Measurable discharge and ground level drainage in the water board streams: Consideration of the usability of discharge standards for determination of the normal discharge, as a result of changes in the water management and the occurrence of ground level drainage.
To determine the status of a watercourse (whether or not to be managed by the waterboard), the Dutch waterboard Vechstromen applied a method where the design-discharge was generated with a typical relation between mean highest water table and this discharge. Evaluation of this approach led to the conclusion that this method has some drawbacks and it was not applied properly. A major omission is the lack of knowledge about the process of surface runoff. During the summer period the infiltration capacity of the soil is the most important property; during the winter period the storage capacity in and on the soil is the most decisive variable. Its spatial and temporal variabilities are high and difficult to determine in the field. For return periods of 1 and 10 years the probability of surface runoff was calculated. The GIS-based maps indicate that surface runoff can contribute considerably to the discharge, and climate change will have an increasing effect.