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<br />Oi}[l739 <br /> <br />wetlands created by long-term irrigation, <br />Within any given basin, wide-scale adoption of conservation practices designed to increase divernion <br />efficiencies has the potential of altering basin hydrology by reducing the magnitude of return flows, In areas <br />where retwn flows fulfill a portion of irrigation demands, their importance is measured not only in magnitude, <br />but also in timing. A significant fraction of the reIum flows from irrigation are delayed in their return to the <br />stream system because the pathway is via porous soil media within the alluvial aquifer, which restricts the <br />rate of water flow, On a basin-wide scale dischmges from the alluvial aquifer increase late-season <br />streamflows, which can meet irrigation demands for extended periods after peak runoff from snowmelt The <br />system, in effect, functions in the same manner as a reservoir, with divernions in excess of consumptive use <br />contributing to storage and return flows functioning as releases, <br />Changes in irrigation practices at the basin level that would significantly increase divernion efficiencies <br />could negatively impact water users who depend on these return flows, In the South Platte, Arkansas, and <br />Rio Grande basins, irrigation diversions greatly exceed streamflows, which demonstrates the dependence of <br />downstream users on return flows and is evidence of a relatively high level of water use efficiency within the <br />basin. Reduced retwn flows are not a concern in the lower reaches of some watersheds where there is no <br />downstream dependence on these return flows if interstate compact issues are not limiting, <br />Another likely effect of basin-wide increases in diversion efficiency is increased consumptive irrigation <br />water use, This would occur as a consequence of irrigators using the savings from more efficient divernions <br />to fill shortages within their decrees, Increased consumptive use would affect basin hydrology and eventually <br />result in reduced return flows, <br />Large-scale transfers of water from irrigation to alternative uses causes a transition from irrigated to <br />rainfed (dryland) agriculture, In many cases, this can cause decreased land values because of limited <br />alternative land uses, greater potential for soil erosion,~ unreliability of dryland cropping practices because <br />of limited and variable natura1 precipitation (Sutherland and Knapp, 1988), If annual precipitation levels and <br />soils are favorable, dryland cropping practices can be successful even though productivity levels are greatly <br />reduced, In many cases, precipitation and soils dictate that land must be converted to rangeland to stabilize <br />production and prevent erosion. This requires carefullong-tenn management of revegetation, which is not <br />generally cost-effective, In the past, the unfavorable economic outlook for dryland cropping and rangeland <br />restoration led to land abandonment after water transfers, This haS been largely remedied by various <br />combinations of government subsidies supporting land revegetation and water court stipulations requiring <br />restoration. Even with these remedies in place, transfern have a significant negative impact on local <br />economies and public institutions because of decreased economic activity and a lower tax base, <br /> <br />\: <br />I <br />I <br /> <br />\ <br />r <br /> <br />,,> <br /> <br />A <br /> <br />46 <br />