Hargreaves and other reduced-set methods for calculating evapotranspiration

Abstract

Globally, irrigation is the main user of fresh water, and with the growing scarcity of this essential natural resource, it is becoming increasingly important to maximize efficiency of water usage. This implies proper management of irrigation and control of application depths in order to apply water effectively according to crop needs. Daily calculation of the Reference Potential Evapotranspiration (ETo) is an important tool in determining the water needs of different crops. The United Nations Food and Agriculture Organization (FAO) has adopted the Penman-Monteith method as a global standard for estimating ETo from four meteorological data (temperature, wind speed, radiation and relative humidity), with details presented in the Irrigation and Drainage Paper no. 56 (Allen et al. 1998), referred to hereafter as PM. The main limitation to generalized application of this methodology in irrigation practice is the time and cost involved in daily acquisition and processing of the necessary meteorological data. Additionally, the number of meteorological stations where all these parameters are observed is limited, in many areas of the globe. The number of stations where reliable data for these parameters exist is an even smaller subset.