Simulation of Nutrient Transport in Streams During High Flows

The occurrence of high flows in watersheds requires models that link solute transport processes with hydraulics of flooded stream cross sections. For instance, nutrient export occurs predominantly during high flows. Important mechanisms include the retention caused by exchange with flood plains, stagnant water in the stream channel and retention in the hyporheic zone. The investigation uses a 1-dimensional network model and parameterizes the result in a form appropriate for nutrient in a compartment model that has been developed for Swedish conditions during decades. The compartment model is a (kinetic) mass-balance model that accounts for a large set of processes, while the stream network model uses the 1-dimensional advection-dispersion equation with transient storage and lateral inflow. The distribution of stream segment within the network is described through the width function, which reflects the frequency distribution of flow distances within the network. The ability to describe these processes in a physically based manner has provided a tool to improve predictability of the nutrient status of watershed and the possibility to trace and explain the problems behind e.g. eutrophication.