Abstract
“Where does water go when it rains on hillslope?” Researchers have worked on this question since studies by Horton in 1930s. Studies by forest hydrologists like Tsukamoto and Hewlett in the 1950s and 60s in forested upland catchments presented an alternative to infiltration excess theory in the form of the variable source area theory (VSA). This theory has been the basis for many topographically based conceptual rainfall-runoff models used today. Recent observations and monitoring of natural hillslopes in many areas of the Pacific Rim have begun to observe that VSA does not adequately explain the types of behaviors now uncovered through combined use of hydrometric, isotopic and geochemical measurements: threshold subsurface stormflow initiation, hysteresis in the hillslope-catchment discharge relation, dominance of vertical and lateral preferential flow and rapid mobilization of old water. Based on McDonnell (2003, Hydrological Processes, 17, pp. 1869-1875), this commentary summarizes advances in hillslope hydrology since 1960's, and examines the applicability of several VSA assumptions used in our current rainfall-runoff models. Further, we present recent challenges to introduce qualitative field information to rainfall-runoff models. Then, we try to summarize some possible ways forward to for how we as a community might develop a “post” variable source area concept.
