The present paper describes an application of the FLUX program to obtain the nutrient loads for the Passaúna Reservoir in Curitiba´s Metropolitan Region, in the state of Paraná, Brazil. The difficulties and advantages associated with model usage are listed, highlighting the tools available to increase the efficiency of monitoring systems which provide the elements for water quality evaluation models. Estimates of loads in the tributary watersheds are presented, and the associated export coefficients are obtained, indicative of soil use and nutrient-related pollution levels. A comparison of these export coefficients with published values indicates that unaccounted-for non-point sources are likely to exist in the area of study.