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Executive Summary Physical Constraints an Groundwater Jse Many of the state's aquifers can supply limited quantities of water on a sustainable basis because of their physical characteristics. One example of this is the Closed Basin aquifer that comprises most of the San Luis Valley in the south-central part of the state. This aquifer supports large-scale agricultural production through center-pivot sprinkler irrigation (Figure ES-14). When the drought of the late 1990s began, and included one of the driest years in recent history (2002), diversions of water by the agricultural ditch systems from the Rio Grande River were severely limited. Likewise the lack of precipitation reduced surface recharge and water directly available to crops, stimulating increased pumping from the unconfined aquifer. The result has been a significant change in the volume of water stored in the aquifer and a decline in the groundwater levels in the San Luis Valley. Figure ES-15 shows that aquifer storage has been on a downward trend since 2000. Figure ES-16 illustrates the drop in water level from January 2002 through August 2003. The Rio Grande Basin is an example of the effects of continued groundwater pumping when recharge by surface diversions is constrained during drought. While this balance between inflows (recharge) and outflows (pumping) is clearly unsustainable, more immediate concerns relate to increased pumping costs and decreased well yields. An increase of flow in the Rio Grande and increased precipitation in the San Luis Valley would increase recharge to the aquifers and help raise water levels, but, as suggested by the water level declines in recent years, the Closed Basin aquifer must be managed and used carefully. A second example of an aquifer that is physically limited is the vast Denver Basin non-renewable bedrock aquifers, located between Greeley and Colorado Springs. Figure ES-17 provides a cross-section through the Denver Basin highlighting the four major aquifers. The aquifers are very thick and contain a significant amount of water. Unfortunately, their yield is low compared to the alluvial aquifers because of the composition of the aquifers. Impervious rock layers exist between and even within each of these aquifers and inhibit the movement of the water. The result is aquifers that are "confined," but likewise are very limited in their ability to produce water and to receive natural recharge. The confining pressure of the overlying rock causes the water in a well, when initially drilled, to sustain an artesian pressure. This upward, artesian pressure caused the earliest of these Denver Basin wells, like the one that serviced the Brown Palace in Denver in the 19th century, to actual flow without pumping. Groundwater withdrawals for suburban communities along the Front Range have increased dramatically in many areas of the basin in the past 2 decades, particularly in the South Metro Denver region in Douglas and Arapahoe Counties and more recently Northern EI Paso County. There are very few sources of renewable surface water supplies available for these areas. In the South Metro Denver area some wells (such as in the more productive Arapahoe Aquifer) are showing declines as much as 30 feet per year (see Figure ES-18) and over 250 feet total decline in the aquifer water level over an area tens of square miles in size. As water levels continue to drop there are concerns about loss in the yield of individual wells. Additional wells will be needed to sustain the delivery of water at the original rate, which will increase pumping costs dramatically. Figure ES-14 Center pivot irrigation crop circ/es in the San Luis Valley ~~ ~J~~a Statew~itle Water Supply Inii'iative ES-26 S:\REPORT\WORD PROCESSING\REPORT\EXEC SUMMARY 11-10-04.DOC Source: CWCB - using 1998 imagery