Application of an integrated framework for estimating nitrate loads from a coastal watershed in south-east Sweden

Nitrate-nitrogen (NO3-N) loading from a 734 ha coastal watershed draining into the Baltic Sea off south-east Sweden was simulated using a simple modelling approach in which the nitrogen model DRAINMOD-N II and a temperature-dependent NO3-N removal equation were incorporated into the Arc Hydro-DRAINMOD framework. Hydrology and water quality data collected during six periods between 2003 and 2007 were used to test Arc Hydro-DRAINMOD and its performance was evaluated by considering uncertainty in model parameters using GLUE methodology. The GLUE estimates (5th and 95th percentiles) and calculated monthly NO3-N loads were in satisfactory agreement. There are some sources of errors that may affect the performance of the framework, such as NO3-N load calculations, soil denitrification and in-stream removal of NO3-N. Although additional measurements may help to improve the understanding of these processes and reduce uncertainty, they cannot completely eliminate the uncertainty in framework predictions. These uncertainties must be evaluated by some methodology, such as the GLUE procedure. Sensitivity analysis showed the framework to be most sensitive to changes in stream baseflow and N removal processes in the stream network. These results show that the Arc Hydro-DRAINMOD framework can be an effective tool to support water stakeholders in managing NO3-N loading from small tile-drained watersheds at monthly time step.