Irrigation Water Management MANAGEMENT 4.6. Irrigation Water Management Water management is an important element of irrigated crop production. Efficient irrigation systems and water
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4.6. Irrigation Water Management
Water management is an important element of irrigatedcrop production. Efficient irrigation systems and watermanagement practices can help maintain farm profitabilityin an era of limited, higher-cost water supplies. Efficientwater management may also reduce the impact of irrigatedproduction on offsite water quantity and quality. However,measures to increase water-use efficiency may not besufficient to achieve environmental goals in the absence ofother adjustments within the irrigated sector. As is oftenthe case, technology is not the whole solution anywhere,but part of the solution almost everywhere.
Why Manage Irrigation Water?. . . . . . . . . . . . . . . . . . . . . 225
Use of Improved Irrigation Technology and Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
Irrigation Technology and Environmental Benefits . . . . 233
Factors Affecting Technology Adoption . . . . . . . . . . . . . . 236
Policies and Programs Promoting Improved Irrigation Water Management. . . . . . . . . . . . . . . . . . . . . . 237
The U.S. Department of Agriculture identifiesimprovements in water management as one of theprimary agricultural policy objectives for the 1990s(USDA, 1994). Irrigation water management (IWM)involves the managed allocation of water and relatedinputs in irrigated crop production, such thateconomic returns are enhanced relative to availablewater. Conservation and allocation of limited watersupplies is central to irrigation management decisions,whether at the field, farm, irrigation-district, orriver-basin level.
Why Manage Irrigation Water?
Irrigation water is managed to conserve watersupplies, to reduce water-quality impacts, and toimprove producer net returns.
Water Conservation. Water savings throughimproved management of irrigation supplies areconsidered essential to meeting future water needs.Irrigation is the most significant use of water,
accounting for over 95 percent of freshwaterwithdrawals consumed in several Western States androughly 80 percent nationwide (see chapter 2.1, WaterUse and Pricing). However, expanding waterdemands for municipal, industrial, recreational, andenvironmental purposes increasingly compete foravailable water supplies. Since opportunities forlarge-scale water-supply development are limited,additional water demands must be met largelythrough conservation and reallocation of existingirrigation supplies (Moore, 1991; Schaible and others,1991; Vaux, 1986; Howe, 1985).
Water Quality. Improved water management can alsohelp minimize offsite water-quality impacts ofirrigated production. Irrigated agriculture affectswater quality in several ways, including higherchemical-use rates associated with irrigated cropproduction, increased field salinity and erosion due toapplied water, accelerated pollutant transport withdrainage flows, degradation due to increased deep
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percolation to saline formations, and greater instreampollutant concentrations due to reduced flows.Strategies to improve the Nations water quality mustaddress the effect of irrigation on surface and groundwater bodies (National Research Council, 1996).
Farm Returns. Finally, improvements in IWM canhelp maintain the long-term viability of the irrigatedagricultural sector. Irrigated cropland is important tothe U.S. farm economy, accounting for about 40percent of total crop sales with just 15 percent of theNations harvested cropland in 1992 (USDC, 1994).Water savings at the farm level can help offset theeffect of rising water costs and restricted watersupplies on producer income. Improved watermanagement may also reduce expenditures for energy,chemicals, and labor inputs, while enhancing revenuesthrough higher crop yields and improved crop quality.
Use of Improved Irrigation Technology andManagement
How producers respond to higher water costs andlimited water supplies is important to policymakers.Producers may reduce water use per acre by applyingless than full crop-consumptive requirements (deficitirrigation), shifting to alternative crops or varieties ofthe same crop that use less water, or adopting moreefficient irrigation technologies. In some cases,producers may convert from irrigated to drylandfarming or retire land from production. Manyirrigators have responded to water scarcity throughthe use of improved irrigation technologiesoften incombination with other water-conservingstrategiesand irrigators will likely look totechnology as one of several means of conservingwater in the future.
Various management practices and irrigationtechnologies are available to enhance efficiency ofapplied water in irrigated agriculture (see box,"Irrigation Water-Use Efficiency"). Irrigationimprovements often involve upgrades in physicalapplication systems, with improved field applicationefficiencies and higher yield potentials. Improvedwater management practices, such as irrigationscheduling and water-flow measurement, may also berequired to achieve maximum potentials of thephysical system. In addition, management ofdrainage flows may be an important concern in manyirrigated areas (table 4.6.1). In some cases, theeffectiveness of improved irrigation practices may beenhanced when implemented in combination withother farming practices such as conservation tillageand nutrient management.
Irrigation Application Systems
Irrigation application systems may be grouped undertwo broad system types: gravity flow and pressurizedsystems. (For an explanation of irrigation systemsdiscussed here, see boxes, "Gravity (Pressurized)Irrigation Systems and Practices," pp. 229-230.)
Gravity-Flow Systems. Many irrigation systems relyon gravity to distribute water across the field. Landtreatmentssuch as soil borders and furrowsareused to control lateral water movement and channelwater flow down the field. Water is conveyed to thefield by means of open ditches, above-ground pipe(including gated pipe), or underground pipe, andreleased along the upper end of the field throughsiphon tubes, ditch gates, or pipe valves. Fields are
Irrigation Water-Use Efficiency
Water-use efficiency measures are commonly used tocharacterize the water-conserving potential ofirrigation systems. Alternative efficiency measuresreflect various stages of water use and levels ofspatial aggregation. Irrigation efficiency, broadlydefined at the field level, is the ratio of the averagedepth of irrigation water beneficially used(consumptive use plus leaching requirement) to theaverage depth applied, expressed as a percentage.Application efficiency is the ratio of the averagedepth of irrigation water stored in the root zone forcrop consumptive use to the average depth applied,expressed as a percentage. Crop-water consumptionincludes stored water used by the plant fortranspiration and tissue building, plus incidentalevaporation from plant and field surfaces. Leachingrequirement, which accounts for the major differencebetween irrigation efficiency and applicationefficiency, is the quantity of water required to flushsoil salts below the plant root zone. Field-levellosses include surface runoff at the end of the field,deep percolation below the crop-root zone (not usedfor leaching), and excess evaporation from soil andwater surfaces. Conveyance efficiency is the ratio oftotal water delivered to the total water diverted orpumped into an open channel or pipeline, expressedas a percentage. Conveyance efficiency may becomputed at the farm, project, or basin level.Conveyance losses include evaporation, ditchseepage, operational spills, and water lost to noncropvegetative consumption. Project efficiency iscalculated based on onfarm irrigation efficiency andboth on- and off-farm conveyance efficiency, and isadjusted for drainage reuse within the service area.Project efficiency may not consider all runoff anddeep percolation a loss since some of the water maybe available for reuse within the project.
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generally rectangular with water runs typicallyranging from one-eighth to one-half mile in length.Gravity systems are best suited to medium- andfine-textured soils with higher moisture-holdingcapacities; field slope should be minimal and fairlyuniform to permit controlled water advance.
Although total acreage in gravity systems hasdeclined by 20 percent since 1979, gravity-flowsystems still account for over half of irrigated acreage
nationwide (table 4.6.2). Gravity-flow systems areused in all irrigated areas, and are particularlypredominant in the Southwest (California, Nevada,Arizona, New Mexico), Central Rockies (Wyoming,Colorado, Utah), Southern Plains (Texas, Oklahoma),and Delta (Arkansas, Louisiana, Mississippi) regions.The predominance of gravity systems in arid regionsof the West reflects early project development onbroad, flat alluvial plains; high crop water-consumption requirements; and increased soil salt-
Table 4.6.1Irrigation technology and water management: conventional methods and improved practices
System and aspect Conventional technology or management practice
Improved technology or management practice
Onfarm conveyance Open earthen ditches. Concrete or other ditch linings; above-groundpipe; below-ground pipe.
Gravity application systems:Release of water Dirt or canvass checks with siphon tubes. Ditch portals or gates; gated pipe; gated pipe
with surge flow or cablegation.Field runoff Water allowed to move off field. Applications controlled to avoid runoff;
tailwater return systems.Furrow management Full furrow wetting; furrow bottoms
uneven.Alternate furrow wetting; furrow bottomssmooth and consistent.
Field gradient Natural