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where; ET inst is the instantaneous ET (mm/hr), 3600 is the time conversion fromseconds to hours, and λ is the latent heat of vaporization or the heat absorbed when akilogram of water evaporates (J/kg).2. The Reference ET Fraction (ET r F) is defined as the ratio of the computedinstantaneous ET (ET inst ) for each pixel to the reference ET (ET r ) computed fromweather data:ETinstETrF = (43)ETrwhere; ET inst is from Equation 41 (mm/hr) and ET r is the reference ET at the time of theimage from the REF-ET software (mm/hr). ETrF is similar to the well-known cropcoefficient, K c . ETrF is used to extrapolate ET from the image time to 24-hour or longerperiods.One should generally expect ET r F values to range from 0 to 1. At a totally dry pixel,ET = 0 and ET r F = 0. A pixel in a well established field of alfalfa or corn can occasionallyhave an ET slightly greater than ET r and therefore ETrF > 1, perhaps up to 1.1.However, ET r generally represents an upper bound on ET for large expanses of wellwateredvegetation. Negative values for ET r F can occur in SEBAL due to systematicerrors caused by various assumptions made earlier in the energy balance process.ET inst and ET r F are computed in model F25 along with 24-hour ET as shown in Section 5below.5. 24-Hour Evapotranspiration (ET 24 )Daily values of ET (ET 24 ) are often more useful than instantaneous ET. SEBAL computesthe ET 24 by assuming that the instantaneous ET r F computed in model F25 is the same asthe 24-hour average. Figure 5 shows a plot of ET and ET r F vs. time. This graph shows howET varies throughout the day while ET r F is relatively constant. Finally, the ET 24 (mm/day)can be computed as:ET = ETrF × ET(44)24 r _ 24where; ET r-24 is the cumulative 24-hour ET r for the day of the image. This is calculated byadding the hourly ET r values over the day of the image.ET 24 is computed in model F25. Enter the files for H, G, R n , T s , and the values for ET r andET r -24. Save the output files and the model.35