Features of landscape morphology—including slope, curvature, and; drainage dissection—are important controls on runoff generation in; upland landscapes. Over long timescales, runoff plays an essential role; in shaping these same features through surface erosion. This feedback; between erosion and runoff generation suggests that modeling long-term; landscape evolution together with dynamic runoff generation could; provide insight into hydrological function. Here we examine the; emergence of variable source area runoff generation in a new coupled; hydro-geomorphic model that accounts for water balance partitioning; between surface flow, subsurface flow, and evapotranspiration as; landscapes evolve over millions of years. We derive a minimal set of; dimensionless numbers that provide insight into how hydrologic and; geomorphic parameters together affect landscapes. We find an inverse; relationship between the dimensionless local relief and the fraction of; the landscape that produces saturation excess overland flow, in; agreement with the synthesis described in the “Dunne Diagram.’; Furthermore, we find an inverse, nonlinear relationship between the; Hillslope number, which describes topographic relief relative to aquifer; thickness, and the proportion of the landscape that variably saturated.; Certain parameter combinations produce features with wide valley bottom; wetlands and nondendritic, diamond-shaped drainage networks, which; cannot be produced by simple landscape evolution models alone. With; these results, we demonstrate the power of coupled hydrogeomorphic; models for generating new insights into hydrological processes, and also; suggest that subsurface hydrology may be integral for modeling aspects; of long-term landscape evolution.
