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.

Interactions between topography and the spatial distribution of cypress give rise to the spatial heterogeneity of surface runoff on steep forested slopes in southwest China. To reduce surface runoff and improve the water conservation capacity of cypress forests, the coupling effects of topography and the spatial distribution of cypress on surface runoff coefficient were studied through the Structural Equation Modeling (SEM) and the Response Surface Method (RSM) based on twelve natural runoff plots. Results showed that the surface runoff coefficient increased monotonically with the increase of the composite index of topography (topographic relief× runoff path density/ surface roughness), and increased first and decreased later with the increase of the composite index of the spatial distribution of cypress (stand density of cypress× contagion index of cypress). To reduce surface runoff coefficient from a larger value (>0.5) to less than 0.3, two strategies of stand structure adjustment could be adopted, including only increasing the stand density of cypress or increasing both the stand density and the contagion index of cypress, and which strategy should be adopted depended on the initial stand density of cypress. When the initial stand density of cypress was relatively low (<20 ind/100m2), the first step was to increase the stand density of cypress, and until the stand density of cypress reached to a moderate level (20-50 ind/100m2), adjusting the spatial structure of cypress from relatively regular to relatively clumped could reduce surface runoff coefficient to a greater extent.