Arid and semiarid regions have particularities that make more difficult hydrological modeling, such as shallow soils, pronounced temporal and spatial irregularity of precipitation, and sometimes, lack of consistent data. In order to contribute to the hydrological studies in these regions, this research used the CAWM IV model (Campus Agreste Watershed Model Version IV), specially developed for applications in these areas. This model was used to simulate the input of natural flows in the Castanh & atilde;o reservoir, the most important reservoir in the state of Cear & aacute;, northeast of Brazil. From this determination, it is possible to analyze the synergistic gain from exogenous water from other basins. Covering the Project of Integration of S & atilde;o Francisco River (PISF), eight sub-basins were modeled up to the Metropolitan Region of Fortaleza, the capital of the state, in the period between 1969 and 2021, depending on the availability of data for each fluviometric station. The sum of the areas of all delimited sub-basins is 60,755.68 km2, corresponding to 40.8% of the state area. The results obtained confirmed the satisfactory performance of the modeling, with emphasis on medium and high flows. Despite the calculation and analysis of water synergy made only to Castanh & atilde;o reservoir, this method can be extended to the system of reservoirs that supply the Metropolitan Region of Fortaleza and other systems of reservoirs in an integrated manner. The research found a synergistic gain between 706 and 850 hm3 per year, as well as an increase in the regularized average flow, corresponding to results previously found by other authors.