This paper introduces a fuzzy rule-based model to estimate runoff in a tropical watershed using the Soil Conservation Service Curve Number model. The data used were collected within a tropical watershed covered by Atlantic forest and pasture and involved map compilation, ASTER multispectral data processing and fieldwork. The computer models were developed using the PCRaster Environmental Modeling Language. Fuzzy theory allowed the mathematical modeling in zones of imprecision and/or uncertainties as fuzzy theory considers the transition zones among geo-objects. The evaluation of runoff derived from fuzzy and boolean methods demonstrated that the former provided calculated runoff closer to the measured runoff in the watershed, confirming the suitability of the fuzzy theory in modeling natural phenomena. (c) 2007 Elsevier Ltd. All rights reserved.232244253