Catchment coevolution and the geomorphic origins of variable source area hydrology

Aspects 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 and modifying role in shaping these same features through surface erosion. These feedbacks suggest that modeling long-term landscape evolution while accounting for hydrology could provide insight into hydrological function. Here we examine the hydrological features that emerge when runoff is equilibrated with topography, focusing particularly on the emergence and persistence of saturated areas. We use a new coupled hydro-geomorphic model that captures saturated and unsaturated zone storage and water balance partitioning between surface flow, subsurface flow, and evapotranspiration, but has numerical efficiency sufficient to drive a landscape evolution model over millions of years. Our results reveal the emergence of perennial and ephemeral stream networks, variable source areas, and even non-dendritic drainage networks under certain circumstances. When capacity for water storage and lateral drainage relative to climate are low, lower relief landscapes emerge with greater variability in the extent of saturated areas, while greater relief and less variability in saturated areas emerge as soil storage and lateral drainage capacity increase. Results from a case study suggest that emergent topography and runoff generation patterns reflect this coevolution in some places.