Revised FAO methodology for crop-water requirements

Abstract

In the early 1970s, the Food and Agriculture Organization of the United Nations (FAO) developed a practical procedure to estimate crop-water requirements that has become a widely accepted standard, in particular for irrigation studies. Since its publication as FAO Irrigation and Drainage Paper, new concepts and advances in research have revealed shortcomings in the methodology and made necessary a review and revision. A consultation of experts organized by FAO recommended the adoption of the Penman-Monteith combination method as a new standard for reference evapotranspiration, and advised on procedures for calculation of the various parameters. By defining the reference crop as hypothetical with an assumed height of 0.12 m, a surface resistance of 70 s m{sup -1} and an albedo of 0.23, closely resembling the evaporation of an extensive surface of actively growing and adequately watered green grass of uniform height, the FAO Penman-Monteith method was developed, overcoming previous deficiencies and providing values more consistent with actual crop-water-use data worldwide. Furthermore, recommendations have been developed for the use of the FAO Penman-Monteith method with limited climatic data, largely eliminating the need for any other reference evapotranspiration methods and creating a consistent and transparent basis for a globally valid standard for crop-water-requirement calculations. (author) 19 refs, 2 tabs