Application of Dielectric Soil Moisture Sensors for Real-Time Automated Irrigation Control

Transcription

1 Applicatin f Dielectric Sil Misture Sensrs fr Real-Time Autmated Irrigatin Cntrl M.C. Kuyper and J. Balendnck Institute f Envirnmental and Agricultural Engineering (IMAG-DLO), department f Instrumentatin and Measurement Technlgy, PO B 43, 6700 AA Wageningen The Netherlands Keywrds: leaching, flushing, temperature dependence, calibratin methds Abstract In this article the irrigatin cntrl system t be develped in the running Waterman prject is utlined. Als, the fllwing prcess mnitring cnsideratins are discussed: what sil misture state t measure, calibratin fr sil water cntent, and the temperature dependence f sil dielectric cnstant and bulk electrical cnductivity (EC). INTRODUCTION This preview article deals with the applicatin f sil misture sensrs in the Waterman prject. Because the difficulties encuntered are a direct result f the specific applicatin, the applicatin is utlined first. Leaching f nutrients and shrtage f high quality irrigatin water are tw majr prblems in plant prductin systems. In arid areas the amunt f high quality irrigatin water is limited. Therefre grwers want t use the available water as efficiently as pssible. In the semi-arid regins f Nrthern Eurpe greenhuse plant prductin can be characterised by high prductivity in cmbinatin with the use f large amunts f water and nutrients. It is cmmn t grw with 50% surplus drain and t flush when t much salt accumulates. A large part f all drainage water cntaining the unused nutrients ends up in the surface water. In 1989 the Dutch gvernment has accepted a law t reduce emissin f nutrients in rder t prtect the envirnment. By the year 2000 almst all greenhuse plant prductin will take place in clsed systems, f which 30% will recirculate drainage water (Annymus, 1989). Fr a number f crps, hwever, making the investments fr new grwing systems can nt be dne cst effectively. It is fr the sil-bund, nnrecirculating plant prductin systems that Waterman wants t develp an autmated irrigatin cntrl methd that minimises water use and minimises leaching. The key apprach is keeping the water available in the rt zne. A dry layer will be kept between the wet rt zne and the water table, making the system virtually clsed. The methd t achieve this situatin is t apply small amunts f water frequently. In this way the sil is given enugh time t absrb the water instead f draining it immediately. Minimised leaching creates a new prblem: salinizatin. T vercme the negative cnsequences f t saline sil the grwer has t flush the accumulated salts every nw and then. Waterman aims at develping a methd fr cntrlled flushing. CONTROL SYSTEM DESIGN Basic Set-Up and Design Cnsideratins Ideally an irrigatin cntrl system shuld: 1) use water efficiently, 2) minimise leaching, 3) be real-time, 4) be autmated, 5) have a facility t flush when t many salts have accumulated, 6) be able t indicate the amunt f leached water and nutrients, 7) need as little input data as pssible, 8) be able t maintain a cnstant sil misture level, 9) be cst effective and 10) be easy t perate. The idea behind the 8 th demand is that the precise sil misture level fr ptimal crp grwth is nt that imprtant, but rather that it stays at a cnstant level at a cnstant place (Bnekamp, 1994). In that way the rt Prc. Sensrs in Hrt. III Eds. I. Shmulevich et al. Acta Hrt. 562, ISHS

2 system des nt have t put a lt f energy in cnstantly seeking water. Frm a cntrl thery pint f view ne can characterise the system n the basis f a basic clsed-lp feedback cntrl system. There is the prcess t cntrl, a device that measures the prcess state, a cntrller that cmpares the prcess state with a setpint and then calculates what the cntrl actin shuld be and an actuatr that cntrls the prcess. Fr an irrigatin cntrl system the afrementined system cmpnents are respectively: the sil water status, sil misture sensrs, the irrigatin cntrller and the watering system (see Fig. 1). Water distracting and delivering mechanisms like evaptranspiratin, upward and dwnward seepage can be cnsidered as disturbances f the prcess, s these difficult mechanisms need nt be mdelled. Therefre the described irrigatin cntrl system relies heavily n input by sil misture sensrs. Fr an advanced cntrl system t wrk prperly it is vital t knw hw the prcess wrks and hw it can be mnitred. Difficulties encuntered in this field are discussed in sectin 3. Available Technlgies A survey f the available literature revealed ne article describing a similar cntrl thery methd fr surface irrigatin (Reddy, 1985). Mst irrigatin methds described are slely based n e.g. crp water use mdels (Valiente et al., 1996), sil misture sensr signals (Mern, 1995 and Annymus, 1997) r infiltratin estimatin (Smith, 1996). A cmbinatin f sme f the afrementined techniques is applied by Thmsn et al (1996) and by Hess (1996). Nne f these techniques meet all ten criteria mentined in sectin 2.1. The best technique available seems t be using tensimeters in cmbinatin with threshld values fr starting an irrigatin event. The principle is simple and fleible. With a tensimeter threshld cntrlled system the grwer can reduce water use and leaching. Als, the system can run real-time and autmated, needs n further input data, is able t maintain a cnstant sil misture level, needs little investments, and is easy t perate. Hwever, n ptimisatin rutines are invlved t really minimise water use and minimise leaching. Furthermre n tls are prvided t cntrl flushing and t indicate the amunt f salts leached. The Final Set-Up The Waterman system will make use f the tensimeter threshld technique in an rdinary feedback cntrl lp. This cntrl lp frms the cre, the inner lp, f the irrigatin system. The tw-ptimisatin rutines, ne fr water use and ne fr leaching, will be cnnected t the cre cntrl lp as an uter cntrl lp each (Fig. 2). The same applies fr the flush cntrl lp. The nly variable available t be manipulated by the irrigatin cntrller is the water supply: when and hw much shuld we irrigate? N cntrlling f the amunt f nutrients in the irrigatin water will take place. The cntrlled variables fr the prcess f sil misture can be summarised as fllws: irrigatin timing and amunt (input) level f leaching (utput) level f water use (utput) sil misture level (utput) level f salinizatin (utput) A tw-dimensinal grund water flw mdel (2D-HYDRA) will be available nline t be used by the ptimisatin rutines when necessary. Fr eample, the leaching ptimisatin rutine will use data f recent sil misture measurements t cmpute the leaching that tk place during that measurement perid. If necessary it can then adjust the sil misture set pints used by the inner cntrl lp. Because the dynamic prperties f grund water flw are highly nn-linear, it is imprtant that these mdel prperties are knwn accurately. T accunt fr this necessity a cntrl lp that finetunes these mdel prperties may be added. This can be dne in a similar way, the leaching algrithm uses recently measured data. The inner lp, which will be eecuted cntinuusly, will have a schedule fr calling the three-ptimisatin rutines (minimum 72

3 leaching, minimum water use, and mdel parameters adjustment). The EC level will be mnitred by the inner lp. As sn as an upper limit is reached the flush cntrl algrithm is given permissin t take ver the irrigatin cntrl. After the flush cntrl algrithm has realised a pre-set EC value the inner lp will regain cntrl ver the irrigatin. PROCESS MONITORING CONSIDERATIONS Sil Misture State t Measure In grund water flw there are tw state variables: pressure head (h) and water cntent (θ fr vlumetric r w fr gravimetric water cntent). When cnsidering what variable t measure it is imprtant t realise that with presently available sensrs there are large differences in perfrmance. Tensimeters have distinct ther perfrmance capabilities than water cntent sensrs. Measuring pressure head is mst cmmn, because it can be measured directly and because cnventinal tensimeters are cheap and easy t perate. Measuring water cntent real-time can nly be dne thrugh an indirect methd, which is measuring anther sil prperty that crrelates strngly with sil water cntent. One f the pssible prperties eligible fr measurement is the sil dielectric cnstant, which can be measured by means f a frequency dmain (FD) r a time dmain reflectmetry (TDR) technique. T btain a sil water cntent value frm dielectric sil measurements ne has t perfrm a sil specific calibratin in advance. In applicatins where sil characteristics, like density changes with time r spatial variability, are large it is nt pssible t btain high accuracy measurements based n ne calibratin. Althugh calibratin f sil water cntent sensrs can be cnsidered a disadvantage, they have a larger wrking range and respnd instantly t sil misture changes. Fr plain irrigatin cntrl, the mst prbable cst-effective apprach is based upn using tensimeter signals (see sectin 2.2). Hwever, in the medium wet area f the water retentin curve tensimeters d nt always have as much reslutin as water cntent measurements d (see Fig. 3). Further, available tensimeters have a large respnse time t changes in pressure head while water cntent sensrs give the eact value instantly. Thus, fr accurately calibrating and/r validating a grund water flw mdel ne might cnsider measuring bth pressure head and water cntent. Calibratin fr Water Cntent The gal is t find the sil specific relatin between measured dielectric cnstant (permittivity ε ) and sil water cntent, the s-called calibratin curve (see Fig. 4a). Whether this cncerns vlumetric r gravimetric water cntent is nt that imprtant. Net, ne has t bear in mind that sil permittivity is dependent n sil density (ρ), irn cntent, teture, rganic matter cntent, temperature, and measurement frequency. Therefre, a calibratin fr sil water cntent is nly valid fr the specific sil used with it's specific prperties. The accuracy with which sil water cntent can be measured depends n the calibratin prcedure and n the accuracy f the permittivity measurements. With sme lss f accuracy, it is pssible t use a calibratin curve fr sils with cmparable prperties. Nw fur ways t btain water cntent values frm permittivity measurements are discussed. The easiest way is t buy equipment with pre-prgrammed calibratin curves. This is nly recmmendable in case ne can judge clearly frm the specificatins that the sil fr which the calibratin was made is cmparable t nes wn sil. Unfrtunately this is hardly ever true. Cmparing fr eample 'mineral sil' with 'mineral sil' is nt advisable. A secnd methd is perfrming a tw-pint calibratin. In general the permittivity at zer water cntent is abut fur r five. The shape f mst calibratin curves is similar t the famus Tpp curve fr sand (a third degree plynminal), but just rtated arund the zer-pint. One then has t make ne additinal permittivity measurement in a sil 73

4 sample and determine it's water cntent The Tpp equatin can then be rtated until it fits the secnd pint. Third, t btain a calibratin curve based n mre measurement pints, ne can use a cylinder t take an undisturbed sil sample. The cylinder shuld be large enugh t nt influence the measurement s electrical field lines and shuld be small enugh t cmpact the sil t much. The sil in the clumn must be wetted carefully. A dielectric sensr is placed in the sil clumn, which then is placed n a balance. During the perid that the sil clumn dries by air every nw and then the sample weight and it's crrespnding permittivity value are registered. Finally, the sil is ven dried and it's water cntent is determined. Tgether with the sample cylinders vlume the calibratin curve can be derived. The last methd is t make density dependent calibratin curves t accunt fr density variatins in depth (thery by Perdk et al., 1996). In the labratry sil sample cylinders are artificially cmpacted t a certain bulk density. This will be dne fr tw series f different gravimetric water cntents and fr a series f different densities (see Fig. 4). Separately the equatins fr permittivity as a functin f gravimetric water cntent, ε'(w), and fr permittivity as a functin f density, ε'(ρ), can be made. The shape f ε'(w) is a third degree plynminal, the shape f ε'(ρ) is a straight line. Further, the slpe f the ε'(ρ) relatin is linearly dependent n the water cntent (J. Kresbergen, persnal cmmunicatin). Cmbining the abve a sil specific ε'(w,ρ) equatin can be derived. Nw fr each dielectric sensr t be installed in the sil specific calibratin curve can be cmputed. When installing the sensr ne determines the gravimetric water cntent at the pint f installatin and ne perfrms a permittivity measurement, s with these values density can be calculated. The advantage f this methd is that n time cnsuming cre samples have t be taken. Cncerning the abve fur calibratin methds ne can say that the first methd is the least accurate, because it is hardly ever sil specific enugh, and the last methd is the mst accurate because it allws fr density dependency. A tw-pint calibratin is the fastest way. The balance methd is less labur intensive than the density dependent methd, but it cvers a lnger perid because the sil has t dry by air. Befre ne decides which methd t chse, ne has t knw what accuracy is desirable and what wrking range the calibratin curve shuld cver. The curve's dry end (< 5-7% gravimetric water cntent) is a prblem with dielectric measurements, due t nt fully understd bund water effects. The same applies fr the wet end. Temperature Dependence f Dielectric Cnstant and Bulk EC Bth dielectric cnstant and bulk EC depend n temperature. We want t crrect fr this influence since the quantities that we are really interested in, water cntent and nutrient cncentratin, are nt temperature dependent. First an impressin is given n what rder f magnitude the dependencies cver. Secnd the directin f research fr finding crrectin frmulas is discussed. During a perid in spring with n irrigatin the maimum differences per day measured in ur greenhuse sandy sil were: T ma =7 C, ε ma =0.4, EC ma =0.03 ms/cm at 5 cm depth and T ma =0.5 C, ε ma =0.1, EC ma =0.01 ms/cm at 45 cm depth (see Fig. 5). In the bserved perid n crps were grwn and n nutrients were applied. Therefre the differences are nt very large. The bserved differences are nt imprtant when cmpared t sensr accuracy, but they are imprtant when cmpared t sensr repeatability. The temperature dependence was investigated as fllws. First, thrugh crrespnding ε, EC and T lines a trendline was fitted. Then the difference f the ε, EC and T values cmpared t their trendline was calculated. Finally the calculated differences f ε and EC were pltted individually against the calculated differences in T. The lines fitted thrugh these plts crssed the rigin fr EC data nly, indicating that when temperature des nt change the EC value des nt change. Fr ε data n trendlines crss the rigin, but crss at a lwer pint n the -ais. This means that sil water 74

5 cntent decreased. Future research cncerning the temperature dependence will be based n the fllwing assumptins. Since we knw that the EC s temperature dependence f a material is dependent n the abslute EC value at a reference pint accrding t: EC = EC ref T ( T) (1) and we knw that ε and EC are linearly dependent n each ther it seems likely that ε is dependent n temperature the same way the EC is. Hwever, there are tw differences between the ε (T) relatin and EC(T) relatins. First, the ε (T) relatin crsses the -ais at zer water cntent ε where the temperature influence is negligible, while the EC(T) relatin crsses the rigin. Secnd the ε (T) relatin will be fr a cnstant water cntent, while the EC(T) will be fr cnstant nutrient cncentratin. DISCUSSION AND CONCLUSIONS Once mre it has been shwn that develping an advanced cntrl system demands a lt f knwledge abut the prcess and hw it can be mnitred. The Waterman irrigatin cntrl system, presented as a cncept in sectin 2.3, shuld be fleible enugh t enable users t chse their wn functinality f interest. The article title One starts measuring sil misture, ne btains data and then what? (Vegter, 1995) clearly shws the prblem when peple want t start cntrlling irrigatin based n sil misture sensrs. In general, n chice whether t measure pressure head, water cntent, r bth is made purpsefully. Slely measuring pressure head seems suitable fr a wide range f applicatins, but fr sandy sils and when using a grund water flw mdel ne shuld cnsider measuring water cntent t. The accuracy f calibratin fr sil water cntent will stay a pint f cncern as lng as there are sensr manufacturers that claim unrealistic high accuracies. Frm the fur methds presented, the Kresbergen (Perdk et al., 1996) methd seems favurable because it prvides density dependent calibratin frmulas. The last remark n calibratin is that fr cntrlling purpses it is nt imprtant at what water cntent value the prcess is kept. The grwer just needs a reprducible set pint that is related t water cntent changes. Cnsidering this, cntrlling the dielectric cnstant is sufficient. Hwever, fr cmparing results f different nurseries with each ther a water cntent value ffers mre clarity. Temperature dependence crrectin frmulas fr ε and EC shuld imprve the accuracy f cntrlling sil water cntent and salinizatin level. Future research will be based n the similarities between ε and EC. ACKNOWLEDGEMENTS The authrs wuld like t thank the fllwing financiers f the Waterman prject fr their cntributin: the Ministry f Agriculture, Nature Management and Fisheries f the Netherlands (prgram Optimisatin f water and nutrient cnservatin in prtected crpping systems, n. 256) and the Eurpean Cmmunity (Fair-4 P0681). Literature Cited Annymus, Stuctuurnta landbuw. Ministry f Agriculture, Nature Management and Fisheries. The Hague, the Netherlands. p77-79 and p135. Annymus, Denar s slide finish. Milieudemnstratie-prject Denar Kas BV. CD- ROM prduced by Vreugdenhil Prducties, Ruys de Beerenbruckplein 7, 2613 AR Delft, The Netherlands. Bnekamp G., Cnsequent nat f drg is het beste. Grenten+Fruit, vakdeel glasgrenten 4(1994)19, p6-9. Hess T A micrcmputer scheduling prgram fr supplementary irrigatin. Cmputers and Electrnics in Agriculture 15: Mern, M., Assaf, R., Bravd, B., Wallach, R., Hallel, R., Levin, A and Dahan, I

6 Sil sensr actuated micrirrigatin f apples. In Prc. 5 th Int. Micrirrigatin Cngress, St. Jseph, Mich.: ASAE. Perdk, U.D., Kresbergen, B. and Hilhrst, M.A Influence f gravimetric water cntent and bulk density n the dielectric prperties f sil. Eurpean jurnal f sil science, 47(3): Reddy, J.M Adaptive cntrl in management f surface irrigatin systems. ASAE Paper Smith, R.J. and McClymnt, D.J Tward real time cntrl f surface irrigatin: estimatin f sil infiltratin parameters under surface and surge irrigatin. Trans ASAE Paper 96C-053. Thmsn, S.J. and Rss, B.B Mdel-based irrigatin management using a dynamic parameter adjustment methd. Cmputers and Electrnics in Agriculture 14: Valiente M., de Juan, J.A.,Tarjuel, J. and Ortega, F Mdel fr ptimum irrigatin water management, applicatin t an etensive hrticulture farm. Prceedings f AgEng, paper 96C-040, p Vegter, B Je gaat vcht meten, je krijgt een cijfer, maar wat dan? Vakblad vr de Blemisterij 12, p Figures sil misture state Sil Misture Set-Pint cmparatr + - Irrigatin Cntrller cntrl instructin Irrigatin System water Sil measured sil misture state Sil Misture Sensrs Cntrller Fig. 1. Basic feedback cntrl lp fr irrigatin cntrl. 76

7 Fig. 2. Cncept f the Waterman irrigatin cntrl system. lg pressure head in cm --> [-] Vlumetric water cntent --> [m3/m3] Fig. 3. Water retentin curve f a medium curse sandy sil. The light grey and the dark grey areas indicate sil misture ranges with high reslutin fr pressure head measurements. Measurements f water cntent will have a lw reslutin. The medium grey area indicates the ppsite. 77

8 dielectric cnstant, e' --> [-] e'(w)=a0+a1*w a2 dielectric cnstant, e'--> [-] gravimetric water cntent, w --> [-] e'(rh)=a3+a4*rh density, rh --> [g/cm3] Fig. 4a and 4b. ε (w) and ε (ρ) calibratin functins fr a sandy sil. Thery frm Perdk et al (1996). Each mark represents an individually cmpacted sil sample. The s indicate a sil sample series f cnstant lw water cntent and the s indicate a series f cnstant high water cntent. The sil sample series with cnstant density is indicated by s. 78

9 a) b) dielectric cnstant --> [-] temperature --> [deg.c] time --> [days] cm -45 cm -5 cm cm time --> [days] -25 cm -15 cm -45 cm -5 cm bulk EC --> [ms/cm] vlumetric water cntent --> [m3/m3] time --> [days] time --> [days] -15 cm -25 cm -45 cm -5 cm -15 cm -25 cm -45 cm -5 cm Fig. 5. Measurements f sil dielectric cnstant bulk EC and temperature, tgether with calculated vlumetric water cntent. The measurements were perfrmed in a sandy lam sil at fur different depths frm t Irrigatin was applied n the eighth day. 79