Modeling Surface Water-Groundwater Interaction with MODFLOW: Some Considerations by Philip Brunner 1 , Craig T. Simmons 2 , Peter G. Cook 3 , and Ren´ e Therrien 4 Abstract The accuracy with which MODFLOW simulates surface water-groundwater interaction is examined for connected and disconnected losing streams. We compare the effect of different vertical and horizontal discretization within MODFLOW and also compare MODFLOW simulations with those produced by HydroGeoSphere. HydroGeoSphere is able to simulate both saturated and unsaturated flow, as well as surface water, groundwater and the full coupling between them in a physical way, and so is used as a reference code to quantify the influence of some of the simplifying assumptions of MODFLOW. In particular, we show that (1) the inability to simulate negative pressures beneath disconnected streams in MODFLOW results in an underestimation of the infiltration flux; (2) a river in MODFLOW is either fully connected or fully disconnected, while in reality transitional stages between the two flow regimes exist; (3) limitations in the horizontal discretization of the river can cause a mismatch between river width and cell width, resulting in an error in the water table position under the river; and (4) because coarse vertical discretization of the aquifer is often used to avoid the drying out of cells, this may result in an error in simulating the height of the groundwater mound. Conditions under which these errors are significant are investigated. Introduction According to Furman (2008) and Barlow and Har- baugh (2006), the most commonly used numerical model to simulate surface water–groundwater interactions is MODFLOW. However, there are also a number of more sophisticated models that include a more realistic phys- ical coupling between surface water and groundwater. 1 Corresponding author: Flinders University, GPO Box 2100, Adelaide, SA 5001, Australia; 61-8-82012346; fax 61-8-82015635; philip.brunner@flinders.edu.au 2 Flinders University and National Centre for Groundwater Research and Training, GPO Box 2100, Adelaide, SA 5001, Australia 3 CSIRO Land and Water, Private Bag 2, Glen Osmond, SA 5064, Australia 4 D´ epartement de g´ eologie et de g´ enie g´ eologique, Universit´ e Laval, Qu´ ebec, Canada This study focuses on the influence of conceptual assump- tions on the simulation of the interaction between losing streams and groundwater using MODFLOW. Such con- ceptual assumptions are embedded in the way rivers are assigned to the model grid or in the equations used to calculate infiltration fluxes. We discuss and quantify the influence of such assumptions and point out in what sit- uations they will affect the modeling outcome. Most of the issues we talk about are also relevant for gaining streams. However, the simulation of gaining streams adds additional complexities (e.g., the influence of neglecting seepage of groundwater along a riverbank of a gaining stream) and their discussion is not within the scope of this paper. Modeling Streamflow in MODFLOW Numerous streamflow packages with different lev- els of complexity have been developed for MODFLOW. We limit our discussion to streamflow packages devel- oped by the USGS because of their availability and their widespread acceptance. The first streamflow package was 1