Functional intercomparison of hillslopes and small catchments by examining water source, flowpath and mean residence time

SummaryIntercomparisons of hillslopes and catchments in different hydrogeomorphic settings are rare. Those comparison studies that have been completed have focused largely on bulk outflow and chemistry. Here, we present a new functional intercomparison of two well studied hillslopes: one in New Zealand at Maimai and one in Japan at Fudoji. Slope angle, slope length, soil depth, climate and vegetation of both hillslopes are very similar. Thus, questions posed include: In what ways are the hillslopes similar or different as expressed via the combination of throughflow rate, tensiometric response, event/pre-event water partitioning, quickflow rates and mean residence time? How does the apparent difference in soil drainable porosity affect the hillslope response to storm rainfall? How do the apparent differences in bedrock permeability affect the residence time of water at the slope base and catchment outlet? Our results suggest that in steep, wet and thin soil hillslopes, bedrock permeability and water retention characteristics combine to form a first order (main or dominant) control on the baseflow hydrograph and its mean residence time. For storm rainfall totals above about 50 mm, soil drainable porosity appears to be a first order control on the extension of upslope subsurface saturated area and the event water ratio of hillslope discharge in steep, wet and thin soil hillslopes. Although this functional intercomparison is a posteriori, it has yielded information that was not possible from the individual hillslope studies upon which it is based. In particular, it has informed a new, more generalizable, conceptualization of subsurface flow for steep wet hillslopes. We argue that intercomparison of other such hillslopes and small catchments may be a pathway forward for defining first order controls of complex hillslope hydrologic dynamics.