constant maintenance i e refilling and calibration If all those conditions are met the
3. values were entered in the CR10 to convert electrical signal from the TDR into volumetric moisture content of the Metro substrate Results and discussion TDR Calibration Calibration of the TDR was done in order to obtain real values of volumetric moisture content for Metro 500 from the TDR readings Volumetric moisture values were calculated using weight differences measured by an electronic scale Using regression analysis a new quadratic function was calculated to correct the TDR readings The final calibration equation form was 0 t 2 644 5 056 t 2 065 t Equation 6 The above equation was developed for the entire range of data collected during the experiment and represents the range of 7 to 55 VSWC It is important to mention that there are several factors that affect the calibration curve for this TDR sensor The original length of the CS616 rods was reduced from 12 in to 6 in to accommodate sensor placement in the containers filled with substrate This resulted in the sampling space reduction by 50 and greatly affected the factory calibration which is designed for 12 in rods The graph representing the equation that fits the sampled values is presented in Figure 6 It shows that there is less resolution in the instrument reading at the extremes of the soil moisture values recorded in this experiment Above 40 and below 20 of VSWC larger changes in moisture content correspond to smaller changes in the recorded voltage
4. A signal is sent down steel probes buried in the media When it reaches the end of the probes it 1s reflected back to the TDR receptor unit The difference in time that takes the signal to return is caused by the dielectric constant that 1s affected by the water content of the substrate that surrounds the probes The levels of voltage received by a TDR probe are converted into volumetric soil water content VSWC Soil Water Content The gravimetric soil water content GSWC is expressed by weight as the ratio of the mass of water to the dry weight of the soil sample To determine this ratio for a particular soil sample the water mass must be determined by drying the soil to constant weight and measuring the soil sample mass after and before drying Black 1965 The water mass or weight is the difference between the weights of the wet and oven dry samples The oven drying technique is probably the most widely used of all gravimetric methods for measuring soil moisture and 1s the standard for the calibration of all other soil moisture determination techniques Black 1965 Volumetric soil water content VSWC relates volume of water in the sample to the total volume of soil and it is a more convenient way to express the soil moisture content for irrigation management VSWC can be calculated by multiplying the gravimetric soil water content GSWC by the soil bulk density mass of soil solids per unit volume Charlesworth 2000 Materials and Methods
5. the probes The CS616 do not directly measure the wave guide signal reflection time like a common TDR Instead the signal returned from the guides causes a circuit to change states between two discrete values The output of the sensor is a frequency that reflects the number of state changes per second Hz CSI 2002 As with all TDR sensors a wetter soil will cause a longer signal return time and will cause the CS616 circuit to vibrate at lower frequency The wave guides can be buried for in situ readings or used as a portable probe The CS616 was inserted in the container under a 45 degree angle to cover more substrate volume with the electromagnetic signal A CR10 datalogger from Campbell Scientific Inc was used to collect the readings from the CS616 probe A program using PC208 software CSI 1997 was developed to control the water content reflectometer CS616 to store voltage data and to record volumetric soil water content and the time of the measurements The program activated the probe at a specified time interval and saved the information collected in a text file in the CR10 memory The program also allowed for setting the range of volumetric soil water content VSWC values at which the CR10 switched ON or OFF the solenoid valve to start or stop irrigation The file stored in the CR10 memory was downloaded to a personal computer for editing and analysis Determining Gravimetric Soil Water Content An electronic scale was used
6. to record the weight of the pot containing the plant Figure 4 The scale had a capacity of 10 kg a readability of 1g and a linearity of 1g Acculab 2002 It also had a RS 232 serial port port that allowed it to communicate with a personal computer through a program specially written for this project using LabVIEW a graphical programming language that uses icons instead of lines of text to create applications LabView 2001 FA Figure 4 Container on electronic scale The program collected the reading from the scale at a specific time interval and showed it on the computer screen using a graphic interface Figure 5 The time interval when the readings are recorded was be adjusted as needed and the data files produced can be imported as a text file easily into any spreadsheet software gt Scale Data Logger vi File Edit Operate Windows Help ui Current Date Current Time Figure 5 LabVIEW Program The weights collected by the scale were used to calculate the gravimetric moisture content that was later converted to volumetric moisture content and compared against the VSWC registered by the TDR Calibration of TDR Using Gravimetric Soil Water Content The general calibration curve to obtain the volumetric soil water content from the CS616 voltage readings has a following form OTD Co tC t OQo T Equation 1 where Oy is the volumetric water contents t the voltage from the CS616 and Cn are coefficie
7. were utilized and tested along with four different irrigation duration times of 60 75 90 and 120 seconds Table 2 Differences in water use between the TDR controlled system and the timer controlled system are presented in Table 2 To measure the applied water the water from one emitter was collected for the TDR controlled system and for the timer controlled plot Drainage water from one container for each plot was collected It can be assumed that the retained water was absorbed by the plant evaporated from the substrate surface or was retained in the substrate Table 2 Comparison of average applied water and drainage per irrigation event Method Irrigation Water Applied Water Drained Water Retained Frequency days in in in Timer 3 times 120 seconds Daily 11 7 9 3 1 26 VSWC 120 seconds 1 4 11 5 5 5 5 26 VSWC 90 seconds 1 6 5 8 0 6 32 26 VSWC 75 seconds 1 6 5 2 0 52 26 VSWC 60 seconds i 4 3 0 4 3 The variation in the intervals between irrigation events depends on the water consumption affected by environmental factors Even when the conditions in the greenhouse are more or less stable variation is present Since the system will automatically turn on at a given substrate moisture content the time when the irrigation occurs is different among irrigation events which also reflects the water use patterns In Figures 10 through 12 different rates of water use due to evapotranspiration and drainage can be o
8. Controlling Irrigation with Tensiometers and Time Domain Reflectometry TDR Final Report Dorota Z Haman Agricultural and Biological Engineering Thomas H Yeager Environmental Horticulture University of Florida Introduction In 1990 agriculture accounted for the largest use of freshwater in Florida Agricultural water withdrawals amounted to 3 805 million gallons per day and provided irrigation water for 2 15 million acres Carriker 2000 It is generally understood that water conservation is the best way to increase the water management efficiency of agriculture and reduce potential negative environmental impacts Because of this need for conservation combined with other factors such as economics labor and product quality requirements irrigators are under increasing pressure to manage water more efficiently More precise irrigation management that uses reliable moisture sensors and irrigation controllers can significantly improve irrigation scheduling techniques that will result in higher irrigation efficiency Irrigation scheduling may be accomplished by a number of different methods that strive to keep the soil moisture within proper limits Proper irrigation scheduling is the application of water to crops only when needed and only in needed amounts This requires the answer to two questions when to irrigate and how much water to apply Smajstrla et al 1997 No irrigation system will apply water without some waste or losses sinc
9. FS or LL F Or B Ad Estr Figure 13 Comparison between TDR and timer controlled irrigation systems 13 A comparison between the TDR and Timer controlled systems is presented in Figure 14 The main difference is that the TDR system fluctuates around moisture content of 27 At the same time the moisture content on the substrate of the plot controlled by the timer keeps increasing while the drainage collected also increases The increase in moisture content is also and indicator that the plant is not using all that water and the irrigation system 1s just saturating the substrate This increase in water content can diminish the availability of oxygen for the roots and also promote development of bacterial and fungal diseases Tensiometer Based Irrigation Control System In the second experiment the tensiometers were also used to control the starting point of an irrigation system The system was programmed to start irrigation when the signal from the tensiometers reached the 17 24 kPa mark The system was less precise under tensiometer control then under TDR control Figure 14 turning irrigation on later than the set limit Tensiometer Controlled Irrigation 20 0 19 0 18 0 17 0 16 0 15 0 14 0 13 0 12 0 11 0 10 0 i PASM TAADA TAWA TADAA TAAA Pred TAAA Pod TA Ta Fi Pode COO T200 U T200 QUU 1200 QOU TZD QOD TUD D T200 kPa Shot Teng Lorg Tens Figure 14 Comparison of tensiometer controlled irr
10. RR ORR Ree eR RRR aR SE BS oe Se ower eg ere eau Pao Te ee OS 1 Figure 9 TDR controlling lower and upper threshold limits The second set of moisture values used as lower and upper limits were 22 to 25 and the third was 25 to 27 The results were similar to the ones noted above The system still was applying more water than was needed to maintain proper moisture in the substrate The time between irrigation events was long varying from 4 to 5 days depending on the conditions prevailing in the greenhouse that affected plant water use Table 1 Comparison of applied water and drainage using VSWC thresholds Irrigation Frequency Water Applied Water Drained Water Retained Method 3 3 3 days cm cm cm 17 26 VSWC 4 5 3975 2850 1125 22 25 VSWC 35 2400 1250 1150 2 5 25 27 VSWC 1900 1260 640 The approach that showed more success was to use the TDR to start the irrigation event and to let it run for a given period of time This was possible by using a program that allows setting the lower limit for the TDR to turn the irrigation system on and allow the user to preset 11 the duration of the irrigation event This system permitted a better control of the amount of water applied diminishing the waste of water as drainage Several attempts were made to establish the best point to turn the system on Analyzing the irrigation and water use patterns observed with the previous method values between 25 and 30 VSWC
11. bserved TDR Controlled System 26 VSMC 120 seconds 1 42 59 PM Tar 56 Pht 1 31 52 AM T2549 AM 3595 56 Pht 9 53 53 FPM 34750 AM 9 41 47 Ah 33544 Pht 3 29 40 Ph 2037 AM 9 17 34 AM 3 14 31 FM 9 05 26 Ph 2 59 24 Ant 8 53 21 AM 25416 PM 8 46 13 FM 2 4210 AM 8 36 07 AM 2 50007 Pht a 2407 PM 2186 07 Abt B10 AM 2 08 07 Pht 6 00 07 FM 1540r Abt e407 Abt Figure 10 TDR at 26 VSWC and 120 seconds per irrigation event 12 5 seconds 7 TDR 26 VSMC 0 28 CEE f AT a9 Wid L 88 Wd TEUEZ f nr 92028 f wr ezziz Wid ZE t0 8 Wid Sea Ait Be OP g iy P OP L Wid St cf s Bid Btt Wir 29 906 Ay SS S2 Wid 6S 6F 2 Wid 20 2F lt lt Wir SO Pe 2 Ait BO tEZ Wd Targ Aid 9L 80 2 0 275 0 2 26 0 26 0 255 IMSA Figure 11 TDR at 26 VSWC and 75 seconds per irrigation event T D D i T uf A a Ct Ce TD IMSA Wet 20 92 65 We LEBRE Wd SETIS Wd BESE We 0005 9 We POEL ck Wd 20 965 We b 2S Ob We PEDF Wd Sb tP 6 Wd 2 90 We See o Wie 62 0 Wd ZEEL Wd SE Pe eb We SE S5 S Wd SE Ob bb Wd S bP P We SELO Figure 12 TDR at 26 VSWC and 60 seconds per irrigation event TOR System vs Timer System Wit 92 265 Wd Ze Sh Ad FS A 12 125 Wd Of 22 1 Wd ics Ait FIZS My 0028 We i0 Et Wir Ote Hy 0228 Hasit F Wy SES bE Ait PEGLE Wd Cre We cS ie OS C
12. e the cost to prevent all losses 1s prohibitive An excellent method to reduce water use consists of utilizing soil moisture monitoring devices in conjunction with rainfall records and knowledge of plant water needs Munoz Carpena et al 2002 The objective of this research project was evaluate two soil moisture sensors and make a progress in development of a useful and easy to use irrigation controller for container production of ornamental plants in Florida s nurseries Irrigation in Nurseries Most of the irrigation systems used in Florida container nurseries are pressurized irrigation systems such as overhead sprinklers or some type of microirrigation However mainly due to the economics the majority of Florida nurseries use overhead sprinkler systems especially for small containers Typically the application efficiencies of these systems are low 20 50 due to container spacing that is necessary for proper plant development Haman et al 1996 Using microirrigation can increase the efficiency however these systems are more costly and require higher management skills A big advantage of microirrigation is a fact that water is delivered directly to the root zone of the plants For plants that are sensitive to foliar diseases dry foliage will also be an advantage Microirrigation systems are suited for chemigation chemical application with irrigation water The water and chemicals are applied directly to the container substrate and
13. igation systems The three types of sensors and a timer used as irrigation system controllers are compared in Table 3 Compared to the Timer plot the TDR set at 18 VSWC applies almost 80 less water with an application efficiency of 0 38 Application efficiency is the ratio of retained water available for the plant roots to the total water applied Smajstrla et al 2002 Table 3 Comparison of average applied water per sensor Method Irrigation Water Applied Change Water Drained Water Retained Application Events in Applied Water in in Efficiency Timer 28 1676 0 1033 643 0 38 TDR 18 VSWC 17 359 78 137 222 0 61 Short Tens 17 24 kPa 19 593 64 309 287 0 48 Long Tens 17 24 kPa 21 824 50 429 395 0 47 14 Recommendations and Future Work Time domain reflectometry can be used to measure volumetric soil water content in coarse organic substrate with some success TDR works more precisely between 20 and 35 VSWC It should be used with caution with VSWC values above or below those limits for this type of substrate Nevertheless the use of a soil water content monitoring device reduces the volume of water applied in nursery production compared to a timer controlled system Using a TDR controlled system reduced water use by more than 60 and with some settings 18 or 20 VSWC almost 80 The savings were approximately 60 and 50 with the short and long tensiometers respectively The efficiencies are 0 61 for short tens
14. iometers 0 48 for long tensiometers and 0 47 for TDR Literature Acculab 2002 VIR 10kg Scale User Manual Acculab Inc Black C A 1965 Methods of Soil Analysis Part I Physical and mineralogical properties American Society of Agronomy Madison WI Burt C M and Styles S W 2000 Irrigation System Series Updating Drip Irrigation Knowledge Irrigation Journal pp 10 March April 2000 Carriker R R 2000 Florida s Water Supply Use and Public Policy FE 207 EDIS UF IFAS FL Charlesworth P 2000 Irrigation Insights Soil Water Monitoring CSIRO Land and Water Lismore Australia CSI 2002 Water Content Reflectometer Model CS616 L Campbell Scientific Inc Logan UT CSI 1997 CR10 Measurement and Control System User Manual Campbell Scientific Inc Logan UT Haman D Z Yeager T H Beeson R C and Knox G W 1998 Multiple Pot Box Container Plant Production J Environ Hort 16 1 60 March Haman D Z and Yeager T H 1997 Irrigation System Selection for Container Nurseries AE 263 EDIS UF IFAS FL Haman D Z Smajstrla A G and Pitts D J 1996 Efficiencies of Irrigation Systems Used in Florida Nurseries Bulletin 312 EDIS UF IFAS FL Irrometer 2003 Low tension and Mini low tension Irrometers remote sensing tensiometer Irrometer Company Riverside CA LabView 2001 Getting Started with LabView National Instruments TX Lebeau B S Barrington and R Bonnell 2003 Micro tensiometers to m
15. m All threshold values could be set individually for each sensor Figure 3 Within each treatment only one sensor was controlling the irrigation system The other sensors where used for monitoring to evaluate the accuracy and variation among each group of sensors Timer Controlled TDR and Tensiometer Controlled erer 2 CICIerere Water supply O Solenoid valve Long Tensiometer K TDR sensor Short Tensiometer Figure 2 Time TDR and tensiometer controlled plots Experiment II The plants used in both experiments were the Spathiphyllum a popular indoor plant that is grown commercially using hot houses covered in plastic even in South Florida Typically the temperatures are kept at 90 F degrees under high humidity conditions and the plants are grown under shade with minimal air movement The plants were planted in 3 gallon containers filled with Metro 500 growing substrate consisting of pine bark Canadian peat and sand 2 1 1 by volume mix Haman et al 1998 Figure 3 Experiment set up TDR Based Irrigation System The CS616 water content reflectometers Figure 1 from Campbell Scientific Inc were used to measure volumetric water content using the time domain reflectometry method The rods where cut from 12 in to 6 in to fit in the 3 gallon container since the manufacturer advice is to leave at least 1 5 in between the probes and the container walls to avoid interference noise in the voltage levels retrieved by
16. nd 40 cb kPa Irrometer 2003 Three tensiometers of each size where installed The tensiometers were placed vertically halfway between the base of the plant and the container s side Both types of tensiometers incorporated a pressure transducer that could register changes of pressure in the instruments as a change in voltage that was transmitted to the datalogger This signal was later converted back to the pressure vacuum units Timer Controlled TDR and Computer Controlled OO G amp G OK CO Gm GO GO aD CO GOI OCW GCE Solenoid valve Water supply 5 TDR sensor 0 Tensiometer Figurel Time controlled and TDR controlled plots Experiment I Experiment II TDR Controlled Tensiometer Controlled and Timer Controlled Irrigation A follow up experiment was conducted to compare Timer TDR and Tensiometer controlled irrigation systems Figure 2 The project consisted of three different irrigation systems each one controlled by one type of sensor TDR Long Tensiometer and Short Tensiometer In addition there was a timer controlled control treatment Each of the sensors was connected to a CR10 that stored the information acquired by the sensors and controlled the solenoid valves for each of the irrigation systems The threshold level of the soil moisture content or the number of atmospheres soil tension at which to start the irrigation and the duration of each irrigation event were also controlled by a CR10 progra
17. nts developed in the calibration process The coefficients in the factory calibration equation used to obtain the volumetric soil water content from the CS616 voltage readings ware originally developed for sandy clay loam soils with bulk density of 1 6 g cm and an electrical conductivity at saturation of 0 4 dS m 0 t 0 187 0 037 t 0 335 1 Equation 2 Since the Metro 500 substrate used in this project has different characteristics new calibration coefficients C were developed to be used in the general calibration equation Equation 1 that was used in the CR10 program that controls the CS616 The C coefficients were developed using gravimetric data and a bulk density From the weight of the moist substrate the weight of the oven dry substrate and the bulk density of the substrate the volumetric soil water content was calculated as follows CSI 2002 Gravimetric soil water content Oy Mwet Mary Mary Equation 3 Where Mwet 1s weight of the wet substrate and Mary 1s weight of the dry substrate Bulk density Pbulk Mary Vary Equation 4 with Vay representing the volume of the dry substrate The original bulk density for the Metro 500 was 0 36 g cm Finally the volumetric soil water content O Og Pbulk Equation 5 After the volumetric soil water content was calculated Equation 5 this values were plotted against the values collected with the CS616 probe Using the best fit quadratic function C
18. ometers are usually capable to read in the range of water tensions between Field Capacity FC and Maximum Allowable Depletion MAD for a substrate used in container production The main problem that can be expected with tensiometers when used with substrates with more than 70 porosity is to maintain enough contact between the ceramic cup and the substrate To improve the contact the substrate around tensiometer must be carefully pressed during sensor installation Short tensiometers are more suitable for small container production since they fit more easily in the container and do not add water to the substrate via capillarity when the substrate is dry Time Domain Reflectometry TDR A precise and relatively new technology to assess volumetric soil water content is the Time Domain Reflectometry TDR TDR voltage readings can be converted into volumetric soil water content VSWC which is a popular method to report the soil water status A fully automatic system containing this kind of sensors requires a datalogger or computer to activate the solenoid valves that control the operation of the irrigation system TDR sensors give very accurate readings however they are still quite expensive approximately 200 per sensor and additional hardware and software is needed to control an irrigation system using TDR Charlesworth 2000 In a TDR probe the speed of an electromagnetic signal passing through a material varies with the dielectric material
19. onitor water retention in peat potted media Soil and Water Division ASAE 19 5 559 564 Munoz Carpena R Yuncong Li and Olczyk T 2002 Alternatives of Low Cost Soil Moisture Monitoring Devices for Vegetable Production in South Miami Dade County ABE 333 EDIS UF IFAS FL 15 Smajstrla A G Boman B J Haman D Z Izuno F T Pitts D G and Zazueta F S 1997 Basic Irrigation Scheduling in Florida Bulletin 249 EDIS UF IFAS FL Smajstrla A G Boman B J Haman D Z Izuno F T Pitts D G and Zazueta F S 2002 Efficiencies of Florida Agricultural Irrigation Systems Bulletin 247 EDIS UF IFAS FL Zazueta F S Smajstrla A G and Clark G A 1993 Irrigation System Controllers SS AGE 22 EDIS UF IFAS FL Zazueta F S and Xin J 1993 Soil Moisture Sensors SS AGE 27 EDIS UF IFAS FL 16
20. opped below acceptable level The acceptable level is determined from recommendations for the type of plant to be cultivated the environmental conditions cultural practice and the type of substrate used in the containers Moisture Sensors for Container Substrate Many current irrigation systems used in container nurseries are based on timers and moisture sensors However few systems are controlled by moisture sensors in real time Most of the soil moisture sensor have been developed for field conditions and function well when used in mineral soils However many sensors that work well in the field are not very reliable in the container substrate due to the poor contact between the substrate and the sensor A good example of the sensor that often has problems due to poor contact is a tensiometer In addition to contact problem the amount of water that filters from the tensiometer ceramic cup to the substrate could be relatively high compared to the total volume of a small container i e 1 gallon container especially when the substrate is dry As a result the container with the tensiometer could show a water content greater than the actual water content of containers without a tensiometer Other devices such as gypsum blocks are also affected by the porosity of the substrate used in containers In addition gypsum blocks dissolve with time and have to be replaced often after one year of operation Munoz Carpena et al 2002 Tenstometer Tensi
21. there are no losses between containers minimizing the runoff from the nursery Furthermore the danger of chemical loss and contamination is reduced Haman et al 1997 However the control of these irrigation systems is more complicated and a reliable device to monitor the water content of the substrate is needed for efficient water application Irrigation Controller A controller is an integral part of an irrigation system It is an essential tool to apply water in necessary quantity and at the right time to sustain agricultural production and to achieve high levels of efficiency in water and energy uses These devices have evolved into complex computer based systems that allow accurate control of water energy and chemicals while responding to environmental changes and various development stages of the crop Zazueta et al 1993 Closed loop controllers for irrigation systems base their irrigation decisions on direct measurement of soil moisture using sensors like TDR The simplest form of a closed loop control system is a timer with a moisture sensor that interrupts an irrigation cycle based on the moisture status or the soil growth substrate or plant water status The system can be set to irrigate at a very high frequency depending of the settings entered in the datalogger controller program When the controller attempts to irrigate irrigation will occur only if the moisture sensor allows it which in turn occurs only when substrate moisture dr
22. y perform with high accuracy However the characteristics of the growing substrate used in the nursery industry presents challenges for the tensiometer technology since low density and large particle size increases the porosity of the substrate and the size of the pores That results in reduced contact between the ceramic cup and the substrate which translates to reading errors when the substrate is to too wet or too dry since the empty spaces are filled with water or air respectively TDR amp Scale vs Tens BO kPa Scale TDR Power Scale Power TDR Figure 8 Volumetric soils moisture compared to tensiometer readings 10 TDR Based Irrigation Control System A part of this project was to develop a TDR controlled irrigation system Several approaches were attempted before desirable results were obtained The first TDR controlled system was set up to use the readings from the TDR to turn the system ON and OFF In one test volumetric soil moisture values of 17 and 25 were used as lower and upper limits respectively A sample of the data collected is presented in Figure 9 This means that if the TDR reading reached 17 the irrigation system turned on and remained on until the TDR reading reaches 25 It was clear from the amount of drainage that under this management there was an excess of water being applied Table 1 TOR 16 24 ez ecePSSC EST FRETS EFS SERS TES SN 2PL rR ZS RRRSERSRRHRRARRSE SERRE GE eRe ER
Download Pdf Manuals
Related Search