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<br />Effective Precipitation <br /> <br />Because not all of the precipitation recorded is effective in meeting <br />consumptive use requirements, the recorded amount must be adjusted. The <br />Bureau of Reclamation's method for detennining effective precipitation <br />in arid and semiarid regions was used. Table 13 summarizes this method. <br /> <br />'. <br /> <br />Carryover Soil Moisture <br /> <br />, The water stored within the root zone during the winter, when the crop <br />is dormant or before planting, is termed carryover soil moisture. <br />Carryover soil moi sture is an important constituent in computing crop <br />irrigation requirements. Since the soil acts as a reservoir, effective <br />precipitation unused during 1 month was considered available for use the <br />following month. <br /> <br />Tables 14, 15, and 16 show the methodology used in detennining the <br />carryover soil, moisture for the three areas. Usable carryover soil <br />moisture was estimated from effective nongrowing season precipitation <br />(November-April) times the optimum percent dep1 eti on of the soil moi s- <br />ture for each crop. The soil reservoir was assumed to be empty at <br />the end of the growing season. The optimum usable soil moisture holding <br />capacity for the soils of the basin was estimated for the crops grown in <br />the three areas. The lesser of these two soil moisture amounts was used <br />as the initial usable carryover soil moisture amount. <br /> <br />Consumptive Use <br /> <br />The modified Blaney-Criddle method was used in detennining consump- <br />tive use requirements for the three study areas. This method is <br />based upon the close correlation between mean monthly temperature <br />and day1 ight hours, wfth the amount of water consumptively used by <br />a specific crop. This method is outlined in Irrigation Water <br />Requirements, Technical Release No. 21, Soil Conservation Service, <br />September 1970. <br /> <br />As mentioned earlier temperature and precipitation data from 1920- <br />1978 for Wray, Colorado; McCook, Alma, and Red Cloud. Nebraska; and <br />Clay Center, Kansas, weather stations were used as input to the <br />model. Daylight hours were estimated accordi ng to 1 atftude wfth <br />40. N latitude being used. The crop distributions used were des- <br />cribed in the crop distribution section. <br /> <br />Table 17 lists the consumptive use by area as detennined by the modified <br />B 1 aney-C ri ddl e techni que. Consumptive use increase s from we st to ea st <br />in the bas i n wi th a difference of 2 inches between Areas I and I I 1. <br />Area I displays a consumptive use for the entire year, while Areas II <br />and III show no consumptive use during the winter. Area I is the only <br />area with significant amounts (greater than 1 percent'of irrigated area) <br />of winter wheat and irrigated pasture, the only crops consumptively <br />using water during the winter months. <br /> <br />11 <br />