In this study, a regulated stream draining the Seydi Suyu watershed where intensive agriculture is practiced has been modeled with respect to nitrogen. Nitrogen is added to the stream from diffuse sources due to agricultural activities in the watershed. The magnitude of diffuse sources has been determined by using export coefficients. In the stream, the nitrogen cycle has been modeled by using the QUAL2EU model. For a period of two years, monthly samples have been taken from two monitoring stations along the stream. Data obtained from the first station have been used as model input and data from the second station have been used to calibrate the model. Observations and model results have been found to agree well. Moreover, sensitivity analysis has been applied to model inputs and parameters.