Representing Bidirectional Hydraulic Continuum Between the Stream and Hillslope in the National Water Model for Improved Streamflow Prediction
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AbstractAlthough hydraulic groundwater (GW) theory has been recognized as a promising tool for understanding the role of the aquifer(s) in the surfacesubsurface hydrologic cycle, the integrated modeling community still lacks a proper hydrologic structure to apply the wellstudied theory to largescale hydrologic predictions. This study aims to present a novel hydrologic structure that enables the Boussinesq equationbased depiction of the bidirectional streamhillslope processes for applying hydraulic GW theory to largescale model configurations. We integrated the BE3S's (Hong etal., 2020, https://doi.org/10.1029/2020wr027571) representation scheme of the catchmentscale streamhillslope continuum into the National Water Model (NWM) and applied the modified NWM (i.e., the NWMBE3S) to three major basins in Texas (i.e., the Trinity, Brazos, and Colorado River basins). Since the NWM currently relies on a single reservoir model for baseflow simulation, we used the Boussinesq aquifer as an alternative subsurface hydrology routine and evaluated its predictive skill and efficacy. We identified that the implemented Boussinesq aquifer(s) in the NWMBE3S yielded noticeable improvements in predicting streamflow for aquifers that exhibited higher nonlinearities in the observed recessions. The varying degree of improvements in streamflow predictions per the recession nonlinearities demonstrated not only (a) the algorithmic enhancement of subsurface hydrology (physics) but also (b) the applicability of the Boussinesq theorybased depiction of the streamhillslope twoway continuum. We diagnosed each stream's state based on the bidirectional streamhillslope exchanges and identified the dominant processes (i.e., river infiltration or baseflow) that were represented spatially in the NWMBE3S.
