Procedures to estimate low flow statistics at ungauged sites and their relation to hydrogeology are presented. The discussion is based on an example of a Danish chalk catchment of 242 km2, within which the climatic variation is small compared to the physiographical variation. The spatial and temporal variation of streamflow was studied using synchronous discharge measurements and the application of a numerical model. The synchronous discharge measurements showed the runoff is unequally distributed within the catchment and depends primarily on the regional hydrogeology, but also on the lithology of the valley deposits. The numerical model was used to obtain an understanding of the hydrogeological effect on the temporal variation of the runoff from small subcatchments. This was not possible from the discharge measurements because of the high uncertainty of estimating the difference in flow between two nearby gauging stations. The temporal variation was found to be strongly related to the hydraulic gradient along the river. A knowledge of catchment processes and variability of runoff from modelling studies is shown to assist in low flow estimation.

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