An Explicit Scheme to Represent the Bidirectional Hydrologic Exchanges Between the Vadose Zone, Phreatic Aquifer, and River

Abstract

Understanding groundwater storage variations as a result of effects from both the vadose zone and the river is of critical importance in the hydraulically connected surface-subsurface system. In this study, we present a novel Bidirectional Exchange Scheme in Surface and Subsurface (BE3S) that represents bidirectional exchanges between the vadose zone, phreatic aquifer, and river. The approach enables explicit representations of each flow regime while conserving mass, and successfully yields solutions of the coupled system for multiple temporal resolutions. We test the scheme by comparing the BE3S-derived outputs against other models and corresponding observational data. We apply the scheme to simulate hydrologic states in a reach of the Brazos River in Southeast Texas, such as soil moisture content, groundwater level, and river stage as well as net subsurface discharge fluxes. Good agreements between the simulated and observed data for all the components show the suitability of the proposed scheme in modeling the bidirectional flows and exchanges. We also assess how the bidirectional hydrologic exchanges are affected by adjacent flow domains. We find that vertical percolation is significantly affected by unsaturated soil thickness, resulting in the spatial variability of vertical percolation across sloping topography. Hilltop-tovalley convergent groundwater flow is also found to impede vertical percolation in river valleys due to shallow water table while facilitating percolation in the hilltops. The capability of the presented scheme that accounts for the topographically driven lateral groundwater flow and drainage dynamics provides the potential to enhance the representation of the surface-subsurface system in Earth System Models (ESMs).

Effects of Heterogeneous Stream‐Groundwater Exchange on the Source Composition of Stream Discharge and Solute Load

“…We recognize that the vadose zone could have an important effect on stream-groundwater exchange (Frei et al, 2009;Hong et al, 2020) and solute composition of stream water (Neal et al, 2012;Zhi et al, 2019). However, limited by the capacity of MT3DMS, we used the STR package to simulate the stream, which does not consider the unsaturated zone.…”

Representing Bidirectional Hydraulic Continuum Between the Stream and Hillslope in the National Water Model for Improved Streamflow Prediction

“…Time‐dependent hydrologic states in each flow domain (e.g., groundwater level [GWL] and river stage) were explicitly used in establishing interfacial boundary conditions between adjacent domains. The BE3S connects two types of interfaces (i.e., the interface between the vadose zone‐phreatic aquifer and the phreatic aquifer‐stream) and simulates the potential‐driven exchanges at the interfaces through hydraulic head‐based coupling (Hong et al., 2020). Since the BE3S handles multiple processes that involve different temporal scales, care needs to be taken when defining initial and boundary conditions at j + 1th time step as the result of various hydrologic states and fluxes at j th time step.…”

“…Therefore, this study aims to represent the catchment‐scale stream‐hillslope bidirectional interactions based on the Dupuit‐Boussinesq assumption, hereafter referred to as the Boussinesq aquifer, in a large‐scale model configuration. As the theoretical basis of hydraulic GW theory, the Boussinesq aquifer can describe the hillslope storage and release dynamics as the response to streamflow variations (i.e., river stage fluctuations) based on catchment‐scale lateral hydraulic gradients (Hong et al., 2020; Hornberger et al., 1970; Lockington, 1997; D. E. Rupp & Selker, 2006). To understand the effects of the implemented Boussinesq aquifer with regard to parametric efficiency and prediction accuracy, we specifically selected the National Water Model (NWM) (i.e., the WRF‐Hydro NWM configuration) (Gochis et al., 2021) and integrated the stream‐hillslope interaction algorithm in the Bidirectional Exchange Scheme in Surface and Subsurface (BE3S) (Hong et al., 2020) as an alternative subsurface hydrology routine.…”

Advancing Process Representation in Hydrological Models: Integrating New Concepts, Knowledge, and Data

“…This finding emphasizes the need for new approaches to observe and evaluate groundwater flow at relevant scales. Hong et al (2020) have developed a bidirectional exchange scheme applied to the Brazos River in Texas, USA, for studying the interaction of subsurface water and streamflow, which allows for a more rigorous representation of interactions of subsurface flow, the saturated zone, and the stream. The impact of root exudation on subsurface processes was considered in Roque-Malo et al ( 2020) with a biogeochemical view on the nutrient cycle in the soil.…”