Remote sensing and in situ-based estimates of evapotranspiration for subirrigated meadow, dry valley, and upland dune ecosystems in the semi-arid sand hills of Nebraska, USA

Water consumed through evapotranspiration (ET) impacts local and regional hydrologic regimes on various spatial and temporal scales. Estimating ET in the Great Plains is a prerequisite for effective regional water resource management of the Ogallala (High Plains) Aquifer, which supplies vital water resources in the form of irrigation for extensive agricultural production. The Sand Hills region of Nebraska is one of the largest grass-stabilized eolian (windblown) sand dune formations in the world, with an area of roughly 50,000-60,000 km ² that supports a system of five major land cover types: (1) lakes, (2) wetlands (with lakes, ~5%), (3) subirrigated meadows (water table is within ~1 m of surface; ~10%), (4) dry valleys (water table is 1-10 m below surface; ~20%), and (5) upland dunes (water table is more than 10 m below surface; ~65%). Fully understanding the hydrologic regime of these different ecosystems is a fundamental challenge in regional water resource assessment. The surface energy and water balances were analyzed using Bowen Ratio Energy Balance Systems (BREBS) at three locations: (1) a meadow, (2) a valley, and (3) an upland dune. Measurement of the energy budget by BREBS, in concert with Landsat remote sensing image processing for 2004 reveals strong spatial gradients between sites in latent heat flux that are associated with undulating topographic relief. We find that daily estimates of ET from BREBS measurements and remote sensing agree well, with an uncertainty within 1 mm, which is encouraging when applying remote sensing results across such a broad spatial scale and undulating topography.