Critical Rainfall and Groundwater Level Regulating Runoff Generation in a Small Forested Catchment, Western Japan

Abstract:

Rainfall-runoff response is an important index for expressing the water storage and drainage characteristics in the catchment. To clarify the mechanisms of runoff generation we investigated runoff ratio (=direct flow volume/event rainfall volume) and hydrometric and isotopic features (δ¹⁸O) of stream water, soil water and groundwater at concave and planar hillslope in each storm event in a small forested catchment (area 5.9 ha), western Japan, and analyzed the relationships between runoff ratio and flow pass and the critical rainfall amount (Rc, mm) controlling runoff magnitude. Results from 30 storm events showed 2 types of hydrograph shapes, i.e., quick peak type with small runoff ratio (<0.03) and delayed peak type with large runoff ratio (>0.10). The peak time of hydrograph coincided with that of groundwater level at concave hillslope rather than soil moisture content or groundwater level at planar hillslope. In addition, analysis of groundwater peak provided the threshold value (-1.06 m below land surface) between the quick peak and delayed peak response. The critical rainfall amount depended on the initial groundwater level (GL, m) and was expressed as a linear equation, Rc = -357GL-379. These results imply that the hillslope topography can be a significant factor for both groundwater dynamics and rainfall-runoff response, especially the groundwater level at concave hillslope as a determinant of runoff ratio, and that its initial level at concave hillslope has a great potential to predict a rainfall amount needed for occurrence of runoff with a large magnitude.