Evaluation of the storage and evapotranspiration terms of the water budget for an agricultural watershed using local and remote-sensing measurements

The understanding of the water budget components, especially evapotranspiration and the amount of water stored in the watershed, is key to water resources management and hydrological modeling in general. In this work, we present the analysis of 13 hydrological years' measurements of discharge, precipitation, evapotranspiration, and soil moisture over two agricultural watersheds in Eastern Nebraska, United States. These data were used to investigate several facets of the hydrological water budget: water-and energy-balance closures, water budget residues (here, the water budget residue is the remaining imbalance after all water budget terms have been considered; see Eq. (13)), storage in the unsaturated and saturated soil layers, and the evapotranspiration derived from the water budget. Remote-sensing data were also used to assess the spatial variability of net radiation and soil moisture over the basins. Adjusting the eddy-covariance measured evapotranspiration to close the energy budget agreed, in the long term, with the water budget evapotranspiration estimates. The evapotranspiration estimated with storage calculated with soil moisture measurements and recession analysis presented the best agreement with the (adjusted) measured evapotranspiration, but considering only either the vadose zone or the saturated zone for storage estimates still produced reasonable results. We found considerable variability in soil moisture over the basins in the final period of the growing season, coinciding with the time of the year when water budget residues were largest.