Field evidence of groundwater ridging in a slope of a granite watershed without the capillary fringe effect

The controls on the rapid mobilization and runoff of pre-event water are an important issue in hillslope hydrology. One of the mechanisms involved is groundwater ridging, based on the capillary fringe effect. The typical conditions for groundwater ridging to occur are as follows: (1) the slope consists of fine- to medium-textured materials with a large extent of the tension-saturated zone, (2) the slope gradient is low, and (3) the initial groundwater level is high. Where these three conditions are met, a further condition, (4) a small amount of rainwater, would then be enough to trigger groundwater ridging. In this study, we detail groundwater ridge formation in the bedrock layers of Akakabe Watershed (Japan) as recorded by detailed field observations using tensiometers and bedrock boreholes under conditions violating the above. The study site consisted of materials showing almost no tension-saturated zone and had a relatively high gradient (22°). Moreover, the initially high groundwater level did not always generate a groundwater ridge, and a large total rainfall (>160 mm) was required to trigger groundwater ridging. Hence the conventional mechanism cannot explain the groundwater ridging at the field site studied here. It seems that the smaller distance from the groundwater table to the ground surface and the higher hydraulic conductivity in the downslope areas compared to the upslope areas caused a more rapid and larger groundwater level rise in the downslope areas, which accounted for the observed groundwater ridging. This study demonstrated that a groundwater ridge can be formed in the absence of the capillary fringe effect and that bedrock groundwater can contribute to storm runoff generation in headwater catchments under the influence of groundwater ridging.