Estimating the daily rainfall thresholds of regional debris flows in the Bailong River Basin, China

Debris flows pose a major threat to life and property in mountainous regions. Rainfall thresholds are an important component for providing an early warning of debris flows. Most rainfall thresholds were calculated based on historical debris flow events and rainfall monitoring data, but in the areas where the debris flow events and rainfall monitoring data are lacking, these threshold models cannot be established. Here, we present a method for estimating the daily rainfall thresholds of regional debris flows according to different underlying surface factors. First, based on 34 underlying surface factors and 652 debris flow catchments with known frequencies, a relationship model between the influencing factors and debris flow frequency was established; second, a fitting function between the frequency of days with rainfall greater than a given amount (15 mm, 20 mm, 25 mm, 30 mm, 40 mm, and 50 mm) was derived. Last, the predicted debris flow frequencies were input into the fitting function to estimate the daily rainfall thresholds of each catchment. The estimated thresholds were verified using historical debris flow events, and the verification results were R-2 = 0.82, MAE = 2.81, RMSE = 3.95. The daily rainfall thresholds of regional debris flows estimated in this study can provide a reference for the early warning of regional debris flows where monitoring data are lacking.