986 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 46, NO. 4, AUGUST 1997

A Low-Cost Water Detection Methodfor Furrow Irrigation Control

David J. Turnell, Gurdip Singh Deep,Senior Member, IEEE, and Raimundo Carlos Silverio Freire

Abstract—The typically low efficiency of furrow irrigationcan be raised by a new control method. In this method, acomputer monitors the water advance along the furrows duringthe initial phase of the irrigation. The distance-versus-time dataobtained are used to calculate the soil’s infiltration coefficients,which, in turn, are used in the water infiltration model. Thecomputer can thus determine the most appropriate water inputrate and application time to achieve maximum efficiency. Thispaper describes a simple low-cost water sensing network thatenables a computer to monitor the water advance in real-time.Each sensing network consists of up to 14 water detectors linkedby a maximum of 1000 m of inexpensive twin-wire. The detectionof water is done by the change in capacitance of a wire element.This alters the frequency of oscillation of a relaxation oscillator.The detectors signal their states to the computer by means ofcurrent pulses generated from the oscillator. The simplicity ofthe network allows easy field maintenance by nontechnical peoplewith minimal tools.

I. INTRODUCTION

FURROW irrigation [1] is composed of three phases:advance, ponding, and recession. In the advance phase the

water slowly advances down the furrows until it has floodedtheir entire lengths. For long furrows, the advance phase canlast for 2 h or more. During the ponding phase the waterinflow is normally reduced to the point where the furrowsremain flooded but excessive run-off of water at their endsis avoided. The ponding phase lasts for several hours. Whenthe desired application of water is obtained, the water inflow isstopped and the recession phase begins. During this last phase,the water level in the furrows quickly drops due to continuedinfiltration and run-off of water.

The main disadvantage of furrow irrigation is its typicalinefficiency. Water is lost due to both run-off at the furrow endsand deep infiltration beyond the root zone. A new computer-based control technique promises to raise the efficiency tolevels comparable to those of other forms of irrigation (suchas sprinkler and trickle). This control technique is based uponfinding a solution to the “inverse furrow advance problem”[2]–[4]. The computer monitors the water advance alongthe furrow during the initial phase of the irrigation. Thisis accomplished by means of a network of water detectorsinstalled along one of the furrows within each irrigated area.These detectors must indicate the presence of water but are notrequired to measure either the depth of the water infiltration

Manuscript received June 3, 1996. This work was supported by the CNPq.The authors are with the Department of Electrical Engineering, Federal

University of Paraıba, Paraıba, Brazil, 10.004 (e-mail: david@dee.ufpb.br;deep@dee.ufpb.br; freire@dee.ufpb.br).

Publisher Item Identifier S 0018-9456(97)06503-0.

or the humidity of the soil. The distance-versus-time dataobtained from the detectors are used to calculate values forthe water infiltration coefficients of the soil. These coefficientscan then be used in the modeling of the water infiltrationinto the soil along the furrows. The results of the modelingallow the computer to obtain maximum irrigation efficiencyby determining both the duration of the ponding phase andthe alteration in water inflow rate.

For best results, this control technique requires at least sixor seven water detectors installed along one of the furrowsin each irrigated area. There are many ways in which thesedetectors could communicate with the computer in the controlroom (which could be hundreds of meters away). One exampleis the use of radio and infrared links between microprocessor-based field station [5]. Such sophistication may be acceptablewithin the bounds of a research project. However, for costreasons, it is relatively infeasible for real-life field applications,especially in most of the regions of the world where irrigationis necessary.

In this article we describe a new low-cost water detectionmethod developed as a part of an irrigation control systemcalled real-time irrigation optimization system (RIOS) [6].RIOS is currently being developed at the Federal University ofParaıba. The RIOS water sensing method should satisfy twobasic requirements:

• Low-cost: Para´ıba is a poor, semi-arid Brazilian statewhere economic reality is an important factor that restrictsthe use of irrigation.

• Ease of maintenance:The detector network must besimple enough so that most repairs can be done bynontechnical people with only basic tools.

II. THE WATER SENSING NETWORK

Each detector network consists of up to 14 water detectorslinked to the computer by up to 1000 m of twin-wire line asshown in Fig. 1. The voltage that the computer applies tothe line determines which one of the detectors is addressed atany given time. To monitor a particular detector, the computerputs its nominal address voltage on the line and waits for400 ms. It then samples the line current through anA/D converter. The interrogated detector generates a streamof current pulses, the period of which indicates whether thedetector is in contact with the water or not.

To illustrate this, Fig. 2 shows the scanning ofdetectorsover time. The voltage addressing is shown in Fig. 2(a) andthe resultant current waveform on the network line is shown

0018–9456/97$10.00 1997 IEEE

TURNELL et al.: LOW-COST WATER DETECTION METHOD 987

Fig. 1. Schematic of a water detection network.

(a)

(b)

Fig. 2. (a) Detector addressing with line voltage and (b) detector response.

in Fig. 2(b). For a correctly configured network, only onedetector responds to each voltage “address” applied to the line.When a detector detects the presence of water the period ofthe current pulses it generates increases by up to 70%. Theamplitude of the pulses is of no relevance.

While rather unorthodox, this addressing and signalingmethod does have some advantages:

• The current pulses generated by the detectors in the lineare relatively immune to noise. In the prototype network,the current pulses pass through 600 m of thin wire withno significant distortion.

• A healthy detector always returns a stream of currentpulses when addressed, whether wet or dry. Faulty de-tectors can thus be detected and eliminated from consid-eration before an irrigation session starts.

• Just two wires are needed for the network. This reducesthe number of electrical connections needed in the field.

• The circuit needed for the voltage addressing and pulsegeneration is simple and inexpensive. Simple detectorsresult in higher network reliability.

A. The Detector Circuit Diagram

Fig. 3 shows the detector circuit diagram. The three op-erational amplifiers (A1–A3) are part of the same integratedcircuit device (LM324). The operational amplifier A3 is con-figured as a relaxation oscillator whose period is determined bythe capacitance of the water sensing element. For the sensingelements used in the prototypes, the dry capacitance wasaround 1 nF and the corresponding period of oscillation around1 ms. The switching of the transistor (when adequately biased)causes the current pulses on the line. The light emitting diodeD2 is used for a visual confirmation of detector addressing.

The operational amplifiers A1 and A2 are configured ascomparators and implement the voltage addressing mecha-nism. Both amplifiers compare the network line voltage (byway of R1 and R2) with a reference voltage. In the case

Fig. 3. Water detector diagram.

(a)

(b)

(c)

Fig. 4. Voltage addressing scheme. (a) Input line voltage, (b) output voltageof amplifier A1 (Fig. 3), and (c) output voltage of amplifier A2 (Fig. 3).

of amplifier A2, the reference voltage of 5.0 V is obtainedfrom the LM336 reference diode. In the case of amplifierA1, the reference voltage is slightly lower than 5.0 V andis obtained by means of the voltage drop across the diode D1.The difference between the two reference voltages determinesthe addressing voltage range for a given detector.

The function of amplifiers A1 and A2 is best demonstratedby examination of Fig. 4, which shows their outputs as afunction of linearly increasing voltage applied to the networkline. As the line voltage rises and reaches the lower limit atwhich the detector is addressed (V1) the output of amplifier A1goes from low to high. As the line voltage rises even more andpasses the upper limit (V2), the output of A2 also goes fromlow to high. From the way that the transistor T1 is configured,the detector only generates current pulses when the output ofA1 is high and output of A2 is low. This only happens whenthe voltage on the line is within the detector address range V1to V2. This range is different and non-overlapping for eachdetector on the network.

B. The Network Interface Circuit

Each detector network requires an interface circuit at thecomputer. This circuit must generate the variable line voltage(0–18 V) used to address or interrogate the various detectors. Itmust also provide a voltage that is proportional to the line cur-rent, from which the computer can detect the pulses generatedby a detector being interrogated. Fig. 5 shows the interface

988 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 46, NO. 4, AUGUST 1997

Fig. 5. The interface circuit between the computer and the detector.

circuit used in the prototype system. The computer controlsthe output line voltage by using an 8-bit D/A converterto set the input voltage . The operational amplifier A1maintains the voltage proportional to . The transistorTIP31 increases the current output capacity.

The output of A2 (configured as a differential amplifier witha gain of ten) is a voltage proportional to the line currentsupplied to the detector network. In the prototype, the outputvoltage of A2 is connected to an 8-bit A/D converter withinthe computer.

The computer interrogates a detector by writing to the D/Aconverter an 8-bit value that corresponds to the detector’snominal addressing voltage. After waiting 400 ms, it thentakes 200 samples of the line current using an 8-bit A/Dconverter. These samples are then analyzed to determine thepulse repetition period. In the prototype network, an 8-bitD/A converter was found to be adequate when the numberof detectors is below ten. A larger number of detectors wouldrequire a 10-bit D/A converter because the detector addressvoltages would have to be closer together. In relation to theA/D converter, a resolution of 8 bits is adequate for thesoftware to distinguish the current pulses from the networkstandby current.

C. The Sensing Element

Most water detectors use the change of resistance betweentwo electrodes to detect the presence of water. This approachis not possible for the present application due to electricalinteraction between detectors in a flooded furrow. Early fieldtrials with resistance-based detector prototypes showed that thelow-impedance electrical path formed by the saline water inthe furrow interferes with the operation of the oscillator circuit.

To avoid this problem, a detector based on the change incapacitance was used to detect the presence of water. Thisallows the detector to be electrically isolated from the water.The sensing element used in the final prototype is an 80-cmlong 26-wire ribbon cable. At the near end, the cable’s wiresare separated into even and odd groups and at the far end, thewires are left open-circuited (waterproofed and isolated witha covering of silicon rubber). The result is a crude form ofcapacitor of about 1 nF that is sensitive to the presence ofwater. This sensitivity arises because the dielectric constantsof air and water differ considerably.

In laboratory tests, the capacitance of the ribbon cable wasobserved to increase by 90% when it was immersed in water.In the field the cable can become partially buried and theexpected capacitance increase is lower, typically in the rangeof 40% to 70%. The period of the current pulses producedby the detector is proportional to the capacitance. Thus thecomputer must be able to detect a change in pulse period of40% or more. The pulse detection algorithms used in the RIOSsystem software can reliably detect changes in pulse periodfrom about 15% upwards.

D. Current Consumption

One of the critical aspects of the detector design is the cur-rent consumption. If this was excessive, the voltage drop alongthe network line would interfere with the voltage addressingmechanism. In the prototype network, the standby current ofthe detector is dependent upon the line voltage in use. At theminimum operational line voltage of 7 V, the standby currentof each detector is approximately 2 mA. At the maximum linevoltage of 18 V, the standby current is 5 mA. When a detectoremits a pulse, the line current increases by a value dependentupon the load resistor R3 (Fig. 3) and the line voltage. Thevalue of R3 is chosen so as to give approximately 10 mA ofcurrent increase at the interrogating voltage of each detector.

Two strategies help reduce the effects of line voltage drops:

• Detector addressing:The majority of water detectionnetworks do not use the maximum of 14 detectors. Fornetworks employing a number of detectors less than themaximum, one should choose those with lower addressingvoltages.

• Detector placement:The detectors with the higher acti-vation voltages should be placed nearer the computer.

In spite of the low consumption, it is still possible for thecurrent pulses to have an adverse effect upon the addressingmechanism. This happens when the line voltage is at the lowerlimit of a detector’s address range. The effect is minimal,however, because the capacitor C in the detector’s circuit filtersout the line voltage “ripple” caused by the current pulses. Tohelp avoid this problem, the RIOS software provides the userwith the network setup window shown in Fig. 6. With thiswindow, the user can visualize the line current waveform ashe changes the network line voltage. He can thus identify eachdetector’s exact addressing voltage under field conditions.

E. Distortion of the Pulse Current Waveform

One of the concerns during the initial development of thedetector network was the potential pulse distortion causedby power-line pickup, noise, and line reactance. The currentwaveform shown in Fig. 6 is from an actual experimentaldetector that was 300 m from the computer. Nearly all ofthis distance was underneath a 15 kV power line with fourtransformers that was supplying the afternoon load of sixlaboratory blocks. This figure illustrates that:

• The rounding of the pulses due to line reactance isnegligible. Even under later tests with 600 m networksno pulse rounding was observed.

TURNELL et al.: LOW-COST WATER DETECTION METHOD 989

Fig. 6. An experimentally obtained line current waveform display usingRIOS control software.

• There is no apparent 60 Hz pickup from the overheadpower line and transformers.

• High frequency distortion is minimal and probably almostentirely attributed to the low-resolution, noisy (wire-wrapped) A/D converter circuit used to sample the linecurrent.

F. Pulse Period Detection

The irrigation control software developed with the prototypedetector network implements two different methods for thedetection of the current pulse repetition period. Each methoduses the 200 samples taken from the A/D converter in theinterface circuit. The two methods are:

• Peak detection, in which the pulses are “smoothed” by asoftware-implemented low-pass filter; the period is thenderived from the number of samples between positivepeaks.

• Edge detection, in which the software detects largechanges in the line current as positive and negativeedges; the period is then derived from the number ofsamples between two consecutive positive edges.

The user is able to switch between these methods usingthe configuration window for each irrigated area. The peakdetection method is useful for situations where there is eitherconsiderable line noise or where there is excessive roundingof the current pulses due to line reactance. The edge detectionmethod is useful when the current pulses are relatively “clean”because it gives more accurate results. For both methods,however, sampling is repeated twice to obtain more valuesfor the pulse period. In our preliminary tests both methodsperformed satisfactorily.

G. Detector Construction and Installation

The original idea for the detector construction was to usesmall plastic boxes. However, this was ruled out due to thecost of the waterproof boxes that are capable of withstanding

the harsh conditions in the field. Instead of boxes, the finaldetector prototypes are encapsulated in clear plastic resin. Thewater sensing ribbon cable emerges from one side of the plasticblock. On the other side are two brass screw terminals thatconnect the detector to the network.

The detector’s light emitting diode can be seen through theclear plastic, allowing detector interrogation to be visualizedin the field. The detectors are installed along the ridge of thefurrow and are protected by small plastic bags. The networkline wire also runs along the ridge of the furrow. The watersensing ribbon cable drops down from each detector into thetrough of the furrow.

III. CONCLUSION

The water sensing method presented here is one of manypossible ways in which a computer-based irrigation controlsystem could monitor the water advance in a furrow irri-gation. The relatively long distances involved in the field,as well as the number of detectors needed, tend to suggestsophisticated solutions based upon microprocessor-based fieldstations, digital networks, radio links, etc. However, as thisinvestigation has shown, the problem can also be solved ina simple and low-cost manner consistent with the economicrealities of many developing countries. The simple natureof the network is a positive factor in terms of reliability,and maintenance is relatively easy and can be performed bynontechnical personnel.

REFERENCES

[1] A. Benami and A. Ofen,Irrigation Engineering. Haifa, Israel: Irriga-tion Eng. Sci.

[2] C. Azevedo, “Real-time solution of the inverse furrow advance prob-lem,” Ph.D. dissertation, Dept. Agricultural Irrigation Eng., Utah StateUniv., Logan, 1992.

[3] W. Walker and J. D. Busman, “Real-time estimation of furrow in-filtration,” J. Irrigation Drainage Eng., vol. 116, pp. 229–317, May1990.

[4] B. Izadi and D. Heermann, “Real-time estimation of infiltration parame-ters for controlling an irrigation,”Summer Meet. Amer. Soc. AgriculturalEngineers, Baltimore, MD, 1987.

[5] E. A. Latimer and D. L. Reddell, “Components for an advance rate feed-back irrigation system (ARFIS),”Trans. ASAE, vol. 33, pp. 1162–1170,July–Aug. 1990.

[6] D. Turnell, G. S. Deep, and R. C. S. Freire, “RIOS, A real-timeirrigation optimization system,”XI Congresso Brasileiro de Autom´atica,Sao Paulo, Brazil, pp. 1751–1756, Sept. 1996.

David J. Turnell was born in Wellingborough,U.K., in 1960. He received the degree in electronicengineering from Bradford University in 1982. Heobtained the M.S. degree in computer science in1991 and is currently pursuing the Ph.D. degree inelectrical engineering, both at the Federal Universityof Paraıba, Paraıba, Brazil.

Since 1993, he has been an Assistant Professor inthe Department of Agricultural Engineering, FederalUniversity of Paraıba. His current fields of interestare automation of irrigation systems and computernetworking.

990 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 46, NO. 4, AUGUST 1997

Gurdip Singh Deep (M’76–SM’84) was bornon December 12, 1937. He received the B.Tech.(Hons.) degree in electrical engineering from theIndian Institute of Technology (IIT), Kharagpur,in 1959, the M.E. degree in power engineering(electrical) from the Indian Institute of Science,Bangalore, in 1961, and the Ph.D. degree inelectrical engineering from IIT, Kanpur, in 1971.

From 1961 to 1965, he worked as an AssistantProfessor at the Guru Nanak Engineering College,Ludhiana, India, and from 1965 to 1972, he was

with the IIT, Kanpur, as a Lecturer/Assistant Professor. He was Consultantfor Encardio-rite Electronics (Pvt.) Ltd., India, from 1969 to 1970. SinceJuly 1972, he has been a Titular Professor at the Centre of Science andTechnology of Federal University of Paraıba, Campina Grande, Brazil,where he is presently the Coordinator of the Electronic Instrumentationand Control Laboratory. His research interests are electronic instrumentationand microcomputer-based process control.

Raimundo Carlos Silverio Freire was born onOctober 10, 1955, in Po¸co de Pedra, Brazil. Hereceived the B.S. degree in electrical engineeringfrom Federal University of Maranhao, in 1980, andthe M.S. degree in electrical engineering from theFederal University of Paraıba, Paraıba, Brazil, in1982. He received the Ph.D. degree in electronics,automation, and measurements from the NationalPolytechnic Institute of Lorraine, Nancy, France, in1988.

He worked as an Electrical Engineer forMaranhao Educational Television, Brazil, from 1980 to 1983. He was aProfessor of Electrical Engineering at the Federal University of Maranhaofrom 1982 to 1985. Since December 1989, he has been on the facultyof the Electrical Engineering Department, Federal University of Paraıba.His research interests include electronic instrumentation and sensors andmicrocomputer-based process control.