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CellI} a usable storage capacity of 54,000 af with an additional 6,000 at reserved for further flood control purposes, Plataro Reservoir lies in the head- waters of the Conejos River, 80 miles upstream from where the Conejos joins the Rio Grande, The Conejos Water Conservancy Dis- trict was formed to help sponsor the project and to enter inlo repayment contracts with the Bureau of Reclama- tion, To date, however, no repayment has been initiated, nor has the reservoir often been used to provide irrigation water to the Conejos district, Due to restrictions imposed by the Rio Grande Compact, the reservoir has been used for flood control, rarely for water supply storage. Because the intended users could not depend on benefit from the reservoir, the Bureau made an administrative decision not to begin requiring payback of the con, sttuction costs or charging for opera~ tion and maintenance expenses. This repayment obligation will be trig, gered, however, at such time as the Conejos district is able to utilize Platoro Reservoir for its irrigation needs. The Bureau currently operates Platoro Dam in a manner ensuring that the compact is not violated and in conjunction with flood control needs, During times of flood control operations, however, the U.S. Army Corp.s of Engineers assumes respon- sibility over dam regulation in accor- dance with the Flood Control Act of 1944. Section 7 of this congressional enactment gives the Corps authority during flood episodes over those Bureau of Reclamation water storage projects that were designed to include flood control capabilities, For Platoro Reservoir, the Corps assumes authority when-stream gages indicate a flooding condition, or when the state water office in Alamosa requests flood control operations. At such times, the Corps remains in daily contact with the Bureau of Reclama- tion office in Albuquerque, which instructs personnel in Chama, New Mexico, who remotely control the gates at Platoro Dam to minimize potential flood damage, COMPACT OBLIGATIONS OF COLORADO A primary driving force in water use and management in the San Luis Val, ley is the Rio Grande Compact. Under the compact, Colorado must ensure that a specified percentage of the Rio Grande flows into New Mex' ico. This percentage is determined by an agreed upon formula that reflects the amount of annual runoff in the area of the headwaters, In a year of average precipitation, when about 1.0 million acre feet flow through the Rio Grande and its major tributaries in the San Luis Valley, Colorado must allow one third of this total to reach New Mexico. In dry years, this requirement drops to about one fourth, while in wet years, Colorado must allow nearly two thirds of the Rio Grande to flow into New Mexico. River gaging stations are strategically placed at upstream portions of the basin and near the New Mexico bor- der to monitor compliance with the delivery schedule. Gaging stations alone, however, do not ensure compliance with lawful requirements. Although the 1938 compact allows Colorado to under- deliver up to a total of 100,000 af, a debt of more than 900,000 af had been accrued by 1966. As a consequence, New Mexico and Texas filed suit in the U. S, Supreme Court to compel Colorado to comply with the compact and to repay the underdelivery of water. After lengthy negotiations, the states reached a settlement in the case. Under the 1968 settlement, Colorado was not required to immediately repay the debt of 940,000 af (which could have severely undermined the local economy), but it had to there, after meet its annual delivery obliga- tions without fail. This promise, which Colorado has strictly adhered to, set in motion several actions that continue to influence water use and management inche San Luis Valley, As explained in the following seC- tions, these include the regulation of ground water diversions, authoriza- tion of the Closed Basin Project, and actions by special water districts to identify new sources of water. GROUND WATER REGULATION One of the main reasons that Colo- rado was underdelivering to such an extent was that ground water pump- ing in the Valley was depleting the flows of the Rio Grande and its tribu- taries. With surface flows fully appropriated, a 1950s boom in local well drilling began as many irrigators searched for reliable late season sup' plies and farmers opened new lands irrigated solely by ground water. Wells soon became an important ele~ ment in the region's agricultural econ- omy, particularly since shallow ground water and a deeper artesian aquifer were readily available under much of the Valley, In the 1960s, pumping experience and increased scientific understand- ing made it apparent that in most instances, ground water use from Val- ley wells eventually diminished the flow of the Rio Grande and its tribu- taries, Ground water was hydrauli- cally connected to the surface stream; thus additional pumping made more water seep from the river to replenish the ground water table. New ground water wells, therefore, were indirectly using the limited surface supply that had been historically relied upon by ~enior water users. Faced withthe need under the 1968 stipulation to fully regulate water use in the San Luis Valley, the Colorado State Engineer drafted rules for regulating well pumping. He based the rules on the prior appropriation doctrine, which commanded that early water appropriators had a sen- ior right that must be protected from injury by later water users.-Conse~ quently, the rules phased out all pumping from junior wells drilled over the past few decades, unless their owners bought and retired sen- ior surface rights or otherwise offset their pumping impact on surface streams. Similarly, no new wells could be drilled in the Valley without a plan for offsetting their indirect sur- face depletions. The rules of the State Engineer also proposed to administer three major 7 :!': "1