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  A Computer-controlled DripIrrigationSystem forContainer Plant

Production

Rico A. González1

,

Daniel K. Struve1

, and

Larry C. Brown2

 Additional index words. red oak,automated irrigation, irrigationefficiency

Summary. An irrigation control sys-tem has been developed and used to

estimate evapotranspiration of con-tamer-grown plants by monitoring

randomly selected plants within a con-

tainer block and watering on an “asneeded” basis. Sensor reliability andoperational ease allows application of 

the system in a wide variety of fieldconditions. First-year tests, using red

oak (Quercus rubra L.) seedlings,showed a reduction of 95% or betterin both total irrigation and leachate

rates with the computer-controlled

treatment relative to a manually con-

trolled, drip irrigation treatmentwithout reducing plant growth.

As concerns about water conser-vation and quality increase,horticulturists must adopt

management practices that decreasewater use and leachate rates. Unfortu-nately, the perception by producers isthat plant growth, and thus yield, isproportional to irrigation rate. A sig-nificant reduction in water use is pos-sible without affecting plant growth

by improving the delivery system; forinstance, by using trickle vs. overheadirrigation. Even greater efficiencies arepossible by coupling irrigation deliv-ery with a system that monitors themoisture status of the crop or the soil/ media.

1

 Department of Horticulture, The Ohio State Univer-sity, Columbus, OH 43210-1096.2

 Department of Agricultural Engineering The OhioState University, Columbus, OH 43210-1057.

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Computer-controlled irrigation

and monitoring systems have been de-veloped for greenhouse crops (Liethand Burger, 1989; Michelakis andChartzoulakis, 1988). There has beenlimited development of these types of systems for field or container nurseries(Smajstrla and Koo, 1986). Field irriga-tion in the horticultural industry in many

cases is based on a traditional rule of thumb; for example, watering plants 1 ha day (Fare et al., 1992). Manualcontrolor simple timers are sufficient to meetthe needs of this type of watering system.Although more advanced irrigationcontrol schemes are available, such asthe use of capillary beds or wateringduration based on empirical evapo-transpiration estimates linked with cropor landscape specific water-use coeffi-cients (Costello et al., 1991; Martin etal., 1989), these methods lack a controlfeedback (monitoring) of actual plant or

soil moisture status. Smajstrla and Koo(1986) described a system that moni-tored soil moisture status but did notpresent data on irrigation efficiency orcrop growth. One serious drawback of this particular irrigation system was theuse of soil monitoring instruments thatrequired constant maintenance andrecalibration. What would be most usefulis a controlled irrigation system thatapplies water to plants based on theactual soil evaporation and plant tran-spiration losses since the last period of irrigation.

The goal of this study was todesign, build, and test an irrigation

control and monitoring system forcontainer production that would: 1)use existing off-the-shelf technology,2) apply water based on actual evapo-transpiration rates, 3) operate under awide range of field conditions, 4) re-quire limited instrument calibration,5) provide some operating flexibilityto allow adjustments during the

growing season, and 6) interface withpersonal computers for system moni-toring, scheduling changes, and datacollection. The research objectives wereto test the reliability of the computer-controlled system and to determine if this system reduced irrigation andleachate rates compared to manualcontrol of a drip irrigation system byan experienced nursery manager. Thispaper describes the hardware andsoftware components of the control andmonitoring system and presents acomparison of plant growth under thecomputer and manual control methods.

HortTechnology ž July/Sept. 19922( 3)

 Design concept. There are fivemajor components to the irrigationsystem: 1) standard trickle irrigationequipment, 2) solenoid valves to con-trol water to the drippers, 3) a fieldmicroprocessor to open and close thesolenoid valves, 4) moisture sensorsunder “indicator” plants for detectingleachate, and 5) an office microcom-

puter to collect data, write programs,and allow general access to the fieldmicroprocessor.

The system is based on a simplefeedback control loop with leachatedetection as the control parameter. Awatering cycle begins when the mi-crocomputer opens a solenoid valvepressurizing the irrigation line. Wateris delivered to a container plant throughthe drip-irrigation equipment untilleachate from the plant is detected bythe moisture sensor. When leachate isdetected, the solenoid valve is closed.

At this point the container is assumedto be slightly above field capacity.

If the plant and container areconsidered as a single system, the re-

lationship between the amount of waterapplied and plant water demand can bedetermined by balancing water inputsand losses for the plant/container. If the container is covered to limit rain-

fall, water into the plant/container issimply the amount of irrigation ap-plied, while water losses from the sys-tem are due to soil/media evaporation,plant transpiration, and leaching. One

can assume that the delay between thetime for the container to reach fieldcapacity and leachate detection for eachirrigation cycle are similar, i.e., there isa “standard” amount of leachate at theend of each cycle. Under this as-sumption, the total amount of waterapplied at the end of any irrigationcycle will be an approximation of theevapotranspiration rate for that plantcontainer system since the previousirrigation period less the standardleachate amount. Therefore, this con-trol scheme applies irrigation based onthe actual environmental and plantdemands that drive the rate of waterloss from the system since the previousirrigation. The control loop is scaled-up to a large block of containers bymonitoring a few indicator plantsrandomly located within the block.

 Plant material and plot design.Red oak acorns were collected in Fall1990, germinated in early Feb. 1991,and grown in a greenhouse in l-gal(3.8-liter) containers until late May

1991. At that time, the seedlings weremoved outside, placed under 80% shadecloth for 2 weeks, and then potted in3-gal (11.4-liter) containers using agrowth media consisting of 3 pinebark : 0.5 peat : 0.5 composted mu-nicipal sewage sludge : 1 sand supple-

mented with dolomite, gypsum, andP. The repotted seedlings were placed

in the test plot and allowed to acclimatefor 1 month. During this time theseedlings were irrigated for 1 h dailyand fertilized once per week by injecting250 ppm N from 20N-20P-20Ksoluble fertilizer into the irrigationsystem. During the test period, July toOct. 1991, the seedlings were fertil-ized at a constant rate of 10 ppm Nusing the same 20N-20P-20K solublefertilizer.

Individual plant containers wereplaced on 18-inch (45-cm) centerswith two 0.5 gal/h (1.9-1iteržh

-1)

emitters (Netafim, Shemin Nurseries,Addison, Ill.) per container. A 0.75-inch (1.9-cm) PVC irrigation header

fed eight separate 0.5-inch (1.27-cm)irrigation lines, individually controlledby solenoid activated valves. Fifty con-tainers were placed on each irrigationline. Six-foot (183-cm) bamboo stakeswere placed in the center of eachcontainer and were used to secure theplants loosely as they grew. The bam-boo stakes were secured to guy wiressuspended above the pots. The fieldmicroprocessor (Campbell Scientific,

Model CR10, Logan, Utah) and as-sociated equipment were centrally lo-cated within the container block in arainproof shelter.

An additional 100 plants wereincluded in the container block usingtwo separate irrigation lines. Theseplants served as a control group andwere irrigated by the container-areasupervisor on an as-needed basis bymanually opening and closing the irri-gation valves to these lines. The intentof the supervisor was to irrigate thesecontrol plants 1 h/day, except when itrained. Workers in the nursery wererequired to log the time of day whenthe valves were opened and closed.

 Indicator pot assembly. Indicatorplants were selected randomly andplaced in a pot-within-a-pot assembly(Fig. 1). A 3-gal (11.4-liter) plant con-tainer was placed inside a standard 5-gal (19-liter) white plastic bucket. Twosupports, made of l-inch o.d. (2.54-cm) PVC pipe, were secured on oneside of the 5-gal bucket to tilt the plant

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container, which ensured that leachatefirst drained from the drainage holeover the moisture sensor. The sensor(Campbell Scientific, Model 237) waslocated directly beneath the drainagehole and secured using a 4-inch (10-cm) length of metal strapping. Thesesensors consist of an electrical gridwhose resistance drops with increased

surface moisture. The contact area of the sensor was positioned at a 45°angle so that leachate hitting the sen-sor would be shed rapidly. Electricalleads from the moisture sensors wererouted along the guy wires.

A 0.5-inch (1.27.cm) hole wasdrilled in the side of the 5-gal bucketsnear the bottom for leachate collection.This hole normally was plugged with arubber stopper and leachate collectedmanually on a daily basis. Rainfall ef-fects were minimized by covering themedia surface of each container at the

base of the plant with white polyethyl-ene (2 mil). Communication, electri-cal power, and instrumentation con-

nections between the field equipment,power supplies, and programming/ data collection computer are illustratedin Fig. 2. The field equipment wasunder the direct control of the mi-croprocessor, which read the moisturesensors through an input multiplexer(Campbell Scientific, Model AM416),activated the solenoids using the 5V/ 

24V relays, and communicated withthe data collection/programmingcomputer (IBM, Model XT) throughshort-haul modems (Campbell Scien-tific, Model SRM6A). All field equip-ment, including the microprocessor,input multiplexer, solenoid relays, andshort-haul modem were located in thewaterproof shelter. Power for the mi-croprocessor and solenoids was pro-vided by a 12V battery and a 24Vtransformer, respectively. The batteryand transformer were remotely locatedto isolate the 110V connections. The

data collection/programming com-puter was locatedin a nearby headhousefor operator convenience and to en-

Fig. 1. Indicator pot assembly used in the computer-controlled irrigation system.

404

sure appropriate temperature and hu-midity conditions for the computer.

Although the field microproces-sor can be accessed on location througha hand-held keyboard, a communica-tion system using short-haul modemswas installed to allow access to the

microprocessor via a personal com-puter (IBM-XT). By accessing the field

microprocessor with a personal com-puter, real-time monitoring of thecontrol program and direct down-loading of information was possiblefrom the convenience of an office.

Software. All programming wasdone on an IB M -PC us ing theCampbell Scientific PC208 softwareand downloaded to the field micro-processor. A simplified outline of thecontrol software, depicting operatorinputs and monitor points, is shown inFig. 3. Two independent control pro-grams were developed, one to scan

inputs and the second to control theirrigation lines. The monitoring pro-gram scanned the sensors every 5 sec,converted input readings to grid re-

sistances, and stored these resistancevalues for use by the control programor for operator monitoring. The highand low values for all sensors werecollected hourly.

Water application was controlledby the irrigation program and is basedon several operator input values. Thesevalues included active control period(time during a 24-h day when the

program is active), irrigation cycle time(irrigation frequency during the activecontrol period), and sensor resistancetrip value (resistance reading at whichthe sensor is considered to be “wet”).These parameters were set during thedevelopment test to irrigate the plantsevery 4 h between 9:00 AM and 7:00 PM

with a resistance trip of 5 kohm. Thisresistance value was determined frompreliminary testing and was highenough to prevent a false reading dueto condensation. At the end of eachwatering cycle, total irrigation time

was stored.To avoid errors resulting from false

sensor readings the following criteriawere used. Each controlled irrigationline included five indicator plants. Duringan irrigation cycle water was applieduntil leachate was detected from at leastthree of these five plants. At the begin-ning of each irrigation cycle, all sensorswere checked; no water was applied if three or more indicators were still wetfrom a previous application.

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Fig. 2.  A schematic diagram showing. the electrical, communication, and instrumentation

 connections used in the computer-controlled irrigation system.

On a daily basis, excepting week-ends, all stored information, includingirrigation times and minimum andmaximum hourly sensor values, were

downloaded from the field micropro-cessor to the personal computer andstored in a standard spreadsheet pro-gram. Leachate that had accumulatedin the bottom of each 5-gal bucket wascollected and measured, and theamount was logged daily.

At the beginning and end of thegrowing season, randomly selectedplants were harvested for growthcomparison. Statistical differences be-tween the plants grown under com-puter-controlled and those grownunder manual irrigation were deter-mined using SPSS/PC+ one-wayanalyses at a 0.05 significance level.

Water use and leachate cum- parison. Water use was reduced by95% to 97% in the computer-controlledirrigation system in comparison to themanual irrigation method (Fig. 4).Due to the large volume of waterinvolved, leachate was not collectedfrom the manually controlled plants.Assuming that the water required toreach field capacity for the manuallycontrolled plants would be similar tothat for the computer-controlled

plants, a 97% reduction in leachate canbe estimated for the computer-con-trolled treatment.

The average measured irrigationrate per container was 3.2 gal/day for

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the manually controlled plants and 0.1gal/day under computer control. Notethat the amount of water applied underthe manually controlled system is highby a factor of two. This is due to the useof two emitters per pot for consistencybetween treatments rather than a singleemitter per pot, more typical of a com-

mercial operation. For the same period,leachate rates were 3.17 gal/day (esti-mated) and 0.08 gal/day for themanually controlled and computer-controlled treatments, respectively.

Plant growth under the two irri-

gation treatments was similar, i.e., therewere no statistical differences in seed-ling height, number ofleaves, total leaf 

area, total plant dry weight, or shoot :root ratio between plants grown be-tween treatments (Table 1).

System reliability. The com-puter-controlled irrigation systemcontinuously operated for 15 weeks(2500 h). During this time only twoirrigation cycles were missed. In bothinstances the interruption of the irri-

gation was due to a worker in thecontainer area shutting the valve to themain irrigation header.

A serious system reliability con-sideration is that the salt buildup onthe moisture sensors could possiblyresult in false wet readings and prema-ture irrigation shutdown. A decreasein sensor resistance values was ob-served during the test period but didnot appear to affect irrigation rates.Under nursery operating conditions,the irrigation rates and sensor resis-tances could be monitored on occa-sion. If resistance readings of “dry”sensors decrease below the trip value,the operator could counteract thisproblem simply by changing this pa-rameter to a lower value. Note that thetrip value used in this study was 5kohm and that a completely wet sensorwill have a resistance of <l kohm. Inaddition, the moisture sensors can becleaned using an abrasive sponge and/ or a mild acetic acid solution to rees-

tablish the maximum dry resistance

Fig. 3. Generalized software scheme used in the computer-controlled irrigation system.

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values. If this type of sensor is used inthe field, a yearly cleaning program atthe end of the growing season is ad-vised.

System modifications. The use oromission of rain shields will not affectthe computer-controlled irrigationsystem. The plastic rain shields used inthis study were labor-intensive to in-stall and, in a commercial operation,

would need to be replaced yearly.Growers primarily concerned with ir-rigation control could omit thesecovers. Rainfall would simply be anadditional environmental parameteraffecting the time required by thecomputer-controlled system to reachfield capacity in the indicator pots.

An additional commercial use of this system would be for the reductionof leachate and/or ferti lizer applica-tion; in that the control of both theseparameters would be affected signifi-cantly by rainfall, the evaluation of 

rigid container covers would be in-cluded in future tests.

Another system modification thatwould not affect the overall controlscheme is the number of emitters usedper pot. In this study two emitterswere placed in each pot to reduce thepossibility of channeling if the mediumbecame very dry. Observations duringplant harvests showed our concernsfor adequate media wetting to be un-founded. These observations indicate

406 

that two emitters per pot are not re-quired and we recommend singleemitters for similar growing media.

Although irrigation and leachaterates were reduced, leachate did occur.This is not necessarily undesirable inthat leaching does provide a means of preventing salt buildup. As a result of the lag time inherent in the moisturedetection method, the overall

le h te

rates were somewhat higher than ex-pected. Further leachate reduction maybe achieved by first irrigating indicatorplants and then irrigating crop plantsat a fraction of the rate applied to theindicator plants. This technique couldbe used to reduce both irrigation andle h te

  rates, although the relation-

ship between plant growth and re-duced irrigation rates beyond that de-termined by the indicator plants isunknown. Furthermore, the relation-ship between salt buildup and plant

growth would have to be addressed.A second approach to further re-

ducele h te

  is increasing the irriga-

tion cycle time. During our test thecontrol system checked the sensorsevery 4 h and irrigated as needed.Under these conditions the mediumin the containers was maintained at afairly high moisture level. The cycletime could probably be increased to 12or 24 h without changing the effec-tiveness of the computer-controlledirrigation system. However, with a pinebark-based medium, the possibility of channeling increases as the mediumbecomes dryer.

 Research application. The con-trol system currently is being evaluatedfor use in studies ofdrought adaptationand plant response to water stress.Although several drought impositiontechniques are available for small plants

(Krizek, 1985), drought studies of larger plants, especially in an opennursery environment, have been lim-ited primarily to periods of applyingand withholding irrigation. These

methods tend to impose water stressrapidly in container-grown plants. Theirrigation system described in this pa-per might be used to estimate theevapotranspiration rates of well-wa-tered plants on a day to day basis and

impose stress treatments based on thisest imate . Using this technique,drought stress could be applied slowly

and thus more closely reflect droughtstress development under field condi-

tions, a valid concern in plant-waterrelation studies (Kramer, 1988).

Overall, the relative simplicity of this control system makes it a reliableoption for many container nurseries.Independent irrigation blocks couldbe set up with randomly located indi-cator plants for each species beinggrown or for species grouped accord-ing to water use. The reduction inirrigation and

le h te

  rates when us-ing the computer-controlled irrigation

Table 1. End-of-season harvest comparison between 1 -year-old red oak seedlings grown under a computer-controlled irrigation system and manually controlled irrigation.

 z

 Mean and standard deviation (n = 10) means in rows fol lowed by same symbol are not significantly

different from each other by Student-Newman-Keuls test (a = 0.05).

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system relative to a manually controlledsystem could result in cost savings in acommercial operation. These costsavings would be seen in reducedpumping, water treatment, and fertil-izer costs.

Literature Cited

Costello, L. R., N.P. Matheny, and J. R. Clark.1991. Estimating water requirements of landscape plantings. The landscape coeffi-

cient method. Coop. Ext. Serv., Univ. of California. Lflt. 21493.

Fare, D. C., C. H. Gilliam, and G. J. Keever.

1992. Monitoring irrigation at container

nurseries. HortTechnology 2(1):75-78.

Kramer, P.J. 1988. Changing conceptsregarding plant water relations. Plant Cell

and Environ. 11(7):565-568.

Krizek, D. T. 1985. Methods of inducing

water stress in plants. HortScience20:1028-1038.

 Lieth, J.H. and D. W. Burger. 1989. Growth

of chrysanthemum using an irrigation sys-tem controlled by soil moisture tension. J.Amer. Soc. Hort. Sci. 114:387-392.

 Martin, C.A., H.G. Ponder, and C.H.Gilliam. 1989. Effect of irrigation rate and

media on growth of Acer rubrum L. in largecontainers. J. Environ. Hort. 7(1):38-40.

 Michelakis, N.G. and K.S. Chartzoulakis.1988. Water consumptive use of green-

house tomatoes as related to various levelsof soil water potential under drip irriga-tion. Acta Hort. 228:127-136.

Smajstrla, A.G. and R.C. Koo. 1986. Use

of tensiometers for scheduling of citrustrickle irrigation. Proc. Florida State Hort.Soc. 99:51-56.

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