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Nine years of groundwater monitoring data has documented the important influence that topographic relief and location in the groundwater flow system have on the hydrologic function of interdunal valleys. The western "wet" valley at the Gudmundsen Sandhills Laboratory in central Nebraska, which is a net discharge area, is more strongly buffered from the effects of annual-scale climatic variability than the eastern "dry" valley. The east valley is generally an area of net recharge and as such is more responsive to climatic variability. This study employed a simple water balance approach to estimate evapotranspiration (ET) from water level measurements in the west valley for four specific time intervals in 1998-99 that included growing and senescence periods. The estimates of ET ranged between 5-6 mm/day in the mid-growing season and 2-3 mm/day during the period of senescence.