Effects of topographic factors on runoff and soil loss in Southwest China

Soil erosion is a threat to sustainable agricultural and regional development in karst regions. In this study, field plot observation method was used to estimate the effects of slope gradient and length on runoff and soil loss in Guizhou, Southwest China. The results showed that runoff and soil loss is nonlinearly related to slope gradient. The increasing trends of runoff and soil loss declined after the slope gradient of 15 degrees. This turning point was affected by both slope gradient and rock outcrops on the 20 degrees-25 degrees slopes, hence it is still unknown whether the slope gradient of 15 degrees is a critical value. Runoff showed a trend of decrease-increase-decrease as slope length increased, and soil loss rate showed an increasing trend as slope length increased. There is a significantly positive linear relationship between soil loss and slope length (P < 0.01). Runoff and soil loss were significantly correlated with rainfall amount (P) and the maximum 30 or 60 min rainfall intensity (I-30 or I-60), which had power function with PI30 on gradient-changed slopes and PI60 on length-changed slopes. Moreover, soil loss has a power function relationship with slope gradient/length and runoff depth. This study is helpful to elucidate the effect of topographic factors on soil erosion and to take effective soil conservation measures in karst regions.