Impacts of increasing precipitation variability on landslide triggering in the San Francisco Bay Area (California, USA) due to climate change

Generation of sufficient soil pore-water pressure to trigger failure of rainfall-induced landslides depends on the combination of antecedent soil moisture and the intensity and duration of infiltrating rainfall during the triggering storm. Precipitation extremes and volatility are both projected to intensify in the future due to climate change, particularly in California (USA), which already experiences considerable interannual variability relative to the rest of the country as well as distinct seasonality. The effects of these changes on rainfall-induced landslides are not straightforward. 

Nominally, increasing precipitation intensities lead to more landslides, however above a certain threshold the efficacy will decrease as precipitation gets diverted from infiltration to runoff due to infiltration excess. Additionally, antecedent soil moisture conditions are likely to be affected by rising temperatures and changes in the timing and sequencing of storms relative to present. 

Here we explore the potential effects of these changes on landslide activity in the San Francisco Bay area using an ensemble of downscaled climate projections coupled to a 1-D hydrological model of transient, variably saturated flow in a 2-layer system. We first calibrate the model to site-specific groundwater observations and identify landslide days based on empirical hydrological thresholds. We then apply the model chain to historical and future (1950-2100) climate projections to assess changes in the characteristics and frequency of landslide triggering conditions due to the interplay of intensifying precipitation, rising temperatures, and changes in the frequency and timing of precipitation events. For shallow landslides, we project an increase in the frequency of both extreme landslide years and years with no landslide days at all, as well as a small increase the average annual number of landslide days overall.