Application of a catchment water quality model for assessment and prediction of nitrogen budgets

The objective of this study is the assessment of a catchment water quality model towards its suitability for simulation of solute transport and nitrogen budgets and application in river basin management. The conceptual semi-distributed water budget and water quality model NPSM (Non Point Source Model) was applied in the meso-scale Brugga catchment (39,9 km²) in southwest Germany. Spatial distribution was achieved by classifying sub-areas based on the concept of hydrological chemical response units (CHRUs).

Parameter values for the runoff and nitrogen simulation were estimated from catchment properties, previous experimental and modelling studies, regional field studies and literature. Optimization of the parameter values, calibration of the model and validation of the simulation was achieved using the runoff curve, concentrations of nitrate and natural tracers. Sensitivity analysis revealed that only a few parameters were important for the simulation of runoff and the dynamics of the nitrogen fluxes. The simulated nitrate concentrations were strongly controlled by the nitrogen input, the water movements and the nitrogen reactions in the different sub-areas. Due to the lack of data, the parameterization of the nitrogen contents and reactions in the soil remained uncertain.

As a result of this study NPSM proved to be applicable in a meso-scale catchment performing satisfactory results for the simulation of stream flow. However the short time dynamics of the measured nitrate could not be fitted by the nitrogen simulation. An improvement of the short time performance can only be achieved using a more detailed soil and agricultural data basis. To evaluate the suitability of the model as a tool for river basin management, simulations for longer time periods and with additional nutrients have to be performed.