Revised FAO Methodology for Crop Water Requirements

Revised FAO Methodology for Crop Water Requirements Martin Smith Land and Water Development Division, Food and Agriculture Organization of the United Nations Via delle Terme Caracalle - 00100 Rome - Italy e-mail: Martin.Smith@FAO.org

In the early 1970's, FAO developed a practical procedure to estimate crop water requirements, which has become a widely accepted standard, in particular for irrigation studies. Since the publication of the methodology as FAO Irrigation and Drainage Paper No24, new concepts and advances in research made a review and revision necessary.

A consultation of experts organized by FAO in 1990 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. On the recommendations of the Expert Consultation reference is made to the Report of the Expert Consultation, which can be downloaded in WP5 format from the FAO-FTP server (ETO-REPT.ZIP)

Revised procedures have been developed by FAO in cooperation with an international working group of high level experts to estimate Crop Evapotranspiration based on the Penman-Monteith approach. Details on the revised calculation procedures are described in the Report of the Proceedings of the FAO Expert Consultation and can be separately obtained by downloading in WP5 format from the FAO-FTP server. (ETO-ANN5. ZIP)

Revised calculation procedures according Penman-Monteith are included also in the CROPWAT computer programme. (FAO Irrigation and Drainage Paper No 46) and the CLIMWAT database (FAO Irrigation and Drainage Paper No49).

To verify the step by step calculation procedures of the FAO Penman-Monteith a spreadsheet was developed, which may be of use for those who develop their own algorithms for the FAO Penman-Monteith equation and want to verify calculation steps. The spreadsheet can be downloaded in MS EXCEL Format from the FAO server. (FAO-PMON.ZIP)

The FAO-Penman-Monteith equation is recommended as the standard method for estimating reference and crop evapotranspiration. The new method has proved to have a global validity as a standardised reference for grass evapotranspiration and has found recognition both by the International Commission for Irrigation and Drainage and by the World Meteorological Organization.

The change of ETo definition to a hypothetical crop with fixed parameters has, to a large extent, eliminated problems related to the previous requirements in measuring a living reference ETo and will further facilitate the calibration of crop coefficients for the estimation of crop water use.

To allow the use of the FAO Penman-Monteith method also in conditions where limited climatic data are available, procedures were developed and tested to estimate missing data. The missing data procedures have proved more accurate than adoption of other empirical methods and will greatly increase the transparency and consistency of reference and crop water requirement calculations and eliminate the use of any other ETo method.

Information on the missing data procedure are included in a recent article presented at the ASAE International Conference on Evapotranspiration and Irrigation Scheduling in San Antonio, USA in November 1996 and can be obtained from the FAO-FTP server (ASAE-PAP.ZIP).

A new FAO publication in the Irrigation and Drainage series on Crop Evapotranspiration is presently under preparation and will contain full details on the new procedures for crop water requirements including revised values and procedures on Kc estimates.