| Sumario: | Reference evapotranspiration (ET0) and rainfall are basic variables for estimating the net irrigation depth (NID). Theobjective of this study was to estimate the NID for designing irrigation systems in Piracicaba, SP, Brazil, using ET0and rainfallprobability distributions. A 30-year ET0and rainfall dataset (1990–2019) was obtained from the ESALQ/USP weather station. Thewater balance between ET0and rainfall indicated July, August, and September as months of higher water defi cit. Based on the fi rstorder Markov chain, August presented the highest water defi cit. Rainfall and ET0were estimated on 19 probability levels, and fourprobability distributions such as normal, log-normal, beta, and mixed gamma were evaluated. The analysis of historical August seriesusing accumulated values in periods of fi ve, ten, or 15 days is recommended for sizing irrigation designs in Piracicaba, SP, Brazil. Thelog-normal and mixed gamma probability distributions presented the best fi t for ET0and rainfall data, respectively. To reach a cropcoeffi cient Kc= 1 in Piracicaba, SP, Brazil in August, the irrigation system should be designed for an NID of 4.1 mm day−1. The useof mean monthly rainfall and ET0values for designing irrigation systems underestimates the NID by a mean of 26.6% compared toestimates made at a probability of 75% at fi ve-, ten-, and 15-day intervals because the mean rainfall values occurred with exceedanceprobabilities of < 36%, and mean ET0values occurred with non-exceedance probabilities of < 56%.
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