Field investigation and modeling of runoff generation in a granitic catchment in Zhuhai, China

Rainwater percolation into bedrock and the contribution of bedrock groundwater flow to streamflow have been reported for several experimental catchments in China, Japan and the US. On the basis of previous studies, the particular runoff characteristics of granitic catchments were determined: (i) fast rainwater infiltration, (ii) obviously saturated throughflow (interflow) at the soil–bedrock interface, and (iii) obvious rainwater percolation into the bedrock and a substantial contribution of bedrock groundwater flow to streamflow. Next, the runoff generation characteristics of a small granitic catchment (0.99 km2) in southern China were analyzed, based on long-term monitoring of rainfall, runoff at the catchment outlet and at different depths in two plots, along with monitoring of soil suction and soil water content at different depths in these two plots (5 m × 10 m). According to the monitoring results, the maximum soil infiltration rate in this granitic catchment was larger than 4.45 × 10−4 m/s. Interflow was found at the soil–bedrock interface in plot B with a soil depth ⩽1 m. In plot A overlain by a thick soil layer ⩾2 m, interflow was mainly generated within an aquitard at a shallow depth (∼1 m). The plot interflow hydrograph was similar, but slightly slimmer and sharper, than that of streamflow, illustrating that interflow is the main source of catchment streamflow at the storm scale. A semi-distributed model was constructed based on a digital elevation model, and the ratio of rainwater percolation into bedrock to precipitation in the study catchment from 1 April 2007 to 31 March 2008 was 0.33–0.47, in agreement with results for other granitic catchments. These results can aid in understanding the particular runoff generation characteristics of granitic catchments.