We propose a new hydrograph separation method for runoff source modeling based on continuous open system isotope mixing using a variable source area and three isotopic reservoirs. The three reservoirs are (1) direct precipitation on saturated areas, (2) a near-stream saturated zone, and (3) subsurface water in upslope areas. The reservoir sizes and isotopic compositions evolve due to continuous, open system mixing. Streamflow is composed of rainfall on saturated areas and water exfiltrated from the near-stream Saturated zone. The contributing source fractions are determined using the observed isotopic compositions of the stream and the modeled isotopic compositions of the contributing reservoirs, which are functions of time. The near-stream saturated zone receives inflow from upslope areas which are not Surface saturated. The surface-saturated area changes dynamically based on the difference between lateral inflow and contributions to streamflow from the near-stream saturated zone. The upslope zone (reservoir 3) evolves by mixing with rainfall. This hydrograph separation method requires (1) a function which relates the amount of water stored in the near-stream saturated zone to saturated area, (2) a function which relates saturated area fraction to total streamflow, and (3) knowledge of the water storage capacity in reservoirs 2 and 3. The hydrograph separation method was applied to oxygen isotope data for rainfall and runoff during a 36-hour storm at the Gardsjon Fl catchment in southwestern Sweden, The hydrograph separation method was used to estimate time-dependent streamflow contributions due to overland flow of storm rainfall and. subsurface flow of mainly preevent water. We also used the separation method to estimate parameters for relations between saturated area fraction and streamflow, and saturated area and subsurface water storage. We show that streamflow contributions from overland flow may be described by a simple analytic function of antecedent conditions and catchment parameters.
