Groundwater affects geomorphic and hydrologic properties of coevolved landscapes

Abstract:

Ground saturation and subsurface flow are important for runoff generation, yet our understanding of how groundwater affects long-term catchment evolution is still rudimentary. Early research with numerical landscape evolution models showed that partitioning precipitation between surface and subsurface flowpaths using a steady state topographic wetness approach produced strong threshold effects on runoff generation and erosion. However, the effects of subsurface drainage capacity on threshold relationships have not been investigated systematically. Furthermore, stochastic precipitation leads to space- and time-variable thresholds in runoff generation that steady state hydrological models are unable to capture. Thresholds vary in time both as a function of precipitation event magnitude and as a function of past events that control antecedent streamflow and ground saturation. Coupled dynamic groundwater and landscape evolution models are rare and have not been used to their full potential for understanding the transient effects of groundwater flow and storage on long-term landscape evolution. Process feedbacks between groundwater and landscape evolution may also have important implications for emergent hydrologic characteristics of coevolved landscapes. In this study, we present a new groundwater landscape evolution model and use a nondimensional framework to (1) explore how the competition between precipitation and subsurface drainage sets geomorphic properties (e.g., drainage density, hillslope geometry), and (2) explore how geomorphic controls are expressed in emergent hydrological properties and processes (e.g., runoff ratio). We examine the effects of storm intensity, duration, and frequency on these emergent properties. We show increasing subsurface drainage capacity leads to longer hillslopes with greater relief, and that emergent hydrologic properties are sensitive to geomorphic parameters. Because groundwater influences runoff generation and erosion throughout the evolution of many landscapes, there may be emergent geomorphic properties that are useful for predicting hydrological function without intensive measurement. This could prove especially useful for parametrizing land surface models over large areas.