A method for the separation of total discharge into base flow, overland flow and channel precipitation for water quality modelling of a small watershed in the Netherlands

Abstract

For surface water quality modelling all contributing discharges, each with different loads of dissolved matter have to be considered separately. Apart from physical and (bio)chemical interactions, water quality is the result of all inputs, both in volume and mass. For this reason dynamic modelling of water quality is possible only when the processes leading to the temporal variability for each different type of input can be modelled as well. In the Netherlands almost all inland watersheds discharge considerable amounts of groundwater. During storm events however, surface runoff is an important factor even in these flat areas. Other discharge sources to be modelled are channel precipitation and effluent discharges. A dynamic one-dimensional numeric discharge model has been developed for a catchment area in the central part of the Netherlands, with distinct subareas where infiltration or seepage is dominant. Model output is dayly discharge of the three most important discharge components (groundwater discharge, overland flow and channel precipitation) and total catchment outflow. From these components groundwater discharge has been calculated using recorded levels of groundwater and surface water. Because precipitation volumes per day can be computed from meteorological data and surface water area, and effluent discharges usually are well known, overland flow discharge modelling was possible.