Quantifying the effects of rainfall intensity fluctuation on runoff and soil loss: From indicators to models

Rainfall represents the main driving force of runoff and soil erosion. Previous studies usually focused on the effects of general rainfall characteristics at event scale on runoff and soil erosion, but the effects of rainfall intensity fluctuation at intra-event scale, i.e., the temporal distribution of intensity bursts across the rainfall profile, have always been difficult to be characterized. In this study, the rainfall intensity profile was disaggregated into high-intensity zone and low-intensity zone using the average rainfall intensity as threshold, and a series of indicators were proposed to describe the features of rainfall intensity fluctuation. Field observations of runoff and soil loss in six grass-cover plots under natural rainfall conditions were conducted from 2011 to 2016 in the Loess Plateau of China. The main rainfall indicators influencing runoff coefficient (RC), sediment concentration (SC) and soil loss coefficient (SLC) were identified using the principal component analysis, and runoff and soil loss regression models were developed using data from 2011 to 2013 to predict RC, SC and SLC during 2014-2016. The results indicated that the natural rainfall events displayed strong intensity fluctuations (e.g., aggregation, intermittency, and variability), with approximate 78% of rainfall amount occurring in only 27% of rainfall duration. Most of the proposed rainfall intensity fluctuation indicators were significantly correlated with RC, SC and SLC (p < 0.05). The average intensity of rainfall peaks (I-ap) and rainfall duration of high-intensity zone (RDh) were the main variables to simulate RC, and the average intensity of high-intensity zone (I-ah) and relative amplitude of rainfall intensity (R-am) controlled SC and SLC. The models considering rainfall intensity fluctuation had good performances in predicting RC, SC and SLC with higher R-2 values (0.64 vs 0.46), lower RMSE values (0.66 vs 0.76) and higher NSE values (0.54 vs 0.42) compared to the models only considering general rainfall characteristics. The prediction accuracy of SLC was higher than that of RC and that of SC was lowest. These results gain an insight into the marked influence of rainfall intensity fluctuation in generating and predicting runoff and soil loss, and shed light on exploring rainfall-erosion relationships.