poster - International Conference of Agricultural Engineering

Calibration of Hargreaves Equation for estimating reference
evapotranspiration in the Southeast of Spain
A. Ruiz-Canales 1 ; J.M. Molina Martínez 2 ; D.G. Fernández-Pacheco 2 ; H.
Puerto Molina 1 , R. López-Urrea 3
1 Agua y Energía para una Agricultura Sostenible (AEAS). Departamento de Ingeniería.
Escuela Politécnica Superior de Orihuela. Universidad Miguel Hernández, Crtra. de Beniel,
km 3,2, 03312 Orihuela (Alicante), Spain.
2 Grupo de Investigación en Ingeniería Agromótica y del Mar. Universidad Politécnica de
Cartagena, Paseo Alfonso XIII 48, 30203 Cartagena (Murcia) Spain.
3 Water Management Research Unit, Instituto Técnico Agronómico Provincial (ITAP) y
FUNDESCAM (Albacete). Spain.
* Corresponding author. E-mail: acanales@umh.es
Abstract
This study calibrates Hargreaves equation in order to estimate daily evapotranspiration
(ETo) in semiarid conditions at the Southeast of Spain. Firstly, the Hargreaves equation
determined by using meteorological data registered daily from several agrometeorological
stations of the Irrigation Advisory Service for Farmers (SIAR) of several nearby autonomous
regions of Spain during period 2005-2009 (Comunidad Valenciana, Región de Murcia and
Castilla-La Mancha) is obtained. Next, these obtained values were compared with the ones
calculated by using the Penman-Monteith FAO56 equation. ETo estimations obtained by
both methods were later compared with a simple regression analysis, and several
comparative statistics were calculated. According to the obtained results, the Hargreaves
equation is precise enough to obtain daily estimations of ETo in the studied zone without a
previous calibration. It would be necessary to extend this study to a biggest number of
agrometeorological stations in order to obtain a local or regional scale calibration.
Keywords: reference evapotranspiration, Hargreaves equation, Penman-Monteith equation,
calibration, semiarid conditions.
1. Introduction
Crop water consumption must be adjusted to the atmospheric demand, which depends on
climatic conditions. Doorenbos and Pruitt (1975) proposed a methodology for computing crop
evapotranspiration (ETc) based on reference evapotranspiration (ETo) and crop coefficients
(Kc). One of the methodologies to estimate ETo is by using empirical equations from
metheorological data. The most extended indirect methodology is the FAO56 Penman-
Monteith method (Allen et al., 1998), which requires a great availability of metheorological
data to calculate ETo. Parameters such as relative humidity, air temperature, solar radiation
and wind speed, among others, are required. While air temperature is available in the
majority of metheorological stations all over the world, the rest of data is collected only in few
occasions and their reliability is doubtful. For this reason, the use of standard methods to
estimate ETo when a direct method is not available is required. The need of a precise and
simple method to estimate ETo has been pointed out by numerous authors.
One of the most simple equation to determinate ETo is Hargreaves equation (Hargreaves
and Samani, 1985). This equation only requires average, maximum and minimum daily
values of temperature and extraterrestrial radiation. Previous works have demonstrated that
Hargreaves equation provides precise estimations of ETo (López-Urrea et al., 2006).
However, this method usually overestimates ETo in humid regions, and underestimates it in
very dry zones. For these reasons, Hargreaves equation has to be evaluated and, if it is
necessary, calibrated for being used with a required precision in a determined zone.
Methodologies that use an equation to calibrate another one have been used by other
authors (Allen et al., 1998). If a direct measuring method, e.g. for lysimeter measurements, is
not available, then these methodologies are needed. In previous works carried out near the