Sonoma Valley Groundwater

Following the 2014 South Napa earthquake near the end of a long drought in Central California, streamflow in Sonoma Creek increased and showed amplified daily fluctuation. No such changes occurred in the shallow groundwater, however, suggesting that the stream was hydraulically disconnected from the subsurface outside the stream corridor. Here we show that the amplified fluctuation reflected increased interaction between the post-seismic rising water table and plant roots in the riparian zone, according to results from numerical simulation constrained by streamflow data and hydraulic properties of riparian sediments. Evapotranspiration during the day kept the water table beneath the riparian zone, lowering the discharge to the stream. At night, the water table rose and increased discharge to the stream. We conclude that substantial local differences occur in earthquake-induced interactions between groundwater and the surface, which may influence our understanding of the spatial scale of earthquake impacts on vegetation and ecosystems.