The objective of this work was to determine the orthophosphate and nitrate loads in runoff at the Ribeirão Fortuna micro-basin, located in the municipality of Timbó, in Itajaí basin. This micro-basin was monitored by the Micro-basin Project, developed by EPAGRI, in the state of Santa Catarina, Southern Brazil. Samples were collected in four fluviometric sections along the main water course and substance concentrations were analysed. The IPHS1 hydrologic model was used for simulating the transformation process of precipitation in discharge. Model parameters were calibrated for a fluviometric station located nearby, which has a historical series of daily average discharge. The annual loads were calculated and correlations were established between the runoff discharge and orthophosphate and nitrate loads. It was noticed that the orthophosphate specific loads are superior to the nitrate ones and the occupied area for the annual crops and pasture in the basin explains the yearly estimated specific phosphate loads. Nevertheless, the use of the agricultural soil in the sub-basins is not related to the nitrate loads in the drainage network.
nutrient losses; runoff; agricultural watershed
