Threshold behavior in a fissured granitic catchment in southern China: 1. Analysis of field monitoring results

An experimental granitic catchment in southern China was instrumented to continuously monitor the hydrologic processes, within which two plots (5 m x 10 m) were equipped. Observed rainfall and corresponding responses of surface water, soil water, and groundwater were used to examine the hydrological threshold behaviors. New hydrological thresholds were determined on the spatial scales of plot and catchment and on the temporal scales of storm event and month, namely, those between (1) the sum of total event precipitation (P) and the antecedent soil moisture index (ASI) (hereafter P+ASI) and the total event interflow at plots A (soil depth: 200-250 cm) and B (soil depth: 50-90 cm), and between P+ASI and deep interflow at plot A, within which an aquitard exists, (2) P+ASI and event peak flow, (3) the normalized maximum well water level (NMWWL) and event peak flow at the catchment scale, and (4) P+ASI and monthly streamflow depth. In addition, thresholds between P+ASI/NMWWL and event quick flow depth were identified, in accordance with findings of previous studies. Based on the hydrometric observations, we believed that the soil moisture deficit and bedrock depression storage were the main reasons for the threshold behavior in the study catchment. Below a specific threshold of P+ASI, none or little interflow was generated on hillslopes and contributed to streamflow, while interflow generated at the soil-bedrock interface on hillslopes started to contribute to streamflow when it was above the threshold, and the runoff contributing areas expanded from the near-stream zone to hillslopes.