Estimation of Reference Evapotranspiration during the Irrigation Season Using Nine Temperature-Based Methods in a Hot-Summer Mediterranean Climate

Abstract

The FAO-56 Penman–Monteith (PM) equation is regarded as the most accurate equation to estimate reference evapotranspiration (ETo). However, it requires a broad range of data that may not be available or of reasonable quality. In this study, nine temperature-based methods were assessed for ETo estimation during the irrigation at fourteen locations distributed through a hot-summer Mediterranean climate region of Alentejo, Southern Portugal. Additionally, for each location, the Hargreaves–Samani radiation adjustment coefficient (kRs) was calibrated and validated to evaluate the appropriateness of using the standard value, creating a locally adjusted Hargreaves–Samani (HS) equation. The accuracy of each method was evaluated by statistically comparing their results with those obtained by PM. Results show that the calibration of the kRs, a locally adjusted HS method can be used to estimate daily ETo acceptably well, with RMSE lower than 0.88 mm day−1, an estimation error lower than 4% and a R2 higher than 0.69, proving to be the most accurate model for 8 (out of 14) locations. A modified Hargreaves–Samani method also performed acceptably for 4 locations, with a RMSE of 0.72–0.84 mm day−1, a slope varying from 0.95 to 1.01 and a R2 higher than 0.78. One can conclude that, when weather data is missing, a calibrated HS equation is adequate to estimate ETo during the irrigation season.

A Simple Procedure to Estimate Reference Evapotranspiration during the Irrigation Season in a Hot-Summer Mediterranean Climate

“…Table 2 shows the validated k Rs for each month and location, as well as the irrigation season calibrated factor as suggested by [14]. Results show that, for most locations, the monthly calibrated k Rs differ, both locally and monthly, from the seasonal coefficient as proposed by [14].…”

“…The region, according to Köppen-Geiger classification, has a Csa climate, and is characterized by a semiarid Mediterranean climate of hot and dry season in the summer and mild temperature associated with annual rainfall in winter. A more comprehensive characterization of the study is presented by [14]. This study was conducted from April to October, the period that covers the growing season of the main crops in Alentejo, where irrigation is essential to maintain crop yields and profitability.…”

“…Table 2 shows the validated k Rs for each month and location, as well as the irrigation season calibrated factor as suggested by [14]. Results show that, for most locations, the monthly calibrated k Rs differ, both locally and monthly, from the seasonal coefficient as proposed by [14]. Locations with the same seasonal coefficient, such as Beja, Castro Verde, Odemira and Redondo (seasonal k Rs = 0.17), tend to have different validated monthly k Rs throughout the irrigation season.…”

“…Other temperature-based methods to estimate ETo are available in the literature [8,[10][11][12][13]. Rodrigues and Braga [14] compared nine different temperature-based methods in order to determine the best model based on the weather conditions of fourteen locations in Alentejo, Southern Portugal. They have shown that, after calibrating the k Rs for a specific location, the Hargreaves-Samani (HS) method showed its appropriateness for the most of part for the locations, leading to acceptable ETo estimations (RMSE = 0.84 mm day −1 ).…”

“…However, the method requires the calibration of a site-specific factor, dependent on average temperature. Still, when assessing its accuracy for the Alentejo region, [14] concluded that this method lacks a scaling capability since it led to an RMSE varying from 2.22 to 4.00 mm day −1 when estimating ETo for fourteen locations across the region.…”