How to construct recursive digital filters for baseflow separation

There is widespread agreement that good correspondence of measured and calculated streamflow at the catchment outlet is not a sufficient criterion for the validity of a physically based hydrologic model, but that additional knowledge concerning catchment-internal processes is needed (e.g. Beven, 1989). Yet, for many catchments such information is not available and the model’s performance can only be assessed by comparing calculated and measured streamflow. However, the information provided by a hydrograph is not limited to the absolute height of the runoff. For example, a separation of the hydrograph allows for identification of runoff components originating from different compartments of the considered system, and responding differently delayed and smoothed to rainfall events. An example of the potential benefit of this information for model
verification has been given by Eckhardt et al. (2002).