Effects of season and man-made changes on baseflow and flow recession: case studies

Discharge hydrographs of rivers carry the cumulated information on the various hydrological processes in catchments and the influences imposed on them. From the analysis of observed baseflow recessions, direct and groundwater flow can be separated and time series of the main components of the underlying groundwater balance, namely discharge, evapotranspiration losses, abstractions, storage and recharge, can be derived. This inverse, or downward, approach was applied to daily streamflow data of rivers in different climate zones (Germany, Western Australia, Turkey) under different influences. The mostly shallow, unconfined aquifers were found to react as nonlinear reservoirs and an appropriate recession function was used instead of the traditional exponential function of the linear reservoir. However, though the actual storage-discharge relationships of aquifers will hardly change, flow recession properties are subject to seasonal variation and changes due to evapotranspiration and other fluxes and abstractions from the groundwater. As demonstrated in the case studies, recession analysis and baseflow separation permits the detection and quantification of these changes. In a separate application, the method is used to identify groundwater flow intruded into leaky sewer lines. Copyright (C) 2003 John Wiley Sons, Ltd.