Measurement of hydrological connectivity between hillslopes and streams in humid areas of southeast China

Abstract:

The study of connectivity between hillslope and stream via subsurface flow system is becoming increasingly important for Critical Zone (CZ) research. However, quantification of this connectivity through field observations still remains unclear. In this study, we investigated the hillslope-stream connectivity in a zero-order sub-catchment located in Hemuqiao experimental station in southeast China. The average annual rainfall in the Hemuqiao catchment is approximately 1580 mm representing humid hilly areas of southeast China. For this study, 25 underground wells and 30 time-domain reflectometer (TDR) probes (within eight excavated soil pits) were installed starting from the top of the hillslope to the stream, and surface and subsurface flow measurements were taken at the outlet with the recording interval of 6 minutes. Other measurements including soil thickness (53 locations), soil texture (silt in the upper layers, silt loam in deeper layers), and saturated hydrological conductivity (k_sat with mean value of 93mm/h) were also collected. Results indicate that during small rainfall events (<30mm), less than 7% of rainfall volumes were converted to runoff at the outlet and major parts of hillslope were disconnected from the stream. For larger rainfall events (30 to 100mm), the hillslope and stream were found to be connected via subsurface flow system with approximately 50% of rainfall converted to runoff at the outlet (only 1-3% of total rainfall resulted in surface flow). For rainfall events greater than 100mm, water table conditions were observed at the surface, suggesting that saturated soil conditions resulted in surface runoff. During these events, typically 70-85% of rainfall converted to runoff while only 14-25% accounted for subsurface flow. Furthermore, data from groundwater wells showed that as rainfall intensity and volume changed during different events, the subsurface flow generation areas varied widely. Finally, by combining k_sat value, groundwater level and TDR data, we found that the dominant hydrological mechanism on this hillslope (and most likely on many other hillslopes in this region) is the saturation excess flow, and hillslope and stream are connected via subsurface systems near the soil-bedrock interface.