Coevolution of Baseflow and Multiscale Groundwater Flow during Prolonged Droughts

Abstract:

Baseflow generation in mountainous watersheds is tightly coupled with the dynamics of the multiscale groundwater flow system (GWFS). However, due to the lack of observations and processes understanding at relevant scales, the dynamics of mountainous groundwater and baseflow are poorly represented in hydrologic models. Here, we study the evolution of baseflow, the baseflow residence time distribution (RTD), and the multiscale GWFS in a Tothian domain with substantial deep groundwater flow during long-term climatic perturbations. Groundwater flow and age are simulated under three prescribed recharge rates representing mild, moderate and extreme drought scenarios, starting from a common fully-saturated steady-state groundwater flow field. Each drought scenario leads to a distinct final equilibrium system, as manifested by the water-table configuration, the groundwater-age spatial distribution, the baseflow RTD, and the connectivity between the groundwater and the surface water. For each drought, the baseflow RTD in each subwatershed evolves as the drought progresses, with distinct patterns caused by subwatershed connection to groundwater flow paths of various scales. The subwatershed baseflow RTDs are similar during the early stages of the droughts, due to the similar groundwater flow field and connectivity between the groundwater and the surface water. However, under different droughts the subwatershed RTDs depart more from each other as the drought progresses; the final equilibrium RTDs are distinct. This indicates that we cannot use the RTD evolution patterns observed under one drought to infer the pattern for droughts of different severity. This is in contrast to the dynamics of baseflow discharge recession, where a short-term recession behavior replicates a segment of a long-term recession. Our results provide a mechanistic perspective of watershed-scale baseflow generation in mountainous terrains and their response to long-term climatic disturbances.