Responses of runoff and sediment yield to rainfall regimes in a typical slope–gully system on the Loess Plateau: A 24-year field monitoring study

Slope–gully system serves as a critical erosion unit on the Loess Plateau, and its slope and gully refer to summit and ridge, and steeper gully wall, respectively. Current understanding of runoff and sediment transport mechanisms in slope–gully systems remains constrained by insufficient field observations and unsystematic laboratory simulations, particularly regarding the complex interactions between slope and gully wall units under varying rainfall regimes. This study observed rainfall, runoff and sediment data in the three topographic units from 1986 to 2010 and analyzed the responses of runoff and sediment to rainfall regimes. The results revealed that the erosion thresholds for different topographic units were determined to be 7.6 mm of rainfall and 7.9 mm h −1 for 60–minute maximum rainfall intensity, with a total of 61 erosive rainfall events selected and classified. Runoff depth and runoff coefficient in the gully wall were significantly lower than those in the slope and slope–gully systems. However, the erosion modulus in the slope–gully system was 2.09 and 2.43 times that in the slope and gully wall, respectively, indicating that the upslope inflow significantly strengthened the erosion in gully wall, with this enhancement effect becoming more pronounced as rainfall intensity increased. Furthermore, the slope–gully system produced finer sediment than the slope, indicating that the runoff from the upper slope deepened the gully and increased the erosion of fine particles within the gully wall. These findings have important practical implications for fully understanding the soil erosion patterns of slope–gully systems on the Loess Plateau.