DISCONNECTION BETWEEN SURFACE WATER BODIES AND GROUNDWATER: THE PHYSICS, SPATIAL AND TEMPORAL ASPECTS

Different flow regimes between surface water bodies and groundwater exist: The flow between the two compartments can be either fully saturated (connected regime), unsaturated (disconnected) or partially unsaturated (transitional). The flow regime has important consequences on how changes of the groundwater table affect the exchange rate between surface water and groundwater. While in a fully connected regime changes in the groundwater table are related linearly to changes of the infiltration rate, in a disconnected regime the infiltration rate is essentially independent of such changes.

We show that it is possible to conceptualize the unsaturated flow regime in terms of the buildup of a saturated groundwater mound and the development of a capillary fringe above this mound. We analyse the transition between connected and disconnected regimes and identify different transitional pathways. We show how hydrological parameters determine the pathway of transition and demonstrate that spatial variations of the infiltration flux through a surface water body strongly depends on these different pathways of transition. Moreover, the spatial distribution of seepage through a surface water body is shown to depend on the state of connection. These findings suggest that the transition zone may be of greater importance than is usually acknowledged.

A comparison of lakes and rivers reveals that the latter are less likely to disconnect in response to a decrease of the regional groundwater table. We relate this behaviour to differences in the build-up of a groundwater mound in 2D and 3D. Finally, we analyze with some simple transient simulations for 3D systems the transient behaviour of surface water groundwater interaction in the context of disconnection and show that the state of connection is a critical variable in the dynamics of infiltration in a non-steady system.