DYNAMICS OF THE INTERFACE BETWEEN STREAMS AND GROUNDWATER SYSTEMS IN LOWLAND AREAS, WITH REFERENCE TO STREAM NET EVOLUTION

Observations reveal a close relationship between groundwater depth and stream net characteristics in the permeable lowlands of the sandy Pleistocene area of the Netherlands. This applies for average conditions as well as for the seasonal expansion and contraction of the network of streams that participate in the drainage process. It is plausible that this observed relation is caused by the close connection between groundwater and surface water in an area where almost all precipitation surplus is discharged as groundwater. Under such conditions a stream system can be considered the outcrop of a groundwater flow system. The hypothesis is that the stream network, as the interface between both systems, must have adapted in response to the discharge capacity that is required to release the precipitation surplus through the continuum of ground waters and surface waters. A theoretical model of coupled groundwater and surface water drainage systems is proposed that gives the stream net density and channel dimensions as functions of the geological and climatic conditions. The theoretically derived relationships of increasing stream density and decreasing channel size with reducing depth to groundwater and reducing topgraphic slope reflect the observed situation. The model further allows for assessment of the response of a drainage network to a changing environment.