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<br />, <br /> <br />November-March period, not supplied by effective precipitation, were <br />satisifed from the carryover soil moisture. <br /> <br />Tables 39, 40, and 41 tabulate the monthly crop irrigation require- <br />ments from 1920-1978 for each of the study areas. The average annual <br />crop irrigation requi rement for the 1920-1978 study period is <br />13.73 inches for Area I, 13.84 inches for Area II, and 12.98 inches for <br />Area I I 1. These amoun ts compare favorably with the crop i rri gati on <br />requirements determined by the Soil Conservation Service. Table 42 is a <br />compari son by a rea of both agenc f es i rri gati on requi rements for the <br />peri od 1941-1970. A 11 of the crop i rri ga ti on requi rements assume a <br />full i rrigati on supply. <br /> <br />'. <br /> <br />The crop irrigation requirements, as determined in the Frenchman- <br />Cambridge Definite Plan Report (Lowry-Johnson consumptive use figuresl, <br />were lower than the irrigation requirement computed in this study <br />(modified Blaney-Criddle consumptive use figures); see tables 43, 44, <br />and 45. Part of the difference can be attributed to the way in which <br />the carryover soil moisture was determined. The project plan does not <br />take into account the usable soil moisture holding capacity when <br />determi ni ng the carryover soil moi sture amount. Instead, the project <br />plan assumes that all of the effective precipitation occurring during <br />the nongrowing season is available to the plant. The crop irrigation <br />requi rements determi ned by the recent study only subtracts the usabl e <br />soil moisture amount. The usable soil moisture amounts were determined <br />as described in the Carryover Soil Moisture section of this report. The <br />usable soil moisture amount 'fSaPproxlmately 50 percent less than the <br />nongrowi ng season effective precipitati on amount. The resulti ng reduc- <br />tion in the carryover soil moisture figure is 1.9 inches of carryover <br />for Area I, 2.2 inches for Area II, and 2.8 inches for Area III, based <br />on the 1920-1978 average. <br /> <br />The crop irrigation requirement is the irrigation water that is <br />required at the field, and does not include conveyance system losses. <br />The fo 11 owi ng secti on di scusses the onfarm porti on of these conveyance <br />system losses. <br /> <br />ONFARM EFFICIENCY <br /> <br />Onfarm efficiency can be defined as the product of farm ditch and <br />field irrigation efficiency. Proper irrigation practices can reduce <br />onfarm losses by as much as 30 percent. <br /> <br />Farm ditch efficiency depends upon the losses that occur from the <br />fann turnout on the main canal system to the irrigated field. The <br />grea test loss is seepage from the di tches. Seepage can be reduced by <br />lining the ditches or conveying the irrigation water in pipe. Other <br />losses occuring within the system are spillage, use by phreatophytes, <br />and leaky fann gates. Proper operation and maintenance of the system <br />can reduce or eliminate some of these losses. <br /> <br />Timi ng of water del ivery to the farm is another important factor. <br />If deliveries occur when water is not needed, such as after a large <br /> <br />13 <br />