Some hidrologic studies were undertaken in the period of June 1945 up to July 1947, on small basins located in the «Dr. Theodureto de Camargo» Experiment Station, of the Instituto Agronômico in Campinas. Two of the basins, respectively of 120 and 180 hectares, were envelopped by a third basin of 522 hectare limited downstream by an earth dam. The inflow and outflow of the impounded water have been measured, as well as the flow of the two basins. The data collected were analysed by means of simple and mass diagrams of surface flow and rainfall. Comparisons of the evapotranspiration data obtained from the hydrologic studies were made with potencial evapotranspiration data obtained by using Thornthwaite's empirical formula, with coincidentally remarkable agreement, with an average monthly evapotranspiration of about 80 milimeters. The percentages of surface flow in relation to total rainfall were found to average about 26.5% from which a small portion only (3% of the total rainfall) was atributed to surface runoff. The remaining 23.5% is believed to be the percolated water which appeared as ground water flow. Total losses (infiltration + evaporation), occurring due to the accumulation of water by the old existing earth dam were estimated to be of the order of 50% of the total inflow, thus indicating that only 50% of the initial inflow could be used, the remaining being accounted as unavoidable losses. It is believed that part of these losses could be minimized if the dam structure were built according to modern techniques. Rippl diagrams were used and its usefulness in solving problems of storage of water for irrigation purpose was shown by practical examples. The method used in this work to determine the total losses in impounding water, if properly applied to measure the total inflow and outflow, is believed to yield the best results, since no estimated data are obtained but actually measured.