Influence of Subsurface Critical Zone Structure on Hydrological Partitioning in Mountainous Headwater Catchments

Headwater catchments play a vital role in regional water supply and ecohydrology, and a quantitative understanding of the hydrological partitioning in these catchments is critically needed, particularly under a changing climate. Recent studies have highlighted the importance of subsurface critical zone (CZ) structure in modulating the partitioning of precipitation in mountainous catchments; however, few existing studies have explicitly taken into account the 3D subsurface CZ structure. In this study, we designed realistic synthetic catchment models based on seismic velocity-estimated 3D subsurface CZ structures. Integrated hydrologic modeling is then used to study the effects of the shape of the weathered bedrock and the associated storage capacity on various hydrologic fluxes and storages in mountainous headwater catchments. Numerical results show that the weathered bedrock affects not only the magnitude but also the peak time of both streamflow and subsurface dynamic storage. In mountainous terrains, precipitation falling as snow and rain not only flows into streams but also infiltrates into the subsurface, replenishing the subsurface water storage. This stored water in the subsurface (including both soil moisture and rock moisture) supports various ecohydrological processes such as groundwater recharge and plant water use. In this study, we use computational simulation to study how the below-ground heterogeneity affects the partitioning of precipitation into streamflow, subsurface water storage, plant water use, and deep groundwater recharge. Here, below-ground heterogeneity refers to the depth extent of weathered rock at the hillslope- or catchment-scale. Simulation results show that both the annual magnitude and weekly variations of the streamflow and subsurface storage change noticeably as the below-ground weathering pattern varies, in particular under a snow-dominated weather scenario. The outcome of this study will potentially help stakeholders develop more informed water management practices in the face of a warming climate and more extreme weather events. Integrated hydrologic modeling of catchments with different subsurface critical zone structures was performed Simulation results show that the shape of the weathered bedrock and associated storage capacity affect various hydrologic processes in the catchment The effect of weathered bedrock on hydrological partitioning is more significant under weather conditions featuring large snow fractions