The process-based INCA model was applied to Dalelva Brook (3.2 km2) and the Bjerkreim River (685 km2) including several subcatchments, in order to test the model's ability to simulate streamwater nitrate (NO3−) dynamics and output fluxes under highly contrasting climatic conditions and nitrogen (N) loading. The simulated runoff volumes and mean NO3−concentrations at Dalelva and Bjerkreim were within +2 to +10% of the measured average during 1993–1995 (–19 to +31% within individual years). INCA to a great extent also reproduced the observed streamwater flow dynamics at both study sites (coefficient of determination, r2> 0.70). Temporal variation of streamwater NO3−during 1993–1995 was captured quite well by the model, especially at small catchments with a distinct seasonal NO3−pattern (r2= 0.46–0.68). At the Bjerkreim River outlet, the relationship were somewhat weaker (r2= 0.26, p < 0.01). Despite a few situations where the model failed to capture the streamwater NO3−dynamics, INCA proved to be a quite robust tool for simulating NO3−dynamics and output fluxes in the two study catchments.