CHARACTERIZATION OF THE WATER TABLE RESPONSE TO CLIMATE AND VEGETATION CHANGES IN THE NEBRASKA SAND HILLS

The Sand Hills are a unique ecosystem that stretches across north central Nebraska and into southern South Dakota and varies widely in climate, geomorphology and biota from east to west. Climate, vegetation, soils, and hydrology have all played key roles in the establishment of the Sand Hills. The Barta Brother's Ranch research site is located near Rose, in north central Nebraska in a semi-arid environment where prairie grasses cover small gently rolling sand and silt dunes.

The goals of this research were to determine the direction and magnitude of regional groundwater flow, and to assess the effects that climate and vegetation have on recharge to the water table. Water levels were monitored continuously using transducers and dataloggers at six wells drilled to various depths. Water levels in wells located in the valleys respond rapidly to precipitation events, as the water table is close to the surface. However, water levels in wells located atop dunes do not typically respond to the magnitude of individual precipitation events, but rather as a function of event frequency and duration. Water temperature data from the wells show a lag in the groundwater response to seasonal recharge. Groundwaters in the valleys have a four month temperature delay. Beneath the dunes the lag may be longer, or may be absent with yearly values equal to groundwater temperature. Chemical and isotope samples were collected in the summer of 2004, and will be again in 2005. Preliminary results of chemical sampling indicate that the groundwaters have low TDS, and tritium and oxygen isotope results reveal that waters are modern of meteoric origin. Ten heat dissipation sensors were installed as one deep vertical profile in the summer of 2005 to characterize water movement within a dune, and to estimate recharge. It is believed that the dunes, with larger surface area, allow greater recharge to occur, while the valleys recharge more quickly, but lose more water to ET.