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COOPERATIVE EXTENSIONCOOPERATIVE EXTENSION

Surface Irrigation Systems

EM4828

Surface irrigation is the introduction and distribu-tion of water in a field by the gravity flow ofwater over the soil surface. The soil acts as thegrowing medium in which water is stored and theconveyance medium over which water flows as itspreads and infiltrates. Common surface irrigationsystems used in Washington are rill or furrowirrigation, and to a lesser extent, border irrigation.Some lands, particularly pastures, are irrigated by

wild flooding, in which there is little control overthe amount of water applied or where it soaksinto the ground.

Factors Affecting Efficiency and Uniformity

To achieve high efficiency and uniformity of irriga-tion using unpressurized gravity water applicationmethods, all parts of an irrigated field shouldreceive water for near equal lengths of time, with aminimum of water lost to runoff or to deep percola-tion below the root zone. Since the soil is quite

porous, the flow rate of water is continually re-duced as it spreads. Thus, getting water uniformlyspread over a field requires careful management.There must be a balance between the size andshape of a field, slope, water flow rate, sod infiltra-tion characteristics, and surface roughness, such assoil clods or vegetation that retard water flow.

Soils with high infiltration rates (usually thesandy and coarse-textured soils) are not well-suited for surface irrigation. Either field sizesmust be kept small, or large flow rates are needed

to achieve even applications. Soils with moderateto low infiltration rates (loams and other fine-textured soils) are better for surface irrigation.

Fields should have a uniform grade in the direc-tion of water flow to obtain best results. A uniform

grade or level surface prepared through precisionland leveling helps to provide equal infiltrationopportunity times. Field slopes in excess of 3% arenot recommended, unless the field is planted to apermanent sod-forming crop or unless othermeans of erosion control are used. Very low flowrates are used in this situation.

The water flow rate into a field or furrow is an

important design factor. It must be carefullybalanced against the soil type and slope so thaterosion is minimized, and against the field slopeand length so that the water reaches the end in areasonable amount of time. Operating the systemat flow rates below or above the design flow ratescan lead to inefficient and nonuniform applica-tions. Once the water reaches the end of the field,growers must manage the inflow to reducetailwater runoff losses.

There should be some means of controlling the

direction of water flow. In furrow or rill irrigationthe furrows act as conveyance channels. The size,shape and spacing of the irrigated furrows affectthe soil volume wetted. When border irrigation orother flooding methods are used, dikes or bordersare constructed to contain the flow within thedesired field area. The State of Washington Irriga-tion Guide provides recommendations on surfaceirrigation system design and operation by soil type.

Management for High Efficiencyand Uniformity

Surface irrigation systems can be as efficient asmost other methods. This requires improving themanagement and control of water, knowing howmuch water is applied and scheduling applica-tions according to soil water levels and crop

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needs. Information on soil moisture monitoringand crop evapotranspiration from WashingtonsPublic Agricultural Weather Stations (PAWS) andWashington Irrigation Scheduling Expert (WISE)are available on the Scientific Irrigation Schedul-ing (SIS): web page http://sis.prosser.wsu.edu

Typical water application efficiencies for unim-proved surface irrigation systems range from45% to 60%. Using careful management, im-proved water control, and re-use of tailwaterrunoff, growers can boost efficiencies to 70% to85%. Differences in the amount of water whichinfiltrates between the top and bottom ends ofthe field are inevitable. Tailwater runoff is alsoinevitable if the bottom ends of fields are to beadequately irrigated. Using rill systems andimproved management, which is enhanced byimproved on-farm water delivery facilities,growers can reduce overwatering losses at thetop ends of fields and the runoff losses at thebottom.

Growers will obtain the best water applicationfrom the top end of a field to the bottom if thewater reaches the end of the field within one-quarter of the planned irrigation duration. Wherepossible, adjust inflow stream size to meet thatschedule for the field length. For instance, if youplan to irrigate with 48-hour sets, water should

reach the end of the field, or advance, within 12hours for best results.

Use the largest, nonerosive stream size possible toachieve the desired advance time. For most condi-tions, the maximum safe stream size in gallons perminute per rill is found by dividing the field slopein percent into 10 (10/S,%). As an example, a fieldwith a 2% slope should have a maximum streamsize per rill of 5 gallons per minute. If watercannot be made to advance to the end of the fieldwithin the desired time using the maximum

stream, then the field is probably too long for thatgiven combination of slope and soil type.

Cut the field in half by adding water distributionfacilities at mid-length (that is, gated pipe, burieddistribution pipeline with risers and valves).Tailwater from the upper half can be used tosupplement the irrigation of the lower half orcaught in a ditch for re-use elsewhere on the farm.

Furrow lengths up to 1/2 mile on relatively flatslopes (less than 0.5%) can be efficiently irrigatedusing large streams. As a practical limit, however,field lengths of 1/4 mile or less are recommendedfor most Washington conditions.

Surge flow surface irrigation, which has received

recent research attention, has provided significantincreases in the uniformity of water applicationfrom the top to the bottom end of a field. Usingsurge flow, researchers have been able to advancewater to the end of the field at the same rate aswith conventional furrow irrigation while usingonly 50% to 60% of the water. Longer fields can bemore efficiently irrigated with surge flow. Appli-cation efficiency of surge flow with tailwater re-use can be as high as 75% to 90%. Please see theWSU Drought Advisory EM4826,Surge FlowSurface Irrigationfor more information.

Once water reaches the end of the field, cut backthe stream size by 1/3 to 1/2 so only a small, butcontinuous stream of runoff water exits out theend of the rill. This will minimize runoff lossesduring the remainder of the irrigation. The cut-back in flow can be achieved by:

1) adjusting the siphon tube setting or using twosiphon tubes during the advance phase andremoving one after the water reaches the end of

the furrow;2) closing the gates part way with gated pipe; and3) closing the valves part way with buried pipe-lines having risers and adjustable valves.

Vegetation in furrows slows water movementalong furrows and rills. Weed control in thebottoms of the furrows is very important forachieving uniform water application and reducingdeep percolation losses. Cultivation and theformation of a loose soil surface with soil clodsimpede water flow and increase water intake. In a

water-short year, control weeds chemically andavoid cultivating if at all possible. Otherwise, usea furrow packer or smoother after the cultivator toreduce deep percolation losses. Also applyingPolyacrylamide (PAM) after soil cultivation willincrease lateral water penetration and decreasesoil erosion. PAM should also allow increasedfurrow flow rates that will improve uniformity ofirrigation.

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