The Coevolution of Topography and Runoff Generation in Humid Landscapes
Johns Hopkins University
Topography is an important control on runoff generation, as slope and relief affect hydraulic gradients, and curvature affects the convergence or divergence of flow paths. Over long timescales, however, runoff also shapes topography through surface erosion. This coevolution suggests that there may be a close relationship between landscape hydrology and topography that could provide insights into both hydrological and geomorphic processes. However, we do not have a strong theoretical framework for how topography and runoff generation should be linked, nor have there been many studies to determine how these links are expressed in the field. Here I address these areas, focusing on humid climates where runoff is primarily generated through groundwater return flow and precipitation on saturated areas. First, I present a new coupled model of runoff generation and landscape evolution that incorporates fluvial erosion driven by runoff from a shallow aquifer, hillslope diffusion, and uplift. Then, I nondimensionalize the model under the condition of steady and uniform groundwater recharge, and provide a mathematical framework for understanding the link between hillslope length, geomorphic process rates, and subsurface hydrological properties. Next, I explore the hydrological function of coevolved landscapes in more detail, focusing particularly on the emergence of variable source area hydrology. For this aim, I extend the model and nondimensionalization to include evapotranspiration and a simple representation of the vadose zone. I show, among other things, that coevolution with subsurface hydrology can explain why steeper landscapes are likely to have smaller variably saturated areas than landscapes with more gentle topography, and link this difference to subsurface properties and climate. Lastly, I test some of the model predictions in the field by exploring the hydrologic and geomorphic differences between two small watersheds in the Piedmont physiographic province near Baltimore that have contrasting subsurface architecture. I show that the site with a thin permeable subsurface has larger variable source areas and shorter hillslopes than the site with a thick permeable subsurface, as predicted by the model. A full parameterization of the model for the two sites suggests that subsurface properties are necessary to explain these differences.
runoff, groundwater, landscape evolution, hillslope