Simulation of runoff from the Navarro watershed in Costa Rica was undertaken using the U.S. Geological Survey's Precipitation Runoff Modeling System, developed and operated within the Modular Mod eling System. A geographic information system (GIS) database of watershed characteristic s, and GIS-based methodology for delineatin g modeling response units (MRU) were utilized to enhance the spatial representation of watershed features and physical processes . Simulations were carried out using a series of MRU resolutions (from 90 m 2 to 1,260 rrr' ) in order to determine the range through which the hydrolo gic response could be accurately modeled. Fractal characterization provided a method to measure the relationship of the MRU patterns (based primarily on land cover) at the different resolutions. The range of self-similarity of the MRU patterns, as determined by fractal characterization, was compared to the range of resolutions through which accurate hydrologic simulation s were achieved. The results from this research revealed a strong correspondence between the two ranges, and indicate that a range of data resolutions may be useful in representing tropical watershed characteristics for hydrolog ic modeling and watershed management.