SEBAL - Dca.ufcg.edu.br

can be flagged. Masking can be done following the processing of ET, but is an importantfinal step.B. Header File InformationTM images are generally created with an associated “header” file. The header file for thesatellite image is a relatively small file that contains important information for the SEBALprocess. The following information must be obtained from the header file for entry intoSEBAL:• The satellite overpass date and time• The latitude and longitude of the center of the image• The sun elevation angle (β) at the overpass time• Gain and Bias levels for each bandThe satellite overpass time is expressed as Greenwich Mean Time (GMT) and must beconverted to local time (see Appendix 5).For Landsat 7, the header file includes the gains and biases for bands 1-5 and 7. Theseparameters are used to convert digital numbers (DN’s) in the original files into energy units.For band 6, the thermal band, both high and low gain images are supplied and the usermust decide which one to use for SEBAL. We recommend using the low gain image, whichyields slightly lower resolution, but is less likely to suffer from saturation . When the lowgain image is selected, use the gain and bias information for low gain. If the header file ismissing, the user must use the internet to find gain and bias data for the image. Appendix 1gives information about acquiring header file information.C. Land-Use MapA land-use map is not a requirement for SEBAL but is highly recommended since it isuseful in estimating the surface roughness parameter (z om ). The land-use map divides thearea of interest into various general classes of land-use such as agriculture, city, water,desert, forest, grassland, etc. Appendix 2 lists the general land-use types used in the Idahoapplication of SEBAL.D. Weather DataThe following weather data are recommended for processing SEBAL:• Wind speed (required, hourly data is preferred)• Precipitation (daily data are recommended for a several week period prior to theimage)In addition to wind speed data, the following weather data are recommended forcalculating hourly and daily reference ET. If the reference ET data are available forthe image area, then the following data are not needed.• Humidity (hourly data such as vapor pressure or dew point temperature)• Solar radiation (hourly is preferred)• Air Temperature (hourly is preferred)12