The primary objective of this study was to determine the average annual net rate and areal variation of natural recharge to the Ogallala Formation in the Northern High Plains of Colorado. This unconfined aquifer underlies approximately 9300 square miles in northeastern Colorado and is capable of yielding large amounts of water to wells. Natural recharge to the Ogallala Formation is derived from precipitation falling within the basin. However, none of the previous investigations has attempted to describe the areal variation of groundwater recharge a Assuming that the Ogallala Formation was in a state of dynamic equilibrium prior to large scale development, the steady state form of the Boussinesq equation was written infinite difference form and a solution for the net recharge rate was obtained on the CDC 6600 digital computer. To obtain the area l variation in net natural recharge, the Northern High Plains was subdivided into six square-mile grids, and a net recharge rate obtained for each grid. A wide variation in net recharge was obtained. In general, net recharge rates in excess of two inches per year are confined to the sandhill portion of the study area. Most of the area with surface exposures of Peorian Loess and Ogallala Formation have net recharge rates of one inch per year or less. The total volume of net recharge to the Ogallala Formation was computed to be 405,000 acre-feet per year or an average of about 0.82 inch per year. The subsurface underflow across the state line into Kansas and Nebraska was 175,000 acre-feet per year and the net groundwater runoff into the North and South Forks of the Republican River and Arikaree River was calculated to be 75,000 acre-feet per year. Another 50,000 acre-feet per year of groundwater is consumed by evapotranspiration. An area of substantial discharge was disclosed along the subsurface outcrop of the White River Group and it is hypothesized that groundwater is being discharged from the Ogallala Formation into the White River Group. Another area of unexplained natural discharge is along the Smoky Hill River near Cheyenne Wells. This may also be the result of subsurface discharge, and a thorough investigation of the subsurface geology in the area is needed.

Also has numbering: AET66-67DLR37.