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Chapter 2Irrigation Water RequirementsTable 2–11Part 623National Engineering HandbookSelected air properties for varying temperatures2–45and elevationsTable 2–12Aerodynamic resistancefor various wind speeds2–47and common configurations of weather stationsTable 2–13Adjustment factor br for the radiation method2–54Table 2–14Value of parameters used with the radiation method2–55Table 2–15Values of adjustment factor at for use in equation 2–502–58Table 2–16Values of adjustment factor bn for use in equation 2–512–59Table 2–17Values of adjustment factor bu for use in equation 2–512–60Table 2–18Daily percent of annual daytime hours2–61Table 2–19Pan coefficients for Class A evaporation pans2–63for different ground cover and levels of meanrelative humidity and 24-hour wind runTable 2–20Basal crop coefficient parameters for field and2–73vegetable crops for a grass reference cropTable 2–21Basal crop coefficients for alfalfa, clover, grass-2–77legumes, and pastures using a grass reference cropTable 2–22Basal crop coefficients for citrus grown2–78in predominantly dry areas with moderate windusing a grass reference cropTable 2–23Basal crop coefficient for full grown deciduous fruit2–79and nut trees using a grass reference cropTable 2–24Basal crop coefficients for sugarcane using a grass2–80reference cropTable 2–25Basal crop coefficients for grapes with clean cultivation, 2–80infrequent irrigation, and dry soil surface mostof the season using a grass reference cropTable 2–26Crop coefficients for paddy rice grown in the2–81United States mainland using a grass reference cropTable 2–27Monthly crop coefficients for some perennial cropsraised in Northwestern United States2–vi(210-vi-NEH, September 1993)2–81

Chapter 2Irrigation Water RequirementsTable 2–28Part 623National Engineering HandbookFraction of the soil surface wetted for various types2–85of irrigationTable 2–29Wet soil surface evaporation decay function f(t)2–86and the persistence factor Pf for typical soilsTable 2–30Average wet soil evaporation factor2–88Table 2–31Hanway’s stages of growth for corn2–93Table 2–32Irrigation water quality guidelines2–100Table 2–33Determinations normally required to evaluate2–101irrigation water quality problemsTable 2–34Salt tolerance of selected crops2–106Table 2–35Chloride tolerance limits of some fruit crop cultivars2–111and rootstocksTable 2–36Relative tolerance of selected crops to foliar injury2–112from saline water applied by sprinklersTable 2–37Boron tolerance limits for agricultural crops2–113Table 2–38Citrus and stone fruit rootstocks ranked in order2–114of increasing boron accumulation and transportto leavesTable 2–39Estimates of the electrical conductivity of drainage2–116water for determination of the leaching requirementsTable 2–40Sprinkling rate necessary for frost protection2–135Table 2–41Temperatures to start and stop overtree frost protection2–135Table 2–42Factors influencing effective precipitation2–144Table 2–43Average monthly effective precipitation as related2–148to mean monthly precipitation and average monthlycrop evapotranspirationTable 2–44Sample calculation of effective precipitation2–152(corn at Raleigh, North Carolina)Table 2–45Sample calculation of effective precipitation2–152(alfalfa at Denver, Colorado)(210-vi-NEH, September 1993)2–vii

Chapter 2Irrigation Water RequirementsPart 623National Engineering HandbookTable 2–46Average ratios applicable to effective precipitation2–153Table 2–47Average values for parameters used in the Brooks2–163and Corey functionsTable 2–48Probable application efficiencies of the low-quarter2–173and the low-half for various types of sprinkler systemsTable 2–49Example water application efficienciesfor furrow2–175irrigation by slope and intake family assuming noreuse of runoffTable 2–50Example water application efficiencies for furrow2–176irrigation by slope and intake family assuming arunoff reuse efficiency of 75 percentTable 2–51Suggested application efficiency for graded border2–179irrigation systemsTable 2–52Design application efficiency of level systems as2–180function of the advance ratio AR, where AR advancetime/net opportunity time Tt/TnTable 2–53Conveyance, field, and distribution efficiencies2–185for various types of systemsTable 2–54Average 5-day ETc data for the log-normal frequency2–202analysis dataTable 2–55Peak period average daily consumptive use2–205as related to estimated actual monthly useTable 2–56Comparison of the effect of an earliest date and latest2–208date irrigation strategy on system capacity and otherperformance criteriaTable 2–57Example of canal sizing problem for the system shown 2–224in figure 2–65Table 2A–1Monthly percentage of daytime hours of the year2–228for northern latitudesTable 2A–2Seasonal consumptive-use crop coefficientsfor irrigated crops2–viii(210-vi-NEH, September 1993)2–230

Chapter 2Irrigation Water RequirementsTable 2A–3Part 623National Engineering HandbookA guide for determining planting dates, maturity2–231dates, and lengths of growing seasons as relatedto mean air temperatureTable 2A–4Values of the climate coefficients for various2–232mean air temperaturesFiguresFigure 2–1Diagram of the soil-water balance of a crop root zone2–2Figure 2–2Flow chart to compute irrigation water requirements2–3Figure 2–3Energy available for evapotranspiration from crop systems2–5Figure 2–4Schematic diagram of three types of lysimeters2–9used to measure crop evapotranspirationFigure 2–5Relationship of vapor pressure and relative humidity2–14to temperatureFigure 2–6Representation of wind speeds within and above a2–18crop canopyFigure 2–7Variation of albedo during the year for selected latitudes 2–23Figure 2–8Effect of date and elevation on clear sky radiation2–24at 40 N latitudeFigure 2–9Extraterrestrial radiation as a function of time2–32for various latitudesFigure 2–10Flow chart for computing net radiation2–35Figure 2–11Changes in ETo with distance from the ocean2–38for three locations in CaliforniaFigure 2–12Effect of advection on the evaporation rate from an2–39evaporation panFigure 2–13Correction factor for evapotranspiration because2–40of advection using data outside or before irrigationdevelopment for different sizes of irrigated areasunder arid and moderate wind conditions(210-vi-NEH, September 1993)2–ix

Chapter 2Irrigation Water RequirementsFigure 2–14Part 623National Engineering HandbookVariation of the average daily ETc as affected2–42by the length of the averaging periodFigure 2–15Definition sketch for variables used to define the2–44aerodynamic resistanceFigure 2–16Flow diagram for computing ETo using the2–49Penman-Monteith methodFigure 2–17Schematic diagram of a Class A evaporation pan2–62Figure 2–18Plot of pan evaporation against Penman-Monteith2–64ETo to determine a pan coefficientFigure 2–19Generalized basal crop coefficient showing the2–67effects of surface wetness, water stress, andleaf area index during the growing seasonFigure 2–20Basal crop coefficient for corn grown for grain in a2–70windy and arid environmentFigure 2–21Process to compute basal crop coefficients2–72Figure 2–22Functions used to reduce evapotranspiration2–83based on soil-water contentFigure 2–23Comparison of basal and average crop coefficients2–90for the average crop coefficient exampleFigure 2–24Example of the relationship of growth stages of corn2–93to cumulative growing degree days since emergencefor western NebraskaFigure 2–25Example of the relationship of crop development2–94to growing degree days for Pioneer 3901 corn grownfor 5 years in western Nebraska and 2 yearsin eastern NebraskaFigure 2–26Variation of growing degree days required from2–95emergence to maturity for six corn varietiesof different maturity ratingsFigure 2–27Crop growth stages for corn related to the fraction2–95of the growing season based on growing degree daysFigure 2–28Example soil-water retention curves for a clay loamsoil at various degrees of soil salinity2–x(210-vi-NEH, September 1993)2–99

Chapter 2Irrigation Water RequirementsFigure 2–29Part 623National Engineering HandbookRelationship among average root zone salinity2–102(saturation extract basis), electrical conductivityof irrigation water, and leaching fraction to usefor conditions of conventional irrigation managementFigure 2–30Relationship among water uptake-weighted salinity2–102(saturation extract basis), electrical conductivityof irrigation water, and leaching fraction to usefor conditions of high-frequency irrigationFigure 2–31Threshold values of Sodium Adsorption Ratio2–103of topsoil and electrical conductivity of infiltratingwater associated with the likelihood of substantiallosses in permeabilityFigure 2–32Divisions for classifying crop tolerance to salinity2–105Figure 2–33Prediction of leaching requirement based on crop2–117tolerance and water salinityFigure 2–34Evaporation from land areas for various temperatures2–120and rates of rainfallFigure 2–35Pattern of salt buildup as a function of bed shape and2–127those effects on the germination of seeds placedat different locations on the bedsFigure 2–36Depth of water per unit depth of soil required to leach2–130a saline soil by continuous or intermittent ponding,or to leach a soil inherently high in boronFigure 2–37Precipitation pathways2–143Figure 2–38Average monthly effective precipitation as related2–149to mean monthly rainfall and average cropevapotranspirationFigure 2–39Frequency distribution of growing season precipitation 2–151Figure 2–40Hydraulic properties for sand and silt loam2–156Figure 2–41Water content and capillary pressure head for two2–157soils that have a steady, upward flow rateof 0.1 inch per dayFigure 2–42Water table contribution to irrigation requirements2–159as a function of water table depth(210-vi-NEH, September 1993)2–xi

Chapter 2Irrigation Water RequirementsFigure 2–43Part 623National Engineering HandbookGraphical solution for the water table contribution2–160using the Anat SolutionFigure 2–44Diagram of the moisture release curve for a sandy2–162clay loam soil using the method of Brooks and CoreyFigure 2–45Procedure to determine the characteristic parameters2–162for the Brooks and Corey functionsFigure 2–46Distribution of field application depth indicating2–170adequacy of irrigationFigure 2–47Distribution for two irrigation systems having equal2–171adequacy but different uniformity and applicationefficiencyFigure 2–48Distribution for two irrigation systems having equal2–172uniformity but different adequacy and applicationefficiencyFigure 2–49Application efficiency as related to the coefficient2–174of uniformity and the percent of the area that isdeficitly irrigatedFigure 2–50Method to estimate seepage losses from irrigation2–186delivery systemsFigure 2–51Frequency distribution of mean daily ETc of ryegrass2–197for each month in a coastal California ValleyFigure 2–52Log-normal probability distribution to smooth2–201extreme values for daily ETc dataFigure 2–53Weibull transform for smoothing annual maximumETc data to predict the design net capacity requiredFigure 2–54Ratio of mean peak and mean monthly ETc2–2012–203for different climates during months of peak water useFigure 2–55Shortage and surplus periods for a system where2–204the capacity is less than the average ETc during apeak water use periodFigure 2–5610-day ETc, rain and soil water deficit, and the soilwater depletion pattern over a growing season asaffected by gross system capacity2–xii(210-vi-NEH, September 1993)2–206

Chapter 2Irrigation Water RequirementsFigure 2–57Part 623National Engineering HandbookDesign net capacity required for corn grown2–207in eastern Colorado to maintain soil water depletionabovea specified depletion for three design probabilitiesFigure 2–58Checkbook scheduling method2–210Figure 2–59Average daily water use during the season2–211for three crops in North DakotaFigure 2–60Irrigation cycle time, or irrigation interval, and2–212its effect on the soil water depletion at the startingand stopping positions of an irrigation systemFigure 2–61Earliest and latest dates to irrigate for a system2–213that applies 1 inch of net irrigation per applicationand has an allowable depletion of 3 inches, assumingthe irrigation interval is 3 daysFigure 2–62Example of regional ETc data for irrigation scheduling 2–215Figure 2–63Processes involved in sizing irrigation projects2–221Figure 2–64System capacity curve for a conveyance system2–222Figure 2–65Delivery system layout for a farm served by a large2–223lateral canalFigure 2–66An evaluation form for water conservation inventories 2–226of irrigation systemsFigure 2A–1Crop growth stage coefficient curve for alfalfa2–235Figure 2A–2Crop growth stage coefficient curve for avocados2–236Figure 2A–3Crop growth stage coefficient curve for dry beans2–237Figure 2A–4Crop growth stage coefficient curve for snap beans2–238Figure 2A–5Crop growth stage coefficient curve for sugar beets2–239Figure 2A–6Crop growth stage coefficient curve for citrus2–240Figure 2A–7Crop growth stage coefficient curve for corn (grain)2–241Figure 2A–8Crop growth stage coefficient curve for corn (silage)2–242Figure 2A–9Crop growth stage coefficient curve for sweet corn2–243Figure 2A–10 Crop growth stage coefficient curve for cotton(210-vi-NEH, September 1993)2–2442–xiii

Chapter 2Irrigation Water RequirementsPart 623National Engineering HandbookFigure 2A–11Crop growth stage coefficient curve for spring grain2–245Figure 2A–12Crop growth stage coefficient curve for grapes2–246Figure 2A–13Crop growth stage coefficient curve for melons2–247and cantaloupesFigure 2A–14Crop growth stage coefficient curve for deciduous2–248orchardsFigure 2A–15Crop growth stage coefficient curve for pasture grasses2–249Figure 2A–16Crop growth stage coefficient curve for peasFigure 2A–17Crop growth stage coefficient curve for Irish potatoes 2–251Figure 2A–18Crop growth stage coefficient curve for grain sorghum2–252Figure 2A–19Crop growth stage coefficient curve for soybeans2–253Figure 2A–20Crop growth stage coefficient curve for tomatoes2–254Figure 2A–21Crop growth stage coefficient curve for small2–2552–250vegetablesFigure 2A–22Crop growth stage coefficient curve for walnutsFigure 2A–23Crop growth stage coefficient curve for winter wheat 2–257Examples Example 2–1 Dew point2–xiv2–2562–15Example 2–2 Vapor pressure deficit2–16Example 2–3 Wind speed computations2–20Example 2–4 Clear sky radiation2–25Example 2–5 Outgoing longwave radiation2–28Example 2–6 Outgoing longwave radiation for a clear sky2–29Example 2–7 Net outgoing longwave radiation2–29Example 2–8 Rs estimate2–34(210-vi-NEH, September 1993)

Chapter 2Irrigation Water RequirementsPart 623National Engineering HandbookExample 2–9Daily soil heat flux2–36Example 2–10Monthly soil heat flux2–37Example 2–11Penman-Monteith method2–48Example 2–12ETo—Radiation method2–53Example 2–13FAO Blaney-Criddle2–57Example 2–14Local calibration of a pan coefficient2–65Example

Measurement of irrigation water Design of pumping plants Design criteria and design procedures for surface, sprinkler, and micro irrigation methods and the variety of systems for each method that can be adaptable to meet local crop, water, and site conditions and irrigation concerns These chapters will come under the new Part 623 .