Geophysics-Informed Hydrologic Modeling of a Mountain Headwater Catchment for Studying Hydrological Partitioning in the Critical Zone

Hydrologic modeling has been a useful approach for analyzing water partitioning in catchment systems. It will play an essential role in studying the responses of watersheds under projected climate changes. Numerous studies have shown it is critical to include subsurface heterogeneity in the hydrologic modeling to correctly simulate various water fluxes and processes in the hydrologic system. In this study, we test the idea of incorporating geophysics-obtained subsurface critical zone (CZ) structures in the hydrologic modeling of a mountainous headwater catchment. The CZ structure is extracted from a three-dimensional seismic velocity model developed from a series of two-dimensional velocity sections inverted from seismic travel time measurements. Comparing different subsurface models shows that geophysics-informed hydrologic modeling better fits the field observations, including streamflow discharge and soil moisture measurements. The results also show that this new hydrologic modeling approach could quantify many key hydrologic fluxes in the catchment, including streamflow, deep infiltration, and subsurface water storage. Estimations of these fluxes from numerical simulations generally have low uncertainties and are consistent with estimations from other methods. In particular, it is straightforward to calculate many hydraulic fluxes or states that may not be measured directly in the field or separated from field observations. Examples include quickflow/subsurface lateral flow, soil/rock moisture, and deep infiltration. Thus, this study provides a useful approach for studying the hydraulic fluxes and processes in the deep subsurface (e.g., weathered bedrock), which needs to be better represented in many earth system models. The precipitation received in a watershed is usually partitioned into streamflow, plant water use, soil/rock moisture, and groundwater. While many of these components can be directly measured in the field or estimated from hydrometeorological measurements, the moisture stored in the subsurface (soil and rock layers) is difficult to quantify and measure. Using the computer to simulate the water flow and storage in a watershed provides a useful tool for quantifying water partitioning in a catchment. Previous research highlighted the importance of including both above-ground and below-ground variations in computational modeling. Compared to above-ground heterogeneity, it is much more challenging to characterize below-ground heterogeneity because we cannot see the variations in the subsurface. In this study, we propose to determine the subsurface heterogeneity/variations with geophysical imaging (more specifically, the seismic refraction method), and the geophysics-obtained subsurface model is then incorporated into the computational simulation of a watershed or catchment. A small mountainous headwater catchment in Idaho is used to demonstrate this method, and the results show that the new modeling well reproduces the field-measured streamflow of the catchment. This new modeling also gives reliable estimates for some properties that are difficult to determine using traditional methods. With the changing climate, the proposed new method will help better understand how a catchment will respond to the projected weather, such as extreme snowfalls or severe droughts. The 3D velocity model of a mountain catchment was created based on the results of a series of 2D seismic refraction testsThe 3D subsurface critical zone structure was extracted from the velocity model and incorporated into hydrologic modelingGeophysics-informed hydrologic modeling provides valuable additional information on the hydrological partitioning in the catchment

Automated summary

Hydrologic modeling is a useful approach for studying water partitioning in catchment systems. This study shows the importance of including subsurface heterogeneity in the modeling and proposes a method of incorporating geophysics-obtained subsurface structures. The results demonstrate that this geophysics-informed modeling approach fits field observations well and accurately quantifies key hydrologic fluxes in the catchment.