Understanding Hydrologic Connectivity in a Relatively Unmanaged Watershed by Combining Integrated Hydrologic Modeling, Remote Sensing, and Isotope Analyses

Scientists struggle to identify the details of hydrologic connections between headwaters and downstream regions because of complex subsurface flow paths and nonlinear ecosystem interactions. This uncertainty in hydrologic connection has important water resource implications in water-stressed regions such as California's Central Valley. Groundwater pumping and irrigation play a key water supply role in this region; better groundwater management is particularly important in the face of recent droughts and a changing climate. Here we present a study in California's Cosumnes River Watershed, the last watershed in California's Central Valley without a major dam, to better understand watershed processes and flow paths across the Sierra Nevada-Central Valley interface.

We present a novel combination of techniques designed to elucidate the main dynamics of the system, including simulations from an integrated hydrologic model with Lagrangian particle tracking, remote-sensing based estimates of evapotranspiration, and isotope analyses. We quantitatively compare the simulation's spatial pattern of evapotranspiration with the spatial pattern determined from the MODIS evapotranspiration estimate downscaled to 30m resolution using a random forest model. This approach demonstrates the importance of both the underlying geologic structure and land cover to properly represent subsurface flow and thus obtain accurate estimates of future water resources. The isotope analyses constrain flow paths determined via a new particle tracking component in the hydrologic simulator that resolves the sources and ages of water that reach the Cosumnes River. The subsurface flow constraints from both evapotranspiration and isotope analyses also build understanding of the ecohydrologic processes in the watershed. Our approach focuses on the most discriminative information from each technique to better understand flow paths and hydrologic connectedness in the Cosumnes.