This paper presents a study about the monthly effective rainfall, using the probable precipitation which is calculated by the Log-normal distribution, for the levels of 75 and 90% of probability, using daily precipitation data from the period of 1914 to 1991, in the region of Lavras - Minas Gerais - Brazil. The data was obtained from the climatologic station of the Universidade Federal de Lavras, employing three methods to calculate the effective rainfall: fixed percentage (PF) (20, 50 and 80%), dependable rain (FAO/AGLW) and Soil Conservation Service Method (USDA). The results for the level of 75% of probability rendered an annual effective rainfall of 125.4, 313.6, 501.8, 330.6 and 485.0 mm for the methods: PF 20, 50 and 80%, FAO and USDA, respectively. For the level of 90% of probability, the annual effective rainfall found was of 79.5, 198.8, 318.1, 188.8 and 327.8 mm for the methods: PF 20, 50 and 80%, FAO and USDA, respectively.
hydrology; effective rainfall; log-normal distribution
