Transmissivity and groundwater flow exert a strong influence on drainage density

Abstract

Abstract. The extent to which groundwater flow affects drainage density and erosion has long been debated, but is still uncertain. Here, I present a new hybrid analytical and numerical model that simulates groundwater flow, overland flow, hillslope erosion and stream incision. The model is used to explore the relation between groundwater flow and the incision and persistence of streams for a set of parameters that represent average humid climate conditions. The results show that transmissivityand groundwater flow exert a strong control on drainage density. High transmissivity results in low drainage density and high incision rates and vice versa, with drainage density varying roughly linearly with transmissivity. The model evolves by a process that is defined here as groundwater capture, whereby streams with a higher rate of incision draw the watertable below neighbouring streams, which subsequently run dry and stop incising. This process is less efficient in models with low transmissivity due to the association of low transmissivity and high watertable gradients. A comparison of different parameters shows that drainage density is the most sensitive to transmissivity, followed by parameters that govern initial slope and stream erosion. These results imply that permeability and transmissivity exert a strong control on drainage density, stream incision and landscape evolution and that models of landscape evolution may need to explicitly include groundwater flow.