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Cristo range. Over many millions of years the valley has continued to grow in depth, while at the same time filling with sediments and layers of volcanic rock shed from the surrounding mountains and carried to the valley by the Rio Grande, the Conejos River, and other streams. Over time, the pore spaces between the grains of sediments, fractures, and other openings in the volcanic rocks beneath the valley floor have filled with water. These saturated sedimentary and volcanic rock layers comprise the aquifers of the valley, from which numerous wells draw water. Ground water continues to recharge the pore spaces in the aquifers by percolation from surface streams, leakage through canals, and recharge from the mountains surrounding the valley along some of the more permeable rock layers. This complex, interconnected aquifer system is, in many areas, in hydrologic connection with the surface water system. The aquifer system is generally comprised of a shallow unconfined aquifer and a deeper, confined aquifer. The geometry and characteristics of both major aquifers are controlled by the geologic structure and stratigraphy (layering) that have developed over geologic time in the valley. The uppermost water-saturated layer of sand and gravel, down to a depth of about 100 feet across most of the valley, is the unconfined aquifer. Below the unconfined in the central part of the valley are a number of clay layers that serve to separate, although not totally disconnect, the unconfined aquifer from deeper water-bearing layers of sand, gravel, and fractured volcanic rocks. The deeper layers, of which there are many, together make up the confined aquifer because of the overlying and confining clays. Water flows from many wells completed in the confined aquifer due to natural artesian pressure. The volume of storage in the aquifer system of the valley is significant. However, not all of this water is recoverable, and in some locations and at selected depths the quality may limit its uses. The unconfined aquifer functions similar to a surface reservoir with a pattern of rising levels in the spring and early summer caused by recharge from streams, canals, and early irrigation season return flows. These increases are followed by a decline as the streamflow decreases and ground water is pumped largely for agricultural purposes. Agricultural activities account for more than 85 percent of basin water consumption with an estimated 638,000 acres under irrigation (HRS 1987). The primary crops are potatoes, carrots, small grains, and alfalfa. Along the edges of the valley there is little, if any, separation between the confined and the unconfined aquifers. This allows both aquifers to be recharged with water from runoff entering the San Luis Valley. However, the relationships between the two aquifers and between the aquifers and the surface water are not well defined. The RGDSS and additional data will improve the understanding of the physical system and improve estimates of potential water yield. 3.0 San Luis Valley Project The San Luis Valley Project was authorized by Congress in 1940 and described in House Document 693 This project included a Rio Grande element (this was not built), a Closed Basin element (the Closed Basin Project), and a Conejos River element (Platoro Reservoir). The two primary purposes of the San Luis Valley Project were to assist Colorado in meeting its commitments to New Mexico and Texas under the Rio Grande Compact (see Section 4.1) and to assist the United States in meeting its commitments to Mexico under the Treaty of 1906. 3.1 Closed Basin Project The Closed Basin Division of the San Luis Valley Project was authorized by Congress on October 20, 1972 (and modified in 1980 and 1984). Designed to pump water out of the Closed Basin, the Closed Basin Project consists of numerous wells and canals that deliver water to the Rio Grande. The total ultimate production capacity of the Closed Basin Project was estimated to be approximately 100,000 acre-feet per year (Leonard Rice Consulting Engineers, Inc. 1990). However, actual production to date has averaged 24,000 acre-feet per year. This low average is partly due to production curtailment in years when the water was not needed to help satisfy compact deliveries (S. Vandiver 1997). Basin water users agree that a more realistic goal would be a maximum of 65,000 acre-feet per year. According to Public