Effects of underlay on hill-slope surface runoff process under different rainfall intensities

Abstract

Clarifying the impact of underlay (i.e., the combination of understory vegetation and surface micro-topography) on the surface runoff process under different rainfall intensities would provide a significant theoretical basis for controlling soil and water loss on steep slopes in mountainous areas of southwestern China. In the current study, the runoff process under different rainfall intensities was observed based on 10 natural runoff plots, and the correlation between the spatial pattern of cypress (Cupressus funebris), micro-topography and runoff characteristic parameters was tested using the Pearson correlation coefficient method. The effects of the spatial pattern of cypress and micro-topography on surface runoff also were analyzed using the Response Surface Method (RSM). The results indicated that the blocking effects of different underlay conditions on surface runoff decreased with the increase of rainfall intensity. The impact of the spatial pattern of cypress and micro-topography on the runoff process was mainly reflected in the impact on peak flow. Under the condition of moderate rainfall (30-50 mm/24 h) or rainstorm (50-70 mm/24 h), topographic relief, surface roughness, runoff path density, contagion index of cypress, and stand density of cypress were the key factors that affected the peak flow, whereas under the condition of severe rainstorms (> 70 mm/24 h) none of the foregoing factors had a significant correlation with peak flow. Under the conditions of moderate rainfall or rainstorm, when the composite indexes of the spatial pattern of cypress and micro-topography were small, the peak flow would not be significantly affected. When the micro-topography reached the conditions required to significantly increase the peak flow, increasing the composite index of the spatial pattern of cypress within a certain range promotes the peak flow, and when the composite index of the spatial pattern of cypress exceeded a certain value, as the composite index of micro-topography increased, the dominant factor affecting the peak flow gradually changed from the spatial pattern of cypress to that of micro-topography.