Representation in regional models of saturated river–aquifer interaction for gaining/losing rivers

River–aquifer interaction is normally included in regional groundwater models using river coefficients (streambed conductances); this approach is necessary because the dimensions of the river are usually smaller than the mesh spacing of the regional model. River coefficients, which are proxy coefficients, are generally estimated from the properties of the streambed deposits since it is assumed that measurable head losses between the stream and the aquifer are limited to those across the streambed itself. Most studies of river –aquifer interaction relate to losses from rivers due to groundwater pumping; however another common situation is that a river gains water from the aquifer along certain reaches and loses water from other reaches, a gaining/losing river. These different forms of river– aquifer interaction are examined using fine-grid numerical models which allow the representation of a wide variety of different conditions at the river channel, in the aquifer and on the boundaries of the aquifer. The concept of the river coefficient is shown to be reliable but the numerical values of the river coefficient do not depend primarily on the dimensions and properties of the riverbed deposits; the hydraulic conductivity of the aquifer is usually a more important factor. When the river coefficients, determined from the fine-grid solutions, are included in regional groundwater models, the representation of river–aquifer interaction is acceptable.