RGDSS_Task8-1_ReviewPreviousWaterBudgets

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4.2.1 Inflows -The Alamosa Basin water budget inflows consist of total inflows from the San Juan Mountains and Sangre de Cristo Mountains, and precipitation. The study does not distinguish mountain surface inflows from ground water inflows. Inflows from the San Juan Mountains are based on a detailed water budget investigation of the San Juan mountain range. The water budget was conducted because a significant amount of water was believed to leave the basin as ground water. In the water budget, it was estimated that precipitation was the only inflow, changes in storage were negligible and outflows consisted of sublimation, Et, and ground water and surface water. The water budget model was operated on a daily basis for each sub-basin, which was further divided into subareas based upon winter precipitation and vegetation type. For each subarea, a daily water budget was conducted to determine if estimated daily precipitation would be added to snow pack or water in the root zone depending on the temperature. If a snowpack existed then sublimation occurred, and if temperature exceeded 34 deg. F, then snowmelt contributed to water in the root zone. When there was no snow pack then water in the root zone was consumed by natural vegetation. When the water holding capacity of the root zone was exceeded, then runoff or deep ground water percolation was estimated. Total average annual surface and ground water inflow to the Alamosa Basin from the San Juan Mountains was reported at 2,027 kaf/yr. In Table 1, we estimated the contribution of surface water and ground water as 1,137 kaf/yr (56%), and 890 kaf/yr (44%), respectively. This was based on Hearne and Dewey's (Table 7) where surface water outflows are compared to estimated total water outflows, and surface water outflows averaged-about 56 percent of total outflows. Surface water inflows from the Sangre de Cristo Mountains were estimated using a regression model based on basin area and mean winter precipitation. The regression model was based on 16 basins, 15 in New Mexico and one in Colorado. The regression model had a correlation coefficient of 0.96, which is a very strong relationship. Ground water inflows were estimated negligible based on geology of the Sangre de Cristo Mountains, and the location of the stream gages used for the regression analysis. The mean annual surface inflow to the Alamosa Basin was reported at 246 kaf/yr. Precipitation inflow (1,086 kaf/yr) to the Alamosa Basin was computed based on an average precipitation of .64 ft/yr times 1,700,000 acres (rounded). The average precipitation value was computed from the long-term mean of six climate stations in the Alamosa Basin as reported by Davis Engineering Service, Inc. in 1977. 4.2.2 Consumptive Uses -Evapotranspiration (Et) in the Alamosa Basin was determined for crop lands, water course and riparian areas, and for five classifications of undeveloped land where the ground water table was less than 2 feet, 2-4 feet, 4-6 feet, 6-12 feet and greater than 12 feet below land surface. Irrigated crop areas were estimated from maps by U.S. Soil Conservation Service and Colorado Sate University Experiment Station. Cropping patterns were derived from reports by Colorado Crop and Livestock Reporting Service. A total of 710,000 irrigated acres was determined from the land use maps, and consisted of 320,000 acres of harvested crops (alfalfa, other hay, grain and potatoes), 180,000 acres of irrigated pasture, and 210,000 acres ofnon-irrigated pasture. Potential crop consumptive use was computed using the modified Blaney-Griddle method as reported by Davis Engineering Service in 1977. Irrigated crops were estimated to have a full water supply from a combination of effective precipitation, irrigation water, and shallow ground water. Non-effective precipitation on irrigated lands was also estimated to be consumed by evaporation. Total Et estimates ranged from 1.6 ft/yr rg_task8-1_2001-08.doc 10 of 19 Apri16, 2001