Testing a "hydrologically active bedrock hypothesis" with a process-based model

Process-based hydrologic models assembles hypotheses obtained in experimental catchments. Recently the role of bedrock groundwater to control the generation of saturated subsurface flow has been investigated and summarized as a "hydrologically active bedrock hypothesis". For example, our previous study (Sayama et al. 2011) based on recession and water budget analyses showed that, steeper slope catchments store more water than the gentler catchments due to the effects of groundwater storage. To test the hypothesis and understand the mechanism, this study uses a process-based hydrologic model with the bedrock groundwater representation. The model was validated based on both observed hydrograph and isotopic signal at a granite rock experiment catchment in Japan. We applied also Time-Space Accounting Scheme (T-SAS), which was originally proposed for analyzing the temporal and spatial sources of stream water, and further improved in this study, to analyze the contributions of flow paths. The analysis suggested that more than half of annual stream flow was originated from bedrock groundwater and the contribution ratio was equally high for individual storm events. Furthermore, we conducted a virtual experiment by changing the steepness of the catchment. The results showed that only for some storm events in the beginning of a rainy season, the steeper catchment generates higher peak discharges due to the faster velocity of saturated subsurface flow. On the other hand, in most of storm events in the mid of a rainy season, the pre-storm discharges were lower in the steeper catchment due to the limited expansion of groundwater seepage areas to the soil layer. As a result, the steeper catchment generates less amount of saturated subsurface flow and inversely stores more water. This is evident also from the higher dynamic storage changes and longer Mean Transit Time (MTT) estimated by the model outputs. Overall this study supports the "hydrologically active bedrock hypothesis" with a process-based hydrologic model and clarifies the mechanism of groundwater dynamics on storm flow generations.