River-aquifer interactions enhancing evapotranspiration in a semiarid riparian zone: A modelling study

The hydrologic flows across the river-aquifer interface play an important role in groundwater dynamics and biogeochemical reactions within the subsurface; however, little is known about the effects of river-aquifer interactions on land surface processes. In this study, we developed a fully coupled three-dimensional (3D) land surface and subsurface model at a high resolution (similar to 1 km) that accounts for high-frequency hydrologic exchange flow conditions to investigate how river-aquifer interactions modulate surface water budgets in the Upper Columbia-Priest Rapids watershed, a typical semiarid watershed located in the northwestern United States where river stage fluctuates in response to reservoir releases changing. Our results show that the spatiotemporal dynamics of river-aquifer interactions are highly heterogeneous, driven mainly by river-stage fluctuations. Adding 6.64 x 10(6) m(3) year(-1) of water over the watershed from the river to groundwater owing to the lateral flow, river-aquifer interactions led to an increase in soil evaporation and transpiration supplied by higher soil moisture content, particularly in deeper subsurface. In a hypothetic future scenarios where a 5-m rise in river stage was assumed, the hydrologic flow exchange rates were intensified, resulting in higher surface water over the entire watershed. Overall, lateral flow induced by river-aquifer exchanges leads to an increase in evapotranspiration of similar to 75% in the historical period and of similar to 83% in the hypothetical future scenario. Our study demonstrates the potential of coupled model as an effective tool for understanding river-aquifer-land surface interactions, and indicates that river-aquifer interactions fundamentally alter the water balance of the riparian zone for the semiarid watershed and will likely become more frequent and intense in the future under the effects of climate change.