Exploring relationship between evapotranspiration and groundwater level fluctuations in different land covers
Department of Civil Engineering, University of Nebraska–Lincoln
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Groundwater is often the most reliable source of water in arid and semiarid regions. In order to achieve sustainable groundwater management, specifically in the agricultural sector, a more detailed accounting of water balance terms in space and time should be investigated. While the impact of evapotranspiration (ET) on stream flow has been discussed in many studies, the relationship between ET and groundwater levels has yet to be sufficiently explored. One of the largest concerns of water resource planners in groundwater conservation is water losses through ET. Some planners believe that by controlling deep rooted plants or replacing them with shallower rooted plants would reduce groundwater losses through ET processes and potentially provide greater water resources for human consumption. In this paper, we investigated the relationship between ET and groundwater level fluctuation in 3 different land covers across a 1.2 km gradient in central Nebraska: cottonwood, wetland, and grassland areas. Groundwater observation data obtained from monitoring wells in the different land cover areas at the study site were used to measure the groundwater level fluctuations. In addition, a Mapping Evapotranspiration at high Resolution and with Internalized Calibration (METRIC) model, a satellite-based image-processing model, was used to estimate actual ET at ~30 m spatial resolution. We used eight Landsat 8 images from the study site to estimate actual ET for each well location from May to July 2014. The results reveal that while there was a strong relationship between hourly actual ET and hourly groundwater level fluctuations in the cottonwood area for most of the days during the study period, ET did not directly affect the groundwater level in the wetland and grassland areas. The results here indicate the diurnal influence that the deep rooted plants have on localized water table elevations and ET fluxes. When combined with detailed groundwater and surface water flow models, the results of this work can be used in aiding decisions for water managers in water-limited environments.