invited Talk, American Geophysical Union (AGU), Fall Meeting, San Francisco, San Francisco: 2009-12-14 - 2009-12-18
In hydrology and water management exchange between groundwater (GW) and surface water (SW) has traditionally been treated in simplified ways or even been neglected all together. Questions of conjunctive use and net river recharge to regional aquifers were often the main concerns. With the growing awareness of the significance of GW-SW exchange for the ecological health of riparian zones and other groundwater dependent ecosystems the spatial and temporal patterns of exchange at multiple scales have become new foci of research. Better understanding the processes that govern these patterns is a prerequisite for assessing the effects of environmental change on aquatic systems. Modelers have predominantly addressed river-aquifer exchange at two scales: the regional scale, where rivers are often treated as simple line sources or sinks to regional, homo¬geneous or layered aquifers, and, more recently, the channel scale with a focus on local flow cells in the river bed and between the channel and shallow groundwater immediately below it. Relatively few modeling studies exist at an intermediate scale that includes a detailed representation of the extended alluvial aquifer. But flow cells within the river bed are strongly affected by upwelling of deeper groundwater and geologic heterogeneities in the alluvial aquifer and landscape topography are important controls on larger-scale patterns of river-aquifer exchange. In most cases it is computationally not feasible to simulate regional river-aquifer systems at a spatial resolution sufficiently high to resolve all important small-scale patterns and dynamics of exchange. Therefore detailed river-aquifer models at the intermediate scale are important to better understand how larger-scale flow patterns affect local exchange processes and vice versa. Results from modeling studies of river-aquifer systems are used to elucidate the importance of geologic heterogeneity at the scale of alluvial hydrofacies and landscape topography as controls on patterns and dynamics of river-aquifer exchange.