On the role of groundwater and soil texture in the regional water balance: An investigation of the Nebraska Sand Hills, USA

Partitioning of precipitation between evapotranspiration and runoff at the basin scale is primarily controlled by climate and basin characteristics. Here we use the Budyko hypothesis to investigate the impacts of soil texture and groundwater ( e. g., baseflow) on annual and long-term mean annual water balances of basins in a semiarid region located in and around the Sand Hills region of Nebraska, USA. Native grasslands are dominant across the study area with soil textures dramatically different for the Nebraska Sand Hills ( primarily sand deposits) and the adjacent regions ( mainly less permeable silt loam). For each basin, long-term hydrologic and meteorological data are obtained, and a baseflow index is calculated using daily streamflow data to quantify the groundwater contribution to streamflow. We found sound field evidence that suggests the control of soil texture on mean annual water balance and the impact of groundwater storage on interannual variability of water balance at catchment scales, which is usually seen at plot scales and in theoretical models. Our results show that compared to the basins located in the adjacent regions, the Nebraska Sand Hills basins experience much higher long-term mean annual surface runoff ratios ((R) over bar/(P) over bar) and thus lower ratios of ((P) over bar - (R) over bar)/(P) over bar, where (P) over bar is the long-term mean annual precipitation. The high baseflow index and its positive correlation with the ratio of (R) over bar/(P) over bar in the Sand Hills basins illustrate the role of lower ( higher) evapotranspiration ( recharge) rates. On annual time scales, the baseflow-dominated basins exhibit a negative relationship between annual ( P - R) / P and annual aridity index (i.e., the ratio of annual potential evapotranspiration to annual precipitation), which deviates from the original Budyko hypothesis. Moreover, with decreasing baseflow index, this negative relationship gradually transits into a positive relationship that follows Budyko's curves. Our results suggest that soil textural differences may strongly modify the impact of climate on regional water balance. These findings may have important implications for managing water resources under a fluctuating climate.