Author(s): Vassilios Kaleris

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Abstract: The reliability of quantification (measurement and calculation) of the exchange rate between groundwater and small streams is discussed. Concerning measurements, two measuring techniques (discharge difference measurements and seepage meter measurements) are investigated. Discharge difference measurements can give reliable results if the length of the reach, at the ends of which the discharge is measured, is appropriate. Infiltrometer measurements are of questionable value when influenced by difficult to detect installation errors. Concerning calculations the following issues are addressed: (a) The sensitivity of the calculated values of the exchange rate to errors in various parameters. This problem is discussed by means of analytical solutions. The analysis shows that, in case of stream beds with relatively small hydraulic resistance, errors in the hydraulic conductivity of the aquifer are important for the exchange rate. In case of stream beds with high hydraulic resistance, exchange rate is strongly influenced by the errors in the hydraulic resistance. In such cases, errors in the parameters characterizing the capillary forces in the porous media can increase the resulting errors considerably. (b) The reliability of estimations of the hydraulic resistance of the stream bed by means of piezometric head measurements near the stream. Investigations in a small stream show that, if the hydraulic resistance of the stream bed is high, its estimation from measurements of the piezometric heads is uncertain. (c) The forecasting of the exchange rate by means of depth integrated models under a hydrologie regime other than the one used for calibration. By means of analytical solutions it is shown that in case of stream beds with high hydraulic resistance and for porous materials in which the capillary forces are negligible, the exchange coefficient used to describe the exchange flow includes only the influence of the clogging layer. In such cases the value of the exchange coefficient estimated from the calibration can be used to forecast the exchange rate under changed hydrological conditions. In cases where the hydraulic resistance of the stream bed is small and for materials with nonnegligible capillary forces, such forecasting can be erroneous.

DOI: https://doi.org/10.1080/00221689809498593

Year: 1998