Incorporating groundwater-surface water interactions into river management operations by linking an object-oriented decision support model (RiverWare) with MODFLOW

Accurate representation of groundwater-surface water interactions is critical to modeling low river flow periods in riparian environments in the semi-arid southwestern United States. A link between the objectoriented decision support model RiverWare and MODFLOW was developed to incorporate critical features such as riparian evapotranspiration, localized variations in seepage rates, irrigation return flows and rule-based water allocations to users and/or environmental flows. Specifically, the link allows variables associated with MODFLOW’s RIV, STR, SFR, and GHB packages to be exchanged with RiverWare’s Reach, Groundwater Storage, Water User, and Aggregate Diversion Site objects and vice versa. Exchange of data is handled using RiverWare’s computational subbasin structure. Since the MODFLOW grid is likely to be at a finer resolution than RiverWare objects, the user is able to specify multiple MODFLOW cells as interacting with a single RiverWare object. Accordingly, spatial interpolation and summation between some exchanged variables is necessary. An interactive time stepping approach is used to link the two models, in which both models run in parallel exchanging data after each time-step. The performance of the linked RiverWare-MODFLOW model is illustrated through applications on the Rio Grande in the vicinity of Albuquerque, New Mexico, where over-appropriation of human water use in the region adversely impacts the habitat of the endangered Rio Grande silvery minnow.