A methodological approach and preliminary results on water resources assessment in large areas are described with the case study of Ceará State (Brazil). The methodology includes analysis and interpolation of climatic and hydrological data, Digital Elevation Model (DEM) production and interpretation, and GIS procedures. The present results include the distribution of historical rainfall and fluvial discharges and the definition of a 7- levels river and watershed hierarchy in the Ceará State. The highest rainfall and discharge average values are located in the northeastern sector of the state confirming the important role of climatic features in hydrological diversity. Water resources management must then consider technical tools for water resources assessment, in the line of other methods for quantitative assessment of natural features either biotic or abiotic.