SEBAL - Dca.ufcg.edu.br

Appendix 3Reference Evapotranspiration, ET r , CalculationThe reference evaotranspiration (ET r ) is the ET rate expected from a well-defined surfaceof full-cover alfalfa or clipped grass. ET r is used in SEBAL to estimate the ET at the “cold”pixel and to calculate the reference ET fraction (ET r F).ET r can be computed for a given weather station using the REF-ET software madeavailable by the University of Idaho:Allen, R.G. (2000). REF-ET: Reference Evapotranspiration Calculation Software forFAO and ASCE Standardized Equations, University of Idahowww.kimberly.uidaho.edu/ref-et/This software is included with this SEBAL manual and contains the ET r program andinstructions.A. Requirements for running REF-ET:Weather data for hourly or shorter time periods surrounding the date and time ofsatellite image is required. The weather data should include air temperature, solarradiation, wind speed, and humidity (in the form of dewpoint temperature, relativehumidity, or vapor pressure). Data for other days can be processed at the same timeand will be useful for predicting ET for adjacent time periods in SEBAL.The data file should be in “text” format (i.e., ASCII), and data columns can be separatedby tabs, spaces, or commas. The data columns can have headings and can be in anyorder and can use a wide range of data units. The order of the weather parameters andtheir units are defined within REF-ET. The weather data file should be created beforerunning REF-ET.The following figure is an example of hourly weather data with headings for use in REF-ET. Weather station information is defined in section B-5 of this appendix.47