This study is located in the sub-basin of the Lwiro. The method of thornthwaite and statistical method were used. The values of monthly evapotranspiration have been estimated 60 mm for the months of June to July and 70mm for the remain months. A simplified outcome of the rain compared with the evapotranspiration for the period of 15 years (1999-2013) has showed that our sector of study is in its whole humid... About the 15 years of our observation period, 8 years have been qualified «dries», and 7 have been qualified «damps or humid». According to the data on the easily reserve usable as well as that of the effective filtrations, the right interval of time to the identification and to the catching of water's springs intended to supply the population in drinking water has been situated between the end of the month of May and August. The minimum pluviometric module in the sub-basin has 95 chances out of 100 so that it shall be included between 1291.98 mm and 1921.98 mm or although 5 chances out of 100 so that it shall be out of this interval. For the annual maximum pluviometric module in the sub-basin, there no chance for the interval of 50 years so that it being taken back. Then, for the civil engineering works, it agreed to take this pluviometric module value of 2182.0 mm like the minimum capacity of collectors. Nevertheless, it will be maximum value in case of technical or financial problems.