Integrated Surface and Groundwater Modeling of Martis Valley, California, for Assessment of Potential Climate Change Impacts on Basin-Scale Water Resources

This study develops a modeling framework to better understand surface and groundwater interactions in the Martis Valley Basin. Prior to the current study there were no physical or statistical models of the Martis Valley Basin that could be used for long term water resource planning in the Basin. Currently, groundwater is the primary source for municipal and industrial water supply in Martis Valley, and surface water from the surrounding watersheds are partly stored in Donner, Prosser reservoirs and allowed to runoff into the Truckee River. This study presents the development, calibration, and verification of the physically based surface and groundwater GSFLOW model for Martis Valley. The model was calibrated for the historical 1981-2011 observed period. Various observations were used to calibrate the model during different periods, which helped verify the model’s ability to capture the dynamics of the hydrologic system. The model was calibrated to multiple fluxes and states in the hydrological system including streamflow, snow water equivalent, groundwater heads, and wetland and spring locations. In addition to comparing fluxes to multiple observations, consistency at internal gauges was also verified apart from simulating fluxes at the outlet. For future projections of hydrology, the GSFLOW model was forced with five future climate scenarios provided by CH2M HILL. Future simulations of snow water equivalent indicate reductions for all scenarios due to increasing temperatures. Groundwater recharge and its contribution to streamflow also decrease in the future for the warmer drier and hotter drier scenarios. The reduction in snowpack and earlier melt complicates the seasonally dependent nature of water resources and when combined, these results will likely add further stress to long term water availability in the Martis Valley Basin and the downstream consumers of the waters generated in the Basin.