High Resolution Simulation of Nitrate and Ammonium From Point and Diffuse Sources in a Small Headwater Catchment

ABSTRACT Catchment water quality models are common tools for assessing hydrochemical processes in catchments. They improve the process understanding and help to identify pollutant sources. However, the spatial and temporal resolution of many models is too coarse to represent processes occurring within minutes or hours, making them unsuitable for use in fast‐responding catchments. Examples of such cases are headwater catchments or catchments influenced by urban agglomerations. ZIN‐AgriTra is a physically based model that allows simulations with fine temporal (< 1 h) and spatial (< 100 m) resolution. As it also allows the implementation of point sources, it is suitable for the simulation of headwater catchments with mixed land use. In this study, we test for the first time the ability of ZIN‐AgriTra to represent nitrogen transport and transformation processes in a point source influenced headwater catchment. High resolution time series of wastewater treatment plant (WWTP) effluent quantities were available as input to the model. For combined sewer overflow (CSO) discharges, only discharge times were measured. However, this knowledge was still valuable during the calibration process and improved the understanding of CSO contributions during events. Our model setup and modelling strategy allowed us to simulate nitrate and ammonium export from the catchment sufficiently. Overall, point sources have a significant impact of the sensitivity of model parameters by influencing the mixing ratio between point sources and stream discharge. As point sources were found to have a large impact on water quality and quantity, not considering them would inevitably lead to incorrect parameterisation of model parameters. Models should become more inclusive in order to be able to represent processes in mixed land use catchments, especially in places, where data availability is limited.