AbstractBaseflow separation from rainfall and streamflow data is a fundamental problem in applied hydrology, unsolved despite extensive investigations. For example, a part of baseflow is interflow, which results from rainfall infiltration, but almost all of the existing baseflow separation methods, such as graphic and filter methods, have nothing to do with it, which is a serious flaw from the viewpoint of science. The objective of this research is thus to present an innovative baseflow separation method based on the recent general unit hydrograph (UH) model and the classic Green-Ampt infiltration equation. Specifically, we divided a rainfall hyetograph into two parts using the Green-Ampt infiltration equation: one for surface flow that generates direct runoff and the other for subsurface flow that recharges groundwater and generates interflow. As with direct runoff from the surface system, we approximated the subsurface system as a linear system and thus applied the general UH model for interflow in the unsaturated soil zone, where excess infiltration is defined by analogy to excess rainfall. We assumed that groundwater flow could be described by the classic recession curve; we then added interflow and the groundwater flow to obtain the baseflow. We validated the proposed method with six real-world case studies representing four interflow patterns. Particularly, we found that, unlike direct runoff, the interflow UH model parameters are not unique, depending on stream stages. This implies that the subsurface system is a time-variant linear system, which can even make a negative interflow if groundwater is recharged from streams. We expect that this research will provide a better baseflow separation method and thus improve our understanding of watershed processes.