Coupled APEX-MODFLOW: Model Development and Application

Assessing the hydrologic and water quality responses of watersheds to natural and anthropogenic perturbations requires capturing the complex interaction between surface and subsurface processes. Specifically, in watersheds where surface and groundwater are in close interaction, lack of a mechanism to properly simulate either may lead to unreliable results. In this study, the agricultural Policy/Environmental eXtender (APEX) model was linked with MODFLOW to create an integrated surface/subsurface watershed model capable of simulating key hydrologic and water quality processes and complex management practices at field to watershed scales. The APEX model serves as the main engine simulating the surface and shallow subsurface processes. The values of deep percolation estimated by APEX are passed to MODFLOW for groundwater simulation, including aquifer recharge, groundwater head, and return flow to streams. The return flow from MODLOW is then passed to APEX channel routines for routing through the stream network in the watershed. The model is applied to two watersheds: Middle Bosque Watershed in Texas, USA, and Price River Watershed in Utah, USA. The integrated model provides major improvements in more realistic simulation of hydrologic and water quality processes which subsequently will lead to better identifying the effects of watershed management practices.