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<br />002056 <br /> <br />When stream flows and groundwater levels are changed, water dependent <br />environmental values are impacted, some being degraded and others enhanced as a result <br />of efficiency improvements. <br /> <br />As explained at Section VIA, the water supply impacts of efficiency changes depend <br />on basin characteristics and the relative priority of the improved right. Clearly, when <br />, <br />consumptive use is decreased additional water is available in the basin. Depending on <br />current and future needs in the basin this water may get consumed by others or may remain <br />in the stream to improve aquatic and riparian values. Where return flows are decreased it <br />is less dear what will occur. If those return flows are reduced by reducing diversion <br />volumes it might be concluded that stream flows are improved. Actually, the lower <br />diversion rate may only make it possible for some other upstream user to now divert and <br />consume more water, in fact decreasing stream flows. If no upstream use of the reduced <br />diversions occurs, the instream flow will be increased below the headgate down to the reach <br />where return flows historically occurred. . Below that point annual flows should be similar <br />to historical levels, but tbe monthly pattern would vary, returning to a more natural flow <br />distribution. In Colorado the pre-irrigation flow distribution often saw peak flows in spring <br />, <br />and dry streams in the surruner and fall. Irrigation return flows have changed intermittent <br />streams to perennial streams with a year-round water supply; improved efficiency may <br />reverse this trend. <br /> <br />Another environmental resource impacted by irrigation efficiency changes is wetlands. <br />Losses from irrigation systems can augment tbe water supply for natural wetlands and more <br />often result in creation of new wetlands entirely dependent on irrigation for their water <br />supply. Water tbat would otherWise return to the surface stream is consumed by wetland <br />vegetation, creating a stream depletion. The incidental consumptive use occurring within <br />an irrigation system is often reduced with a corresponding loss of wetland acreage. Indeed, <br />the Federal Salinity Control Program has had difficulties in mitigating the wetlands impacts <br />associated With its projects. <br /> <br />29 <br />