Hydrological model of weathered granite watershed based on prototype monitoring and DEM

The aim of the research is to investigate the particular runoff generation mechanisms of granite watersheds. The small coastal basin within Zhuhai campus of Sun Yat-sen University was selected for runoff generation study. With dominant sandy loam soil to a depth of 2 m and weathered granite bedrock in the basin, two plots of 5 m×10 m slope with bush and tree cover respectively were built to monitor the runoff processes of varied layers from surface, subsurface to the bedrock. Moisture profiles by time domain reflectometry (TDR) indicated that preferential flow and excess infiltration flow usually occurred, and subsurface flow, including runoff from the interface of soil and bedrock could contribute a large component to the hydrograph, especially during a heavy rainfall event. Based on these observations and flow discharge at two weirs, a hydrological model with three sources from overland flow, interflow and bedrock fissure flow was developed. The whole basin was separated into 2 736 cells (20 m×20 m) based on a digital elevation map (DEM) with runoff generation calculated for each cell. The linear reservoir concept was used to derive runoff components and route runoff to the next cell with the steepest slope among eight flow directions, while water was routed in the channel by solving one dimensional Saint-Venant equation. Nine rainfall events were used to calibrate the parameters by the trial-and-error method, and the other four were used to verify the model; hydrographs of both calibrated and verified events were reproduced relatively well using the model. According to the simulated result, the overland flow, particularly from the zone next to the channel was found to be the main contribution to the rising hydrograph, while the fissure flow to the recession processes. The monitoring and modeling results illustrated that obvious subsurface flow and bedrock fissure flow were the runoff generation characteristics of granite watersheds in southern China.