Regional hydroclimatic drivers of variability in surface water diversions throughout the Central Valley, California

Increasing water demand in California's Central Valley (CV) and the volatility of precipitation (P) has exposed the vulnerability of water resources in the region. Poor accounting for surface water use prevents development of accurate conjunctive budgets, leading to uncertainties in how groundwater storage responds to extreme P or droughts, and therefore how to manage water resources. Added complexity stems from the large number of farms and local, state, and federal agencies that use and manage water. We analyzed the long-term record of spatially distributed surface water diversions for > 500 locations in the CV, aiming to understand the influence of regional hydroclimate over the historical period 1979-2010. We incorporated Sierra Nevada snow water equivalent (SWE), surface reservoir storage in the region, gridded P, and basin inflows, all on monthly time steps to investigate their spatio-temporal linkages with diversions.

Despite the complexities of water management throughout the CV, variability during the seasons when the majority of water (>85%) is extracted from the surface (Apr-Sep) is still strongly correlated with regional hydrological components. The first half of the main diversion season (Apr-Jun) is responsible for 30% of the annual diverted volume, and there is a clear latitudinal gradient with southern basins (i.e. Tulare) extracting more water than northern ones (Sacramento). This pattern is regionally correlated most strongly with P (r 0.8), and particularly in the south (r 0.9), which suggests that wet springs deliver enough P in the Sacramento basin to reduce diversions, whereas in the Tulare basin, Apr-Jun P is still a small fraction of demand, but excess flows allow more water to be diverted. In contrast, the latitudinal gradient diminishes in Jul-Aug, and diversions are high throughout the valley during wet years. Demand peaks, and P is a decreasing proportion of it, including in the Sacramento. The resulting variability in Jul-Sep diversions is strongly correlated to water year accumulated P, as well as Sierra SWE and reservoir storage throughout the CV (r > 0.8 for all terms). This season accounts for 60% of the annual diverted volume, thus the regional behavior of water diversions is remarkably coherent with hydroclimate variables, regardless of the dominant land-use or managing agency.