Fourteen years of IA-58 sunken pan evaporation data were analysed for predicting irrigation needs. The measurements were carried out in Ribeirão Preto, State of São Paulo. The data were grouped in 12 monthly series which followed the normal distribution with small skewness. All series had significant values for kurtosis, but April, May and June presented strong leptokurtic distributions, i.e., there was a piling up of scores in the center of the distribution. For each month the frequency distribution of mean daily evaporation as well as the comparison between the 75% level of probability of evaporation (recommended by FAO) and the 50% level (commonly used), are presented. Differences of up to 35%, during the rainy season, can occur between the two criteria and might result in improper design of irrigation system.