The Effects of Rainfall Regimes and Land Use Changes on Runoff and Soil Loss in a Small Mountainous Watershed

Abstract:

This paper analyzes runoff and soil loss in relation to the rainfall regimes and land use changes in a small mountainous watershed in the Three Gorges Area (TGA) of China. Based on 10 years of rainfall measurements and K-means clustering, 152 rainfall events were classified into three rainfall regimes. The mean statistical features of different rainfall regimes display a marked difference. Rainfall Regime I is events of medium amounts (31.8 mm) and medium duration (1371 min). Rainfall Regime II is events with high amounts (54.0 mm), long duration (2548 min), and an infrequent occurrence. Rainfall Regime III is events of low amount (22.2 mm), short duration (494 min) and high frequency. Each rainfall regime results in differing levels of runoff and erosion and Rainfall Regime I causes the greatest proportion of accumulated discharge (368.7 mm) and soil loss (4283 t). In the different rainfall regimes, the values of the mean runoff coefficient and the mean sediment load were ordered as follows: Rainfall Regime II > Rainfall Regime I > Rainfall Regime III. These results suggest that greater attention should be paid to Rainfall Regimes I and II because they had the most erosive effect. In the Wangjiaqiao watershed, the changes in land use primarily affected the paddy fields, where the cropland decreased significantly and the forest and orchards increased by 9.9% and 7.7%, respectively, during 1995-2004. The ANOVA shows land use changes caused significant decreasing trends in the runoff coefficients (P<0.01) and sediment loads (P<0.01). In order, the most sensitive response of runoff and erosion to land use was Rainfall Regime II > Rainfall Regime > Rainfall Regime III. Rainfall characteristics are decisive for the relative importance of different storm runoff generation mechanisms. The land use changes in the study watershed have considerably decreased runoff and soil loss.