Residence times and flow paths of water in steep unchannelled catchments, Tanakami, Japan

Based on previous studies, it can be hypothesized that mean residence times of water increase with increase of both sampling depth and upslope contributing area. To test this hypothesis, mean residence times of waters were evaluated with conceptual flow models using nearly sinusoidal variations of δD in rainfall and throughfall in two adjacent steep unchannelled catchments in the Tanakami Mountains of Japan. Groundwater levels and hydraulic head gradients were monitored, simultaneously. In both catchments, perennial groundwater was formed within 2 m from the locations of spring discharge, whereas areas more than 6 m upslope from the springs were only temporarily saturated during storm periods. In the upslope area, hydraulic head gradients indicated the dominance of downward water flux within the soil layers. The estimated mean residence times of soil waters (0–27 days) and the transient groundwater (TGW) in the upslope area (23–67 days) were related to the sampling depths, regardless of the upslope contributing area. The mean residence times of perennial groundwater (PGW; 93 and 179 days) was significantly longer than TGW, independent of sampling depths. The mean residence times of baseflow stream (>1 year) was more than twice that of PGW even though the stream was sampled only 5 m downslope from the sampling well of PGW. These results indicated that mean residence times of soil water and TGW were mostly described by soil depth, whereas, PGW and stream water, which were strongly affected by water flow through bedrock, can be described by the upslope contributing area. The results also demonstrated that the transit times for the lateral water movement through bedrock were significantly greater than those for vertical movement of water through the soil. These results showed that the aforementioned hypothesis cannot be applied for most areas of catchments and instead indicated that mean residence times of water can be described either by soil depth or upslope contributing area.