Evaluation of irrigation requirement for the design of an irrigation system using a probabilistic approach for the estimation of evapotranspiration and rainfall

ABST RACT - Reference evapotranspiration (ET 0 ) and rainfall are basic variables for estimating the net irrigation depth (NID). The objective of this study was to estimate the NID for designing irrigation systems in Piracicaba, SP, Brazil, using ET 0 and rainfall probability distributions. A 30-year ET 0 and rainfall dataset (1990–2019) was obtained from the ESALQ/USP weather station. The water balance between ET 0 and rainfall indicated July, August, and September as months of higher water de fi cit. Based on the fi rst-order Markov chain, August presented the highest water de fi cit. Rainfall and ET 0 were estimated on 19 probability levels, and four probability distributions such as normal, log-normal, beta, and mixed gamma were evaluated. The analysis of historical August series using accumulated values in periods of fi ve, ten, or 15 days is recommended for sizing irrigation designs in Piracicaba, SP, Brazil. The log-normal and mixed gamma probability distributions presented the best fi t for ET 0 and rainfall data, respectively. To reach a crop coe ffi cient K c = 1 in Piracicaba, SP, Brazil in August, the irrigation system should be designed for an NID of 4.1 mm day − 1 . The use of mean monthly rainfall and ET 0 values for designing irrigation systems underestimates the NID by a mean of 26.6% compared to estimates made at a probability of 75% at fi ve-, ten, and 15-day intervals because the mean rainfall values occurred with exceedance probabilities of < 36%, and mean ET 0 values occurred with non-exceedance