Operating

Controlled Drainage

and Subirrigation Systems

Prepared byRobert Evans, Extension SpecialistandWayne SkaggsWilliam Neal Reynolds Professor

Department of Biological and Agricultural Engineering

Contents4 Design Considerations6 System Operation

7 When to Raise/Lower the Water Level Control Structure8 Level of the Water Table Control Structure8 Adding Water to the System

10 Monitor the System

Coastal Plain Soils

OPERATING CONTROLLED DRAINAGEAND SUBIRRIGATION SYSTEMS

IntroductionDual-purpose drainage/subirrigation systems

are now being used on poorly drained soils to reducewater-related stress on crops. The mechanics of thisdual-purpose system is discussed in AgriculturalWater Management for Coastal Plain Soils, AG-355. During wet periods this system operates as adrainage system (Figure 1). Here, excess water isremoved from the field through a system of under-ground drain tubes which outlet to a main draintube or open ditch. When a structure (such as aflashboard riser) is used in the outlet ditch to regu-late the drainage rate, the system may function ineither the controlled drainage or subirrigationmode. Usually a weir is placed in the control struc-ture so that the water level in the drainage outlethas to rise higher than the weir crest before the

water will flow out of the field.The process called controlled drainage occurswhen the structure is used to conserve water byreducing drainage outflows and when no additionalwater is pumped in. During dry periods, water maybe pumped into the control outlet where it movesback through the drainage network, thus raisingthe water level in the field. In this mode the systemis being used for subirrigation.The dual-purpose system will normally fluctuatebetween the drainage, controlled drainage and sub-irrigation modes several times duringone croppingseason. Because the role of the system oftenchanges, intensive monitoring and management ofthe system is necessary for effective operation.

Design Considerations

The best crop response for a drainage/subirriga-tion system will be realized with a system that isspecifically designed to serve both purposes. Sys-tems whose designs focus on drainage are, in gen-eral, not adequate to function in both modes. Thephysical limitations of the system can, however, beovercome somewhat. One way would be to increasesystem management. Another way to overcomesome limitations would be to increase the drainageintensity of the system by installing additional tilelines between existing lines. This additional drain—age capacity is needed when the water table rises, acondition which decreases the drainage capacity ofany subsurface system. Land shaping to improvesurface drainage may also be needed, especially onan existing system with limited drainage capacity.If a new system is to be installed for both drainageand subirrigation, the system’s size and layout mustsatisfy the water management needs of the specificsite. Many factors influence this size and layout. For

example, crop rooting depth and tolerance to waterstress are important considerations. Several soilproperties such as water-holding capacity, hydrau-lic conductivity, and location of layers within theprofile should also be measured for each site. Fur-thermore, rainfall distribution within the geogra-phic region under consideration will influence thedesign and operation of the system. Finally, farmoperators should carefully evaluate the time avail-able to “look after” the system. To reduce the initialcost, design the system for an intensive level ofmanagement. However, intensive management maynot be realistic since the subirrigation system willoften compete for time with other vital farm opera—tions.The final design of the drainage/subirrigationsystem should be evaluated using the water man-agement simulation model, DRAINMOD. For ini-tial planning purposes, subirrigation/drainage re—quires about 65 percent of the spacing normally

needed for drainage alone. Drain tubing depthshould be about 3 to 4 feet which is somewhat shal-lower than the depth for conventional drainage.Subirrigation offers little advantage to placing thetubing deeper unless the lower layer is more per-

System Operation

meable than the overlying layers. The control struc-ture should be sized and managed so that the ditchor waterway can carry its full capacity during highflow periods.

In eastern North Carolina, removing excesswater from the field is the system’s most importantrole. Thus, in areas where droughts are temporary,the system functions mainly as a drainage system.During the traditionally wet winter and earlyspring period, the water-level control structureshould be set below the level of the tile outlet toprovide maximum drainage capacity. This waterlevel should be maintained until after the crop hasbeen planted and most of the trafficable operationshave been completed. To maintain water quality inreceiving streams, it may be desirable to operate inthe controlled drainage mode during most of thewinter in order to reduce nitrate outflows on somesites.Once the system is operational, the most impor-tant management decisions include:

1. when to raise/lower the control structure;

2. at what height to maintain the weir in the con-trol structure; and

3. when to add water to the system.These decisions not only may differ for every site,they may also vary with the crop. Depending on soiltype, root depth, drain depth and control structuresetting, from 1 to 4 inches of plant available watercan be held in the profile that would otherwise drain(Figure 2). (Plant available water is any waterretained in the soil that plants can use.) This amountrepresents from 3 days‘ to 2 weeks of water storagethat would not need to be supplied by pumping.How important this water is to the crop depends onthe capabilities and limitations of the system. Thefollowing situations based on system capabilitiesshould be considered as a guide for the initialdecisions.

Figure 2. Water retained in the soil profile as a result ofmanaging the drainage outlet.

Figure 3. Root development in response to good and poor water table management.

When To Raise/Lowerthe Water-LevelControl StructureControlled drainage—Conserving water by con-trolled drainage is most critical on systems wheresupplemental water is not available. If dry condi-tions are anticipated, it would be desirable to raisethe weir soon after planting to conserve as muchwater as possible. On the other hand, the systemshould be operated so that prolonged saturation ofthe root zone does not occur, especially when thecrop is young. In North Carolina, late April and

early May are sometimes “rainy periods.” There-fore, on systems which do not have adequate drain-age capacity, water table management too early inthe season may result in excess water stress whenthe crop is young.Raisingthe water table too soon or controllingthewater table at a shallow depth below the soil surfacewill also discourage deep root growth, an effectwhich could make the crop more susceptible todrought later (Figure 3). Holding the water tabletoo high will also encourage denitrification whichcould result in a nitrogen deficiency later in theseason. Thus, the long-term average productionbenefit of controlled drainage greatly depends both

on the system’s drainage capacity in addition to theseverity of the drought stress.Consider, for example, a conventional, predomi-nately surface drainage system with inadequatedrainage capacity during wet periods. In a very dryseason, little benefit would be realized from con-trolled drainage (as compared to drainage alone) ona crop such as corn since very few soils can storeenough water to grow a crop without rainfall for 6 to8 weeks. However, acontrolled drainage system canstore enough water to reduce drought stress forshort periods. For this reason, the greatest benefitwill be realized in years which have frequent smallrains (often less than 1 inch) with only shortdroughts. But in a very wet year, yields will proba-bly be reduced if the water table is held too highbecause of inadequate drainage. The long-termaverage benefit of this type of system (as comparedto conventional drainage alone) would be minimal.Operating the system in the controlled drainagemode can provide considerable yield responses ifthe system is designed for subirrigation or con-trolled drainage. This situation occurs when thesystem has adequate drainage for water table man-agement, but the supplemental water supply hasnot been developed. As before, a very dry yearwould offer little or no benefits. But in years withfrequent rains and short droughts, the potentialyields could be 10 to 20 percent higher than for

drainage alone. This situation will occur about 1year in 4. In a very wet year, few or no benefitswould be realized from saving water, but sincedrainage is adequate, there would be no harm incontrolling the water table elevation as in the ear-lier example. Over the long run, this type of systemwould produce average yield increases of 2 to 5 per-cent above yields with a conventional drainage sys-tem for a crop such as corn.Subirrigation—Saving water early in the season

is not as important for high yields if the system isdesigned for drainage and subirrigation and if ade-quate supplemental water is available. Any watersaved and used later in a dry period will delay theneed for irrigation and reduce associated pumping

‘ costs. On the other hand, saving water might in-crease the risk of some early wet stress and discour-age root development.The risk associated with raising the structureeither too soon or too high on any system can bereduced by increasing the intensity of system man-agement. For example, in case of heavy rains andsystems which do not have adequate drainage,operators must be prepared to manually lower thestructure to increase the drainage rate. This drain-age control is critical when the crop is young andshould be done as soon as it is clear that too muchrain will fall. Controlling such drainagecould meanlowering the structure in the middle of the night.The structure should not be raised again until thewater level midway between drains or ditches hasdropped at least 12 inches below the soil surface.This level, however, depends on soil type and cropage as discussed earlier.On systems which are designed for controlled

drainage or subirrigation, the water control struc-ture does not need to be lowered after heavy rains 'provided the system is beingoperated and managedas it was designed. Lowering the level will increasethe drainage rate and may reduce slightly the riskof excess water stress if the crop is young; however,this effect is likely only in unusually wet seasons. Assoon as the rain stops and the water table in the fielddrops below the soil surface, the structure should bereset to the design height. Drain spacings can beincreased slightly (up to 10 to 15 percent) on a sys—tem if the operator wishes to devote more time tomanage the system.Local rainfall patterns will influence the timingand positioning of the control structure level. Froma conservation and utilization standpoint, the struc-ture should not be raised until the water table in thefield has dropped to the design level. Such timing

would reduce the potential damage due to earlyexcess water and still conserve water for a subse-quent dry period. Of course, this situation is neverknown in advance, and the longer you delay raisingthe structure, the greater the risk that rainfall willnot adequately fill the storage capacity of the pro-file. Therefore, the best approach is to compile pastrainfall records for your specific location and usethis information together with weather forecastinformation to decide when to begin raising thestructure.

Level of the Water TableControl StructureThe optimum water table control level will de-

pend on the crop, stage of growth and soil type. Thislevel will need to be higher for sandy soils orshallow-rooted crops. For corn, the water level atthe control structure needs to be between 18 and 30inches for most soils. At planting, begin with thestructure adjusted to the bottom of the drainageoutlet. This level will usually be between 3 to 5 feet.Until experience is gained and the crop response tothe system can be observed, raise the level of thestructure incrementally about 6 inches every 7 to 14days in order to reach the final desired elevationabout mid-May. In most years this practice willallow the profile to store its maximum potential atthe onset of the drought period and reduce the riskof early crop damage due to inadequate drainage.

Several years of system operation may be re-quired before learning the “final” and best setting.Keep accurate records of any apparent wet stress,dry stress and yields at several locations in the fieldto determine the best setting. Compare yields atpoints directly above the drains to yields midwaybetween drains. The greatest water stress should beobserved midway between drains.

Adding Water to the SystemThe main concern here is not to let the soil get “too

dry” before starting to irrigate. The crop respondsmore slowly to a subirrigation system than it does toa conventional overhead system. This is because asthe soil dries out, the hydraulic conductivity of thesoil decreases drastically and the volume of water

Figure 4. Cyclic method of water table control during snbiw'igation. Pumping raises the water table into thedrainage outlet, then the water table recedes due to evapotranspiration.

needed per unit rise of water table height will alsoincrease. In comparison, 2 to 3 days are normallyrequired for water in a saturated (wet) soil to movefrom the-drain to midway between drains. On theother hand, the water may require 2 to 3 weeks totravel this same distance once the soil dries out.Most subirrigation water travels laterally in a

zone 3 to 6 feet below the surface. In general, thewater table should not be allowed to drop below thiszone during any part of the growing season so thatwhen irrigation becomes necessary, only a few dayswill be needed to “start up.” In most poorly drainedsoils in eastern North Carolina, the water table willnot drop below 6 feetexcept during dry periods thatbegin in late May and continue into the summer.Two approaches can be used to manage the sub-irrigation system. First, the water level in the outletis maintained near a constant level, for example, 20inches deep. This level is allowed to fluctuate only 1to 2 inches and is easy to manage by using floatswitches to start and stop the irrigation pump.However, in this case, the crop relies on irrigationfor much of its water, so this approach could presenta problem. If the water table is held too high whenthis method is used, it would reduce the profilestorage space above the water table, storage space

that is at a minimum. Consequently, most rainfallthat occurs during the irrigation season drainsfrom the profile.With the second approach, the water table israised to the desired level, then the pump is shut off(Figure 4). The water table will then be allowed torecede due to evapotranspiration until it drops tosome lower allowable limit. At this point, pumpingstarts and the water table is again raised to theupper level where the process repeats itself. Withthis method, the water table is allowed to fluctuate12 to 18 inches depending on soil type. This ap-proach has greater potential to store and use therainfall that occurs when the water table is in thedeeper range.In using either method, the water table should bemaintained at the greatest depth that will stillsupply adequate water to the crop. This level willprovide the maximum storage potential for—andmore efficient use of—rainfall. To take advantage ofthis rainfall, delay pumping as long as possible,especially when the growing season begins. Itshould not be necessary to add water to a crop suchas corn before mid- to late May in most years, butdonl’t let the soil get too dry for reasons discussedear 1er.

Monitor the System

Observation wells should be established at sev-eral locations in the field to monitor the position ofthe water table. These wells should be located mid—way between the drain lines and at least at twodifferent locations in the field. Additional wellsshould be set up for each soil type being irrigated.Small diameter pipe 4- to 5-feet long can be used forthis purpose (Figure 5). Small holes should be boredin the sides of the pipe so water can move easily intoand out of the pipe. This pipe should extend above‘ the ground surface, and the soil should be crownedaround the sides and at the top to prevent surfaceWater from running in along the sides of the pipe. If4-inch diameter pipe is used, a typical toilet bowlfloat with a Wire or small rod will just fit inside thepipe and can be used to measure the water level.The water level should be measured dailythrough-out the first growing season that the system oper-ates. The wells can be monitored less frequentlyonce (1) experience has been gained, and (2) theresponse of the water table to rainfall and changes

Figure 5. Observation well used to monitor position of water table on a controlled drainage or subirrigationsystem.

in the control structure level have been observed. Atleast for the first season of operation, maintainrecords of rainfall, water table position, controlstructure level, pumping rate and crop perfor-mance. This information will tell how quickly thesystem responds and will help managethe system infuture years.As mentioned earlier, water table managementwill respond differently for various soils and crops.For this reason, it is impossible to suggest precisesettings and/or operational guidelines that willwork at every location. Until experience is gained inoperating the system, seek professional advice. TheNorth Carolina Agricultural Extension Serviceand Soil Conservation Service in each county areavailable to help with your water managementdecisions. The SCS can design the system to satisfyparticular water management needs, and they canprovide more specific recommendations based onsystem capacity and management intensity.

Published byTHE NORTH CAROLINA AGRICULTURAL EXTENSION SERVICE

North Carolina State University at Raleigh, North Carolina Agricultural and Technical State University at Greensboro, and theU. S. Department of Agriculture, Cooperating. State University Station, Raleigh, N. 0., Chester D. Black, Director. Distributed infurtherance of the Acts of Congress of May 8 and June 30, 1914. The North Carolina Agricultural Extension Service offers itsprograms to all eligible persons regardless of race, color, or national origin, and is an equal opportunity employer.11/85/2M/VH AG-356