WATER-BUDGET MODEL OF THE SOUTH-CENTRAL SAND HILLS OF NEBRASKA

The Sand Hills of Nebraska are typified by dunes separated by broad valleys, many of which contain lakes and/or wet meadows. The area is underlain by a vast amount of groundwater in the hydraulically well-connected Miocene, Pliocene, and Plio/Quaternary sediments that constitute the groundwater reservoir. Development of irrigated agriculture in the Sand Hills, since the early 1970s, has led to concern over the future of the area\u27s groundwater and lake-water resources. An excellent way to understand the groundwater/lake resource is to examine the water budget of the area and to define the nature of the linkage between the lakes and the groundwater reservoir. Herein is a study of the magnitude of the components of the water budget and an analysis of the nature of the connection between the two water resources. Two complementary models were used to quantify flow relationships. The Hubbard and Hanks Soil Moisture Model was adapted to permit year-round evapotranspiration estimates and was employed to determine drainage to the groundwater reservoir and the soil moisture deficit. The Regional Aquifer Simulation Model, RAQSIM, was adapted to accommodate lakes and the complex evapotranspiration calculations necessitated by the subdivision of the study area into evapotranspiration zones. The adapted RAQSIM was used to simulate the groundwater flow system. Most study-area lakes were determined to be net groundwater sinks and all were found to have some degree of hydraulic connection with the groundwater reservoir. During a 3$1\over 2$-yr simulation period, 44.68 in. of precipitation fell in the study area. During the same period, groundwater recharge from precipitation, the groundwater contribution to stream base flow, and groundwater discharge to evapotranspiration, were determined to be 30%, 16% and 5% percent of precipitation, i.e. 13.26 in., 7.21 in., and 2.13 in., respectively