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Dellve~' R lIabllitv Criteria Streamflow in the study area vary significantly from year to year because of the natural variation in precipitation, In addition runoff occurs in the spring and early summer and may not be available in August and September for irrigation. 11is variation is shown in Figure III-l for Beaver Creek, With improvements to the existing irrigation system suc'as storage reservoirs, delivery variations can be reduced and reliability of irrigation delivery improved, In the 196 USBR "Feasibility Report - San Miguel Project, Colorado," alternatives were developed to deliver a specified q antity of water 80 percent of the time. That is, with a selected cropping pattern and fixed irrigated area, a full ater supply could be delivered in eight years out of ten to meet or exceed the irrigation demand. In two years ( t of ten, irrigation demands would not be entirely met. Economics generally dictate the reliability criteria to used for a particular irrigation project. Irrigation reliability can generally be increased by increasing the size of torage facilities, however, this increases project costs. Irrigation reliability can also be increased by decreasing he irrigated area. An 80 perc nt reliability criteria has been used for this study for all crops except a portion of the grass hay/pasture lands. Thi criteria is reasonable considering the selected cropping pattern, area to be irrigated, and irrigation system imp ovement costs. In eight out of ten years, water deliveries would meet or exceed water demand. In those years that water deliveries exceed demand, the excess water could be applied to the (marginally irrigated) grass hay/p ISture which does not get a full water supply 80 percent of the time. In years of excess water, the marginally i rigated crop could be harvested for hay and then grazed. In water short years, the crop could be dry grazed with ut a hay harvest, Figure IV-2 shows this concept graphically for the cropping pattern selected for the Norwood-] edvale area. ]rrinatlon "ater Demands Optimal cr p growth results when natural precipitation is supplemented with sufficient irrigation water to meet crop water emands (evapotranspiration requirements), referred to as the crop consumptive use requirement. To estimate th total irrigation diversion requirement for a particular area, allowances for application efficiency and conveyance losses in the irrigation system are added to the consumptive use requirement. The total is then adjusted fo return flows or flows intercepted within the irrigation system to arrive at the amount of water which must be div rted at the source to satisfy the crop consumptive use requirement. N.5 I I I I I I I I II I I I I I I I I I