Groundwater is being extensively developed for domestic, agricultural, and industrial use in a 2,320-square mile area in Laramie County, WY., bounded approximately by Horse Creek on the north, Nebraska on the east, Colorado on the south, and pre-Tertiary outcrops on the west. Currently (1977) about 47,300 acres of land are irrigated with groundwater. Groundwater levels are declining in some areas as much as 4 feet per year. The investigation was made to provide State water administrators with data on water-level changes resulting from present (1977) groundwater withdrawals and to provide a means of predicting the future effect of groundwater development. A digital model was developed of the hydrologic system in the post-Cretaceous rocks. The ability of the model to simulate the hydrologic system was determined by comparing the water-level changes measured at 37 observation wells located in areas of irrigation pumping with the water-level changes calculated by the model for 1971-77. Comparison of the measured and calculated changes showed agreement with a root-mean-square deviation of + or - 3.6 feet with 8 feet as the maximum deviation. It is concluded that the model adequately simulates present hydrologic conditions in the post-Cretaceous rocks and may be used to predict the effect of applied stress to the system. (USGS)
