GROUNDWATER IN AN EVOLVING LANDSCAPE: A SIMPLE, EXTENSIONAL BASIN CASE

We present a case study of groundwater in a simple but dynamic extensional basin to better understand how groundwater flow changes in response to tectonic forces and landscape development over a hydrogeologically long period of time (up to 15 Ma). A numerical model that utilizes two tectonic blocks is developed to mimic the time-varying changes in geology and topography induced by the motion of a listric fault in an extensional regime. As a result, the model generates a half-graben structure filled with eroded sediment. From an inclined, flat surface at the beginning of the model to a fully developed basin, the hydrologic history of both surface water and groundwater are tracked by several important metrics. In particular, we examine evolving patterns of groundwater behavior in terms of water-table configuration, discharge and recharge, nested groundwater flow scales, and surface hydrologic connectivity. We hypothesize that the transition of the hydrologic system from topographically-driven to recharge-driven can be indicated by surface hydrologic connectivity (i.e., spatial continuity of streams) that simultaneously represents both surface and subsurface “wetness”. This study will explain how a regional-scale groundwater system responds to tectonics, landscape evolution, and spatiotemporal changes in surface-water reservoirs (e.g., lakes and streams).