WHAT DO YOU GET WHEN YOU CROSS A HYDROGEOLOGIST, GEOCHEMIST, PHYSICIST, PLANT PHYSIOLOGIST, AND A GRASSLAND ECOLOGIST: AN INTERDISCIPLINARY PROJECT IN THE NEBRASKA SAND HILLS

The juxtaposition of an abundant underground water supply with a semi-arid, desert-like landscape creates a fascinating and complex ecosystem in the Nebraska Sand Hills. To document the dynamic interaction between the biosphere, evapo-transpiration and ground water, a pilot study was initiated in 2001 to measure fluxes of water vapor, energy and groundwater in two hydrologically distinct interdunal valleys at the Gudmundsen Sand Hills Laboratory north of Whitman, Nebraska. These valleys are representative of the two primary types of interdunal valley ecosystems in the Sand Hills, a dry short-grass valley and a sub-irrigated wet meadow. In each valley a study site was established that included an Energy Balance - Bowen Ratio (EBBR) measurement system for water vapor and energy fluxes and multi-level transducers for the measurement of groundwater levels and temperature. Two EBBR experiments were conducted from 9/19/02 to 11/16/02 and 3/12/03 to 4/29/02. Groundwater fluctuations have been measured every half hour since 7/26/02.

Groundwater measurements in the wet valley support previous work that indicate significant upward, but variable, vertical gradients ranging from 0.023 to 0.073. Evapotranspiration (ET) rates were initially about 3.5 mm/day and decreased almost linearly to about 0.5 mm/day by the end of the first experiment. At the beginning of the second experiment ET was nearly 1.0 mm/day and increased linearly to about 5.0 mm/day. The dry valley site is characterized by variable downward vertical gradients ranging from –0.004 to –0.014. ET rates in the dry valley in mid-September were 0.5 mm/ day and were zero about 10/17/02. During the spring, ET was variable, but had a mean of 0.5 mm/day and a generally progressive increase after 4/20/02. Individual daily values were as high as 4.0 mm/day.

Our preliminary results indicate that the biosphere and groundwater are strongly coupled in the wet valley, but less so in the dry valley. Our measured peak ET rates of about 3.5 mm/day from the wet valley are similar to calculated potential evapo-transpiration (PET) rates of between 3.4 and 4.2 mm/day for the same time of year. This suggests that calculated PET may be representative of wet valleys. However, the PET values are not representative of the more extensive dry valleys and related uplands in the Sand Hills.