The difficulties involved in calibration of physically based erosion models have been partly attributable to the lack of robust optimization tools. This paper presents the essential concepts and application to optimize channel and plane parameters in an erosion model, with a global optimization method known as Repulsive Particle Swarm (RPS), a variant of Particle Swarm Optimization (PSO) method. The physically-based erosion model that which was chosen is called WESP (watershed erosion simulation program). The optimization technique was tested with the field data collected in an experimental watershed located in a semi-arid region of Brazil. On the basis of these results, the recommended erosion parameter values for a semi-arid region are given, which could serve as an initial estimate for other similar areas.