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.

2014年南纳帕地震发生于加利福尼亚州中部长期干旱临近尾声之际，震后索诺玛溪（Sonoma Creek）的径流量显著增长，且日波动幅度进一步增强。但浅层地下水并未出现此类变化，这表明溪流与河道廊道以外的地下地层之间存在水力脱节现象。本研究基于河道流量数据与河岸沉积物水力特性约束的数值模拟结果，证实上述波动幅度增强现象实则反映了震后上升地下水位与河岸带（riparian zone）植物根系之间的相互作用强度提升。日间蒸散发作用使河岸带地下水位维持低位，进而降低了向溪流的补给量；夜间地下水位回升，溪流补给量随之增加。本研究最终得出结论：地震引发的地下水与地表相互作用存在显著的局域差异，这一发现或将改变我们对地震对植被与生态系统影响空间尺度的认知。