The objective of this study was to determine the temporal and spatial variability of rainfall and to evaluate the influence of the relief as a potentiator of the orographic effect of rains in the Jacaré Guaçú river basin, located in the interior of the state of São Paulo. Rainfall, topographic data, statistical analysis and Geoprocessing techniques were used to achieve the proposed objectives. The results show that the average annual rainfall in the basin varies between 1313 and 1680 mm. The lowest values are found in the western, northwest and southeastern sections of the hydrographic basin and the largest in the east, in the upper Jacare Guaçú. For the seasonal period the average values vary between 1082 and 1329 mm in the rainy season (October to March) and 301 and 364 mm in the least rainy (September to April). The spatial distribution of seasonal data is similar to that of the annual period. The relief was shown to influence the variability of rainfall in the river basin. The rainfall gradient is 0.8 mm per meter of altitude from west to east, 1.1 mm from northwest to southeast, and 1.6 mm from north to south. The results can help in the management of the water resources of the municipalities that compose the hydrographic basin.
