Evaluation of precipitation intensities conducive to shallow landslides in California

Shallow landslides occur periodically in California often in association with prolonged wet periods. These potentially destructive events occur in many areas of the state, but most frequently in the steep terrain of the Coast Ranges of central and Northern California and the Transverse and Peninsular Ranges of southern California. Soil saturation sufficient for shallow landslides often occurs once accumulated water year precipitation has reached 250-350 mm, variable across the state. Once antecedent moisture conditions are achieved, shallow landslides can be triggered even in cases of relatively moderate intensity rainfall. Intensity-duration thresholds for precipitation triggering shallow landslides are an ongoing topic of research; here we focus on three hourly thresholds suggested by USGS: 15 mm/h, 10 mm/h for two hours, and 5 mm/h for six hours. We explore how the frequency of precipitation meeting these thresholds varies across the state by season, location, time, and elevation. Extreme hourly precipitation values are also evaluated in a similar manner. Additionally, as atmospheric rivers (ARs) are an important feature in California cool season precipitation, we assess each extreme and observation over threshold for AR conditions.

This exploration uses precipitation data from the Remote Automated Weather Station (RAWS) network for the period 1995-2016. This network was selected for its placement of mid-elevation stations in complex terrain where landslides are likely to occur. Precipitation data from the RAWS network is known to have issues, thus a robust and iterative quality control method with substantial manual checks against radar and other station data is applied.

Findings thus far indicate that cool season observations over the 15 mm threshold are most frequently seen in the Transverse and Coast ranges, and with less frequency in the Sierra Nevada. Warm season cases over the 15 mm threshold are most frequently found in southeastern California and far northern California. A majority of the most extreme hourly precipitation values are tied to summertime thunderstorms in northern or southeastern California. Most shallow landslide inventory maps for California do not incorporate historic precipitation intensities associated with the event. Results of this study can allow for integration of these data into inventory maps as well as the comparison of precipitation intensity data with landslide distribution. This information will also inform where shallow landslides are less likely to occur due to relative rarity of precipitation over threshold. The results of this work can also support the siting of soil moisture sensors, which are currently sparse in the state, at the most relevant locations where precipitation of triggering intensity is likely to occur.