Representing Hydraulically-Based Stream-Hillslope Continuum in the National Water Model for Improved Hydrologic Predictions: Catchment-Scale Application of Boussinesq Theory

Abstract

Although hydraulic groundwater theory (i.e., Boussinesq flow) has been understood as a viable approach for catchment-scale understanding of the role of the aquifer(s) in the terrestrial water and energy cycle, the land surface modeling (LSM) community still lacks a proper hydrologic structure to utilize the well-studied theory for large-scale terrestrial predictions. This study aims to present a new LSM framework that enables the Boussinesq equation-based depiction of the stream-hillslope two-way continuum (i.e., bidirectional interactions among the vadose zone-phreatic aquifer-river) for applying the Boussinesq groundwater theory in a large-scale model configuration. Specifically, this study developed a new numerical scheme representing the catchment-scale stream-hillslope continuum and integrated it into the National Water Model (NWM). The NWM-BE3S LSM was applied to three major basins in Texas (i.e., the Trinity, Brazos, and Colorado River basins) to test the effects of the novel physics on improving the predictability of terrestrial water/energy fluxes. We identified the implemented stream-hillslope continuum scheme showed 'more' noticeable improvements in land surface water/energy fluxes (e.g., evapotranspiration) as well as streamflow dynamics as aquifers exhibited higher nonlinearities in the observed streamflow recession curves. The varying degree of improvements in model outputs according to the streamflow recession characteristics demonstrates (1) the applicability of the Boussinesq theory-based depiction of the stream-hillslope exchange processes and (2) the algorithmic enhancement in the NWM-BE3S's structure/capability (compared to the original NWM). Overall, our results revealed the importance of catchment-scale conservation of hydraulic consistency among the vegetation-land surface-critical zone-aquifer-river for the improved predictability of terrestrial water and energy cycles.