Agricultural

Water Management

for Coastal Plain Soils

Agricultural Water Managementfor Coastal Plain Soils

Contents

3 How Water Stress Impacts Crop Production4 Traditional Ways To Reduce Crop Water Stress6 Total Water Management Approach

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A Dual System Reduces Both StressesHow Subirrigation WorksHow Water Management Impacts Water QualitySite Evaluation Is CriticalSite Requirements for Drainage/SubirrigationGood Water Management Increases YieldsSeeing Is BelievingChoose Your System Carefully

11 Advantages of Dual Drainage/Subirrigation Systems11 Disadvantages of Dual Drainage/Subirrigation SystemsSeek Professional Advice

Prepared byRobert Evans, Extension SpecialistandWayne Skaggs, William Neal Reynolds ProfessorDepartment of Biological and Agricultural Engineering

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AGRICULTURAL WATER

MANAGEMENT

FOR COASTAL PLAIN SOILS

IntroductionNorth Carolina has historically been blessed withan adequate source of good quality water to supplyall of its water demands for industry, agriculture anddomestic and recreational users. However, if con-sumption continues to increase at its present rate,water shortages will become prevalent in the nearfuture. Water use in some areas is already ap-proaching critical limits, and shortages in other areasare likely to follow.The best solution to this increasing demand in-volves developing management strategies to conserve

and use the existing water resources more efficiently.These management strategies can also have apronounced effect on water quality. Thus, allmanagement decisions should consider strategiesthat maintain or improve water quality. However,the best management strategies are often geo-graphically sensitive: what might work in one regionmight not work in another. Each managementstrategy, therefore, should be selected based onminimizing the specific local problems.

How Water Stress Impacts Crop Production

Soil water, either too much and/or too little, isrecognized as the single most limiting factor for cropproduction in both North Carolina and much of thesoutheastern United States. This is especially true inthe Coastal Plain and Tidewater regions where bothexcessive and deficient soil-water conditions fre-quently are found in the same field.These soil-water conditions cause problems forcrops grown on about 2.3 million acres of poorlydrained soils in North Carolina, soils that make upabout 40 percent of the state’s total cropland. Yieldreductions of more than 50 percent may develop fromstress caused by excessive soil-water conditions onthese poorly drained soils. This may result either (1)from the inability to plant and tend the crop at theright time due to poor trafficability or (2) from directdamage to the crop when water stands too long in the

field. Excess water may also cause nutrient deficien-cies due to eventual leaching or denitrification.On the average, the Coastal Plain of NorthCarolina receives more than 48 inches of rainfall eachyear. Since only 36 inches of water are needed tosatisfy potential evapotranspiration, this means thatthere are more than 12 inches of excess rainfall an-nually. Yet, droughts ranging from a few days up toseveral weeks occur in most years during June, Julyand August. Because this is when many crops aremost susceptible to deficit soil water, yield reductionstake place often. In fact, droughts have producedalmost total crop failures in some years even onpoorly drained soils. Drought reduces average cornyields by 20 to 30 percent on these traditionally “wet”soils.

Traditional Ways To ReduceCrop Water Stress

Several techniques are available which improvedrainage and reduce excess water-related stress.Traditionally in a surface drainage system, openditches are installed on 300- to 600-f00t intervals, andthe field is crowned and graded to eliminate potholes(Figure 1). Drop inlet pipes and grassed field bordersare needed to minimize ditch-bank erosion whichdevelops when surface water is allowed to enter theditch uncontrolled. Normally, bedded rows are usedto elevate the seedbed and help reduce water stresswhile the plants are small.Surface drainage systems require the least initialinvestment and are often more effective in “tight”clay-textured subsoils; however, maintenance is highand ditch space is nonproductive. Also, high runoffrates characteristic of surface drainage systems con-tribute greatly to soil and nutrient losses from thefield.

Subsurface drainage systems using corrugatedplastic tubing may also be used to remove excesswater (Figure 2). In general, 4- to 6-inch diametertubing is buried 3 to 5 feet deep at intervals of 50 to200 feet. These systems are designed to remove ap-proximately one-half inch of water per day (Figure3). Compared to an open ditch surface drainagesystem, the subsurface drainage system is initiallymore expensive and results in greater nitrate trans-port. But this system’s benefits include lower main-tenance, less interference with tillage operations and.lower peak outflow rates which may be moredesirable environmentally.Many types of above-ground irrigation systems areavailable to help minimize drought-related stress.These systems range from labor-intensive hand movealuminum pipe systems to self-propelled mechanicalmove systems capable of irrigating 200 acres in asingle pass.

Figure 1. Schematic of a typical surface drainage system for eastern North Carolina.

" Figure 2. Schematic of a typical sub-surface drainage system for easternNorth Carolina.

Figures 3(A) and (B). (A) Outlet pipe for a typical subsurface drainage system. (B) Thesesystems were designed to remove approximately 1/2 inch of water per day.

Total Water Management Approach

A Dual System ReducesBoth StressesOn poorly drained soils such as many soils found in

the Coastal Plain, it may be possible to satisfy bothdrainage and irrigation demands with one system.Where plastic drainage tubing is installed to provideartificial subsurface drainage, control techniquesmay also enable water to travel back through the un—derground tubing to satisfy irrigation needs. Thispractice is called subirrigation. In contrast, whendrainage outlets are managed to conserve drainagewater but no additional water is pumped in, the prac-tice is called controlled drainage.

How Subirrigation WorksThe process might best be described by seeing whathappens when water is added to a flower pot through

a shallow pan at its base rather than added from thetop of the pot. As Figure 4 shows, when water isadded through the bottom, the water rises in the potthrough the small passages between soil particles.This is referred to as capillary rise or capillarywater. This capillary water becomes available for theplant’s use once it rises to the plant’s root zone. The

Figure 4. Water rises into the root zone using sub-irrigation in ways similar to water rising in aflower pot when water is added to the bottomof the pot.

height of rise is controlled by the water level in theshallow pan and also the size of the passages be—tween soil particles. Therefore, the height of theshallow pan should be chosen to maintain just thedesired distance of the water table from the rootzone. Then, if too much water is added to the pot

Figure 5 (A) and (B). (A) Flashboard riser control struc-ture before installation and (B) after installation con-trolling the drainage outflow.

Figure 6. Schematic of a controlled drainage system. Drainage stops when the water tabledrops to the same level as the top of the control structure (weir). The water table can-tinues to recede, however, due to evapotranspiration.

through either the top or bottom, it will overrun thesides of the shallow pan and, consequently, reducethe risk of overwatering the plant. This action wouldrepresent the subsurface drainage process.In the field a structure, such as a flashboard riser,is placed in the drainage ditch or tile outlet to controlthe rate of subsurface drainage (Figure 5). The con-trol structure functions in the same manner as theshallow pan discussed earlier. With this type ofsystem, drainage continues as long as the water tableremains in the root zone. 0n the other hand, once thewater table in the field drops below the root zone,

which is normally 12 to 18 inches below the soil sur-face, drainage stops (Figure 6). The water table willcontinue to recede as the crop removes water byevapotranspiration. Without rainfall or irrigation toreplenish this water, the water table will eventuallydrop too low to supply sufficient water to the crop.

Controlled drainage may save enough water in thesoil profile to delay drought stress for a few days.But, in most years, this method will not be sufficientto eliminate all drought stress for the entire growingseason.

Figure 7. Water being pumpedinto a drainage outlet toprovide subirrigation.

In order to provide an irrigation function, it will benecessary to add additional water to the system(Figure 7). Rather than adding this water through anoverhead irrigation system, the needed water couldbe pumped into the ditch containing the controlstructure. This water would move into the fieldthrough the underground tubing and maintain awater level as determined by the position of the con—trol structure (Figure 8). (The actual height of thewater table needed to eliminate drought stress is afunction of the soil and crop but normally will be be-tween 18 to 36 inches from the surface.) The waterwill then move upward from the water table to theroot zone due to capillary rise. This process is knownas subirrigation.

How Water ManagementImpacts Water QualityConserving agricultural runoff water offers anadded benefit: improved off-site water quality. Thethree major agricultural pollutants in drainage out-flow in North Carolina are pesticides, sediments andnutrients—primarily phosphorus and nitrogen.

Coastal counties (such as Hyde and Pamlico) also ex—perience water-quality problems when fresh water isdischarged into estuarine areas which are primarybreeding and nursery areas for shellfish. Salt waterdamage to crops may result when high wind tidesforce brackish water from sounds back throughdrainage ditches and onto fields.Several studies have shown that drainage watercharacteristics can be influenced by the type ofdrainage system used. For example, a subsurfacedrainage system will reduce peak flowrates by morethan 50 percent compared to a surface system. Sub-surface drainage also reduces movement of sediment,phosphorus and pesticides. However, subsurfacedrainage increases nitrate levels in the drainagewater flowing from the site. In these cases it is Vitalto manage the specific system. For example, control-

ling the drainage rate on a subsurface system hasbeen shown to reduce the nitrates leaving the site bynearly 50 percent compared to conventional subsur—face drainage.

Site Evaluation Is CriticalSubirrigation and controlled drainage practices arenot suitable for all sites due to excessive slope, seep-age or low permeability. An adequate water supplycould also limit subirrigation. Estimates show thatover 1 million acres of cropland in North Carolinacould incorporate a subirrigation system. On siteswhere subirrigation can be used, it is probably thebest method now available to reduce water-relatedcrop stress. On some sites, however, subirrigationmay not be practical, and other alternatives fordrainage and/or irrigation should be applied.

Site Requirements forDrainage/SubirrigationConsider several general site requirements whenevaluating the feasibility of a subirrigation system. Adrainage/subirrigation system will likely be the mosteffective and economical alternative on sites whichsatisfy these general requirements:1. The site needs improved drainage (at least inits natural state).2. The site is relatively flat with a surface slope ofless than 1 percent.3. The site has moderate-to-high hydraulicconductivity: Ks > .75 inch/hour.4. The site has a high natural water table orshallow impermeable layer within 10 to 25 feetof surface.5. The site has a good drainage outlet at least 3feet lower than the average land surface.6. An adequate water supply is available.

Figure 8. Schematic of a subirrigation system. Water is pumped into the outlet ditchthen moves through the underground tubing due to gravity flow. Water movesfrom the water table into the root zone by capillary flow.

Field sites should be evaluated before setting upany water management system. One of the bestmethods available to evaluate the potential benefitsof subirrigation or other water management alter-natives is the water management model, DRAIN-MOD, developed at North Carolina State University.This model allows the user to simulate the crop’sresponse (yield) to a water management practice,such as surface or subsurface drainage, controlled

drainage or subirrigation. The actual design and siz-ing of a specific system can also be evaluated. Themodel evaluation considers such factors as crop, siteand soil characteristics, and weather conditions, allof which are important when selecting and sizing aspecific system. The model user can then recommendthe best water management alternative based on op-timizing long-term simulated yields.

Good Water ManagementIncreases YieldsThe potential benefits of any water management

alternative definitely depend on the severity of thewater—related problem. On poorly drained soils inNorth Carolina, improved drainage will increaselong-term average yields by 30 to 50 percent com-pared to yields without improved drainage (Figure 9).More specifically, one study on Rains sandy loam pre—dicted average corn yields of just over 80 bu/a wherea conventional 330-foot open ditch system was used.

For good subsurface drainage with drain tubesspaced 80 feet apart, the predicted yield was 135bu/a.Irrigation may increase long-term yields an ad-ditional 15 to 30 percent on the same soils (Figure 9).Considering the same Rains sandy loam, when thedrains were moved closer together (50 feet apart) toprovide both drainage and subirrigation, the pre-dicted yield was over 165 bu/a.Good water management not only increasesaverage yields, it also reduces the year-to-year varia-tion in yield and the risk associated with growing thecrop. Finally, undesirable off-site effects can also bereduced by selecting the appropriate alternative.

INCREASINGSYSTEMCOST

25 50RELATIVE YIELD , percent

75 IOO

Figure .9. Long-term relative yield in response to different water management options on apoorly drained soil in eastern North Carolina.10

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Seeing Is BelievingSeveral sites have been established to demonstrate

the potential benefit of total water managementsystems. A fabridam structure was installed onMitchell Creek in Edgecombe County to control thedrainage rate during the growing season. Fields adja-cent to the creek have shown a direct response to thestructure. For example, corn yields over a three-yearperiod increased an average of 28 bu/a per year inresponse to the controlled drainage. The structurealso conserved over 90 million gallons of water whichwas used in sprinkler irrigation systems in theneighboring area. One farmer in Scotland County re-ported an increased corn yield of 40 bu/a per yearover a two-year period in response to subirrigation ona 40-acre field.Seven subirrigation demonstration sites were es-tablished recently in Camden, Hyde and Pamlicocounties with federal cost/share assistance providedthrough the Resource Conservation Act. Tours ofthese sites will be conducted routinely todemonstrate the performance and general operationand management of these systems.

Choose Your SystemCarefullyAs with any water management system, find out

which advantages and disadvantages should be con-sidered as part of the evaluation process. Comparedto conventional drainage and irrigation systems, adual drainage/subirrigation system offers the fol-lowing advantages and disadvantages:Advantages of Dual Drainage/Subirrigation Systems1. Satisfies both drainage and irrigation

needs at lower initial costs;2. Needs less energy, labor and maintenance;3. Lowers operational costs due mainly to

reduced energy consumption;

4. Conserves rainfall better depending onmanagement strategy;

5. Reduces evaporation during water addi-tion;6. Provides less obstructions in the field sooperators can continue their cultural prac—tices with no interference;

7. Offers more flexibility in managingdrainage water;8. Improves quality of drainage water.

Disadvantages of Dual Drainage/Subirrigation Systems1. Lacks potential for frost/freeze protection,

crop cooling, chemigation or fertigation;2. Causes underground seepage to sur-

rounding areas;3. Offers poor accessibility for repairs and

isolating malfunctions since this dualsystem is immobile;4. May result in a slower crop response if thesoil is very dry.

Seek Professional AdviceRoutine management ofany water management

practice is vital for the system to perform efficientlyand effectively. This is especially true with adrainage/subirrigation system. If you are looking foroptions that will give you more control over waterutilization on your farm, consider this type ofdrainage/subirrigation system.But before getting started, first seek trained help.Your county Agricultural Extension Service and SoilConservation Service are available to help evaluatethe potential benefits of any water managementalternative for your farm. Their staff has beentrained to measure the field properties—such ashydraulic conductivity—necessary to evaluate yoursite. Drainage/subirrigation equipment suppliersand contractors can also help.

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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.9/85/3M AG-355

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