The Ground-Water/Surface-Water Flow (GSFLOW) Model is a new U.S. Geological Survey model for simulating ground-water/surface-water interactions that will be released near the end of 2006. GSFLOW couples PRMS (Precipitation Runoff Modeling System) to MODFLOW (Modular Ground-Water Flow Model) with a new family of modules for simulating processes in the unsaturated zone. GSFLOW includes enhancements to both PRMS and MODFLOW to facilitate their dynamic coupling. GSFLOW uses physically based equations to describe critical processes in the soil zone (the uppermost part of the unsaturated zone), including infiltration, runoff generation, and lateral flow in temporarily saturated material. Flow through the unsaturated zone beneath the soil zone is based on a 1-day kinematic-wave approximation to the Richards’ equation, implicitly coupled to MODFLOW. Recharge varies spatially and temporally in GSFLOW. Precipitation is partitioned between evapotranspiration, runoff, infiltration, and storage by balancing daily energy and mass budgets of the snowpack, soil zone, and unsaturated zone. Test runs used data from the 6,672-acre volcanic-rock Sagehen basin in the Sierra Nevada, near Truckee, California, USA. Simulated ground-water recharge in this humid watershed depends on several basin properties; however, simulated recharge was most sensitive to the interdependence of subsurface hydraulic properties and the thickness of the unsaturated zone. The hydraulic conductivity was varied by two orders of magnitude to evaluate the effects of hydraulic conductivity on recharge and ground-water discharge. The thickness of the unsaturated zone became large in the upland areas when hydraulicconductivity values were high, which was unrealistic for Sagehen basin as demonstrated by the presence of several springs in the basin. Thus, the hydraulic properties of the model were constrained to a relatively narrow range by the location of springs and the discharge at the basin outlet, despite the lack of available ground-water level data.