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<br />.' <br /> <br />. <br /> <br />. <br />. <br /> <br />. <br /> <br />:r\'" ">(1'8 <br />",,; J.{;.:.... ..r <br /> <br />Rechargeable aquifers are usually hydraulically connected, or tributary, to the sutface <br />water system. Colorado water law recognizes this physical connection and tributary <br />groundwater is allocated and administered on the same basis as surface water. These <br />tributary aquifers are recharged in many areas by irrigation water that infiltrates into the <br />soil and deep percolates down to the aquifers. The water table then rises and groundwater <br />flows to surface streams that intercept the aquifer. Reduced irrigation losses produce less <br />deep percolation and less recharge. Declining recharge rates reduce the rise in the localized <br />groundwater mounds and the regional water table, with potential impacts on well purriping <br />levels and return flows. Pumpers are willing to reduce their well diversions to save on <br />energy consumption and cost of agricultural chemicals (lower water application rates allow <br />reduced applications of fertilizer). When tributary well pumpers become more efficient,they <br />have little absolute impact on the annual water table, because reduced pumping offsets any <br />loss of recharge. However, when surface irrigators become more efficient they can decrease <br />the supply of water available to the tributary aquifers, impacting well users who have come <br />to rely on those return flows. <br /> <br />Non-tributary aquifers are geologically isolated from significant surface recharge. The <br />incentive to improve the efficiency of uses of non-tributary aquifers, such as the Ogallala, <br />is to conserve a vanishing resource. Significant efforts are now under way to reduce well <br />pumping by using water more efficiently and thereby extend the life of this finite supply. <br />The allocation and conservation of non-tributary groundwater is beyond' the scope of this <br />analysis, but may merit further attention and consideration if the General Assembly is going <br />to create incentives to encourage improved irrigation efficiency. <br /> <br />D. Environmental Impacts <br /> <br />When stream flows and groundwater levels are changed, water dependent environmental <br />values are impacted, some being degraded and others enhanced as a result of efficiency <br />improvements. <br /> <br />As explained above, the water supply impacts of efficiency changes depend on basin <br />characteristics. and the relative priority of the improved right. Clearly, when consumptive <br />use is decreased additional water is available in the basin. Depending on current and future <br />needs in the basin this water may get consumed by others or may remain in the stream to <br />improve aquatic and riparian values. Where return flows are decreased the effect is less <br />clear. If those return flows are reduced by reducing diversion volumes it might be <br />concluded that stream flows are improved. Actually, the lower diversion rate may only <br />make it possible for some other upstream user to now divert and consume more water, in <br />fact decreasing stream flows. If no upstream use of the reduced diversions occurs, instream <br />flow will be increased between the headgate and the point(s) where return flows historically <br />entered the stream. Below that point annual flows should be similar to historical levels, but <br />the monthly pattern would vary, returning to a more natural flow distribution. In Colorado <br />the pre-irrigation flow distribution often saw peak flows in spring and dry streams in the <br /> <br />21 <br /> <br />, <br /> <br />-" ~ <br /> <br />',-ii-,_...;-.,.,),;;.- <br /> <br />, <br />, <br />- :{11 <br />. ,;i*~1J <br />"~' <br />