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1833 non-Project water might be spilled from an if-and-whlln storage account in Pueblo Re6ervoir in a wetyear if the space were needed by the USBR to; store yields of native water rights. This operational analysis is not extended to cover such 11 contingency; however, estimates of such I requirements are not complex and could be included i1} the operating requirements of the legal and administrative authorizations of the FLWB. i Anticipated Environmental Effects of Changes in Return Flow Patterns Changes in the way return flows reach the river migh~ result in some minor effects on parts of the environment. Since water would be held in surface ~torage reservoirs rather than underground, evaporation would be increased. However, reservoir levels would also be increased with potential benefits accruing to fish, wildlife and recreation at th~ reservoir. Depending upon the location of the fallowed land, soil moisture and associated runoff; to hedgerow plant species might be reduced temporarily. Effects due to reduced seepage in the distribution canals should be minimal. Since return flow releases would be made upstream from the normal location of accretion to the river, river flows would increase, with resulting benefits to upstream river reaches. No mitigation actions anticipated, but observations to identify and monitor ~ny effects should be planned and carried out on a periodic basis. i Water Bank Administration of Return Flows For the purposes of operating the FL WB, a comprehensive evaluation of return flow characteristics of the FLCC system would be necessary. Engineering analysis and data collection to map the soil and aquifer characteristics would be needed to deter~ine return flow obligations for each specific participating acreage by location and irrigation practices. Most of the needed data are alreaQy available, but synthesis of the data and formation of th~ supporting details of administrative methods is beyond the scope of this stuQy. Tools for quan~fication and administration of return flow obligations would be contained in the authorizing plan$, agreements and decrees for the water bank. Daily accounting would probably be required. The format and essential variables would be determined in the detailed engineering phase of the ;substitute supply plan or change decree. I Fallowing Requirements i In certain areas, crops such as alfalfa can produce a p.rtial crop of hay or a good crop of alfalfa seed without irrigation by drawing upon a beneficial water table connected to return flows. This could injure other water rights on the Arkansas River and bea cause for objectors. For this reason, the I identified fallowed fields must be kept free of any liying vegetation. i , I ! i , I //" I 7-281