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

Abstract

Reference evapotranspiration (ET0
) 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 ET0
and rainfall
probability distributions. A 30-year ET0
and rainfall dataset (1990–2019) was obtained from the ESALQ/USP weather station. The
water balance between ET0
and 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 ET0
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 ET0
and rainfall data, respectively. To reach a crop
coeffi 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 use
of mean monthly rainfall and ET0
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 ET0
values occurred with non-exceedance probabilities of < 56%.