Elucidation of the relationship between geographic and time sources of stream water using a tracer approach in a headwater catchment

Tracer approaches have been used worldwide to clarify time and geographic sources of stream water in catchments, although the relationship between these two sources is poorly discussed. We considered the mean residence time (MRT) and its spatial distribution to determine the relationship between geographic source components and time source components. There were clear differences in solute concentrations and MRT among shallow, middle, and bottom layers along the vertical profile of the riparian groundwater body. Those in the stream water were intermediate compared to those in the shallow and middle layers; thus, the consistent geographic sources were groundwater in these layers. In the context of end-members mixing analysis (EMMA), however, the end-members were rainfall, hillslope groundwater, and riparian groundwater in the bottom layer. The other riparian groundwaters were a mixture of end-members. The discrepancy between the geographic sources and the end-members was resolved by considering the MRT, the time required to move the end-member or geographic source within the catchment. Our approach clarified the relationships among the geographic sources, time sources, and hydrological pathways, which are the essential factors of runoff generation processes and hydrochemical processes. Therefore, to go beyond previous applications of EMMA on the basis of systematic learning from observed data, it is insightful to combine the common approaches to analyze landscape heterogeneity and process complexity and to reconsider the framework of the hydrobiogeochemical models in various regions and at multiple spatial scales.