Modeling Surface Water - Groundwater Interactions in the Tulare Lake Basin, California, USA

The Tulare Lake Basin (TLB) of California, a vital agricultural hub that covers the southern part of the Sierra Nevada and the Central Valley, is experiencing severe water scarcity due to climate change, rising water demand, and extensive groundwater depletion. This study leverages an integrated hydrological model to quantify Surface Water-Groundwater (SW-GW) interactions in the TLB. We focus on understanding seasonal water balance trends, variability in groundwater recharge, and the impact of snowmelt on groundwater storage. The integrated SWAT+gwflow model was calibrated and validated using observations of streamflow, evapotranspiration, snow water equivalent, and groundwater head. Our results revealed a decreasing trend in groundwater storage (-4.77 mm/year), with a greater deficit during prolonged droughts.  Most of the groundwater fluxes have negative trends, including SW-GW exchange, saturated excess flow, and lateral flow. Boundary inflow exhibits a positive trend due to inflow from adjacent regions driven by hydraulic gradients caused by local groundwater depletion. Snowmelt emerged as a critical driver of groundwater recharge in the TLB, showing a stronger correlation in the spring (R² >0.58) than fall and winter seasons (R² < 0.5). These findings suggest exploring alternative means for groundwater recharge, as mountain snowpack is expected to decline in a warmer climate, such as capturing winter flood flows and the need for adaptive strategies to mitigate long-term water stress in the basin.

Keywords:  Coupled Model; SW-GW Interactions; TLB