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: