Exploring catchment connections with integrated hydrologic models: system interactions and responses to groundwater extraction and climate change in the San Joaquin River basin

Abstract

The water that we rely on exists as the result of complex interacting processes that occur over a range of temporal and spatial scales. Historically hydrologic research has been directed at the study of individual processes or components of the larger hydrologic or water resource system, but the advent of sophisticated integrated hydrologic simulation tools in recent decades has enabled a more comprehensive approach to studying the factors that control the quantity, timing, and location of water in the environment. Specifically, such tools provide a platform through which to quantify fluxes and stores in a physically consistent way across large spatial and temporal extents and to test and isolate the impact of specific anthropogenic changes within and among connected hydrologic domains like the subsurface, land surface, and the atmosphere. This dissertation makes use of the integrated hydrologic modeling platform built around ParFlow (i.e. ParFlow-CLM and ParFlow-WRF) to explore the nature of hydrologic connections and the propagation of change across the regional-scale San Joaquin River basin system. Simulations of the integrated land surface and subsurface system expose the dependence of the Central Valley aquifer on the adjacent mountain blocks while highlighting the dynamic interaction of surface processes in supporting streamflow and groundwater-surface water interactions. Analysis of atmospheric response over a region of declining groundwater levels shows that boundary layer height is directly tied to water table depth, although local increases in boundary layer height depend on the regional configuration of the water table. Atmospheric perturbations translate to changes in terrestrial hydrology as well. Simulations of uniform warming across the basin shows that increasing temperatures initiate a cascade of effects that starts with a shift from snow to rainfall and declining snowpack and propagates through an elevation-dependent shift in runoff, evapotranspiration, and saturated groundwater recharge. These local scale effects produce an aggregate response characterized by precipitation-dependent decreases in streamflow and increases in evapotranspiration.