Hydrologic analysis of the Rio Grande basin north of Embudo

Large areas of Colorado, New Mexico, and Texas are connected by the Rio Grande, which is the major channel for surface water and the major drain for a variety of geohydrologic regimes. Five regions represent differing hydrologic regimes in the 10,400 square miles of the Rio Grande basin above Embudo, New Mexico. For each of the five region's flow regimes, the water yield, or difference between precipitation and evapotranspiration, was estimated. The three regions in which precipitation exceeded evapotranspiration and the estimated water yield of each region for 1950-80 were: the San Juan Mountains, 2,800 cubic feet per second; the Sangre de Cristo Mountains, 780 cubic feet per second; and the Taos Plateau, 28 cubic feet per second. Evapotranspiration exceeded precipitation in the Costilla Plains by 150 cubic feet per second and in the Alamosa Basin by 2,400 cubic feet per second. The sum of these estimated flows and the decrease in storage in the Alamosa Basin (87 cubic feet per second) results in a calculated discharge from the study area (1,145 cubic feet per second) that was 69 percent greater than the measured discharge from the study area (680 cubic feet per second). This difference probably results from inaccuracies in estimating flow rates for the individual regions. A three-dimensional digital model was constructed to represent the aquifer system in the Alamosa Basin. Conclusions based on a preliminary analysis using a two-dimensional section model had been: (1) A seven-layer model representing 3,200 feet of saturated thickness could accurately simulate the behavior of the flow in the basin; and (2) the 1950 flows and water levels were approximately stable and would be a satisfactory initial condition. For the three-dimensional model, values of aquifer characteristics were assumed or estimated from the work of previous investigators. Recharge and discharge represented in the model included changes since 1950 in ground-water withdrawals, return flow, salvaged evapotranspiration, and captured streamflow. Reasonable modifications to ground-water withdrawals allowed simulated changes to approximate the measured data small declines in water level from 1950 to 1969 and water-level declines from 1970 to 1979 that were within about 5 feet of measured water-level declines. During 1980, the source of water withdrawn by wells was estimated to be 80 percent salvaged evapotranspiration, 14 percent depleted storage in the aquifer, and 6 percent captured streamflow. Sensitivity tests indicated that salvaged evapotranspiration was the major source, accounting for 69 to 82 percent of the ground-water withdrawals. Simulated hydraulic-head declines were most sensitive to ground-water withdrawals. The three-dimensional model of ground-water flow was used to estimate the response to withdrawals from the Closed Basin Project (U.S. Bureau of Reclamation, 1963) in the Alamosa Basin. Evapotranspiration salvage was the source of most withdrawals, and hydraulic-head declines of more than 2 feet were simulated in the upper confined aquifer. The reliability of these projections is limited by the lack of calibration data in nonirrigated areas.