At these scales, deterministic, gridded approaches become complicated by a combination of numerical, observational, parameterization and calibration challenges (Zehe et al., 2014). Intermediate scales (~5–200 km 2), however, can pose challenges for either of these approaches. Examples include VIC (Gao et al., 2010), PRMS, (Leavesley et al., 1983), SWMM (Metcalf, 1971), CANOE ( SWAT (Arnold et al., 2012b), and MOdel for Urban SEwers (MOUSE) (MOUSE, 1996), among many others. Thus, at large scales, gridded representations are often replaced by the use of semi-distributed models in which hydrological response units (HRUs, typically watersheds/subwatersheds) form the smallest scale on which either landscape properties or hydrological processes are resolved (Flügel, 1996). Gridded models enable direct representation of both the physics of flow and the properties of the landscape at the scale of the grid, (e.g., TOPMODEL (Beven, 1997 Beven and Kirkby, 1979 Kirkby, 1997), MIKE SHE (Abbott et al., 1986), and ParFlow (e.g., Jones and Woodward, 2001 Ashby and Falgout, 1996 Kollet and Maxwell, 2006 Maxwell, 2013)), but can become computationally intractable as model scales increase. The scale of hydrological modeling determines which hydrological processes dominate the hydrological response and which processes can be resolved in the model (Blöschl and Sivapalan, 1995 Zehe et al., 2014). Our findings suggest that hillslope curvature may play an important role in hydrological processes on real landscapes. This curvature is substantial enough to increase peak flow predictions by up to a factor of 3 on some hillslopes, and can be exaggerated at larger scales as multiple hillslope hydrographs are superimposed. We found that, for these two subwatersheds, divergent, convergent, and uniform-width hillslopes account for 43%, 55%, and 2% of subwatershed areas, respectively. We apply the new tool to subwatersheds in the Walnut Gulch Experimental Watershed and Las Trampas Creek Watershed to investigate properties of hillslope curvature and impact of curvature on peak flows. We present Arc Hydro tools to delineate hillslope outlines, identify hillslope width functions and implement an overland flow model that accounts for hillslope curvature. Hillslopes are a fundamental unit of surface hydrology, mediating the flow of water to fluvial networks through overland flow and subsurface pathways.
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