Analysis of a steady-state model of groundwater discharge in a river valley without and with evapotranspiration

Riparian evapotranspiration is an important process that constrains water exchanges within a well-connected river-aquifer-vegetation hydrological system. In this study, we developed an extended analytical Dupuit-Forchheimer (DF) model to consider the additional water flow resistances due to both the vertical deformation of flow lines caused by evapotranspiration and discharge into the river screened by bottom sediments in which a river does not fully penetrate the aquifer. This analytical model can describe the groundwater discharge process in the presence of riparian evapotranspiration. We validated our analytical results with numerical simulations of a 2-D flow model that explicitly considers vertical hydraulic resistances, demonstrating that the surface water-groundwater exchanges in such hydrological systems are strongly dependent on the hydrogeological parameters, river width, riparian zone width, and parameters related to the hydraulic connection between evapotranspiration and groundwater. A sensitivity analysis of our extended DF model reveals that the water balance in hyperarid regions is determined by riparian evapotranspiration, and evapotranspiration most likely causes river flow losses. However, in humid regions, most groundwater (80% or more) is discharged into rivers rather than via riparian evapotranspiration. Our results confirm that riparian evapotranspiration in arid regions is an important process that affects surface water-groundwater exchanges and that evapotranspiration from riparian zones not only reduces groundwater discharge into rivers but also potentially causes disconnections within river-aquifer systems. The obtained analytical dependencies allow the griddependent parameters of the RIV and EVT packages to be calculated in the MODFLOW simulations of regional groundwater flow discharge in a river valley using coarse grids.