Effects of Different Flood Regimes on Soil Erosion and Sediment Transport in Typical Small Watershed of Loess Hilly-Gully Region

Abstract

Soil erosion responses under different rainfall and runoff patterns are fundamentals for the studies of soil erosion mechanisms. To investigate the influence of flood regimes on soil erosion and sediment yield at small watershed scale, a typical small watershed—Shejiagou catchment was selected to conduct data collection and analysis, and Shejiagou is a first order tributary of Chabagou drainage basin lying in the hilly and gully region of Chinese Loess Plateau. Based on 45 individual flood events recorded at Shejiagou Hydrological Station during 1961 to 1969, all the flood events were categorized into three regimes through a combined approach of K-mean clustering and discriminant analysis with three grouping variables, including flood duration, event flood runoff depth and peak discharge. Regime A was characterized by short duration, small flood runoff depth, low flood variability and medium peak discharge, which was the most common regime. Regime B was featured with medium duration, medium flood runoff depth, medium flood variability and small peak discharge, which was of medium frequency. Regime C mainly included flood events of long duration, large runoff depth, high variability, as well as large peak discharge, which was of the lowest frequency. The regime of flood events was mainly controlled by flood duration at the studied scale. Area-specific sediment yield, mean suspended sediment concentration and maximum suspended sediment concentration driven by different flood regimes can be ranked in the order of C＞B＞A, C＞A＞B, C＞A＞B, respectively. However, no significant difference was found among the three flood regimes for the variables examined (P＞0.1). The runoff-sediment relationship was relatively constant at Shejiagou watershed, the variations in suspended sediment concentration can be well described by the logarithmic function of instantaneous discharge. The sediment output at watershed outlet was mainly controlled by event-based total flood runoff. Given that event flood runoff depth was kept constant, the ratio of area-specific sediment yield driven by different flood regimes (A∶B∶C) was 1∶0.93∶1.22. If the flood duration was increased by 1.7 times, the flood regime-based increase for sediment yield reached the maximum, and the maximum increase rate for area-specific sediment yield was 22%. The results may provide beneficial evidence for categorization of individual flood events, and overall rational-based evaluation on the soil and water conservation benefits brought by runoff regulation systems at watershed scale.

Keywords: flood regime, small watershed, soil erosion and sediment yield, runoff regulation