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Therefore, the daily ET is obtained by multiplying ETrF by ETr-24; the latter being ETr for the day, obtained by summing hourly reference ET for the entire day. This approach is capable of capturing most of the advection impacts and any other change in weather conditions during the day.
formula
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The METRIC approach assumes that the ET for the entire area of interest changes in line with ETr calculated for the representative weather station (Allen et al. 2007a, 2007b). Estimation of ET for non-processed image days (whether cloudy or clear sky) by using clear-sky processed satellite image data is an effective approach (Tasumi et al. 2005). Generally, one satellite image per month is sufficient to construct an accurate ETrF curve for estimating seasonal ET. However, more frequent image intervals are needed in periods of rapid vegetation growth. In this study, the intervals of images were chosen as 10 to 20 days for better representation of all vegetation growth period in the study area (Table 2).
Table 2

Processed image dates

Date (year: 2010)Julian daysDateJulian days
23 April 113 14 July 195 
3 May 123 2 August 214 
14 May 134 17 August 229 
30 May 150 29 August 241 
15 June 166 11 September 254 
2 July 183   
Date (year: 2010)Julian daysDateJulian days
23 April 113 14 July 195 
3 May 123 2 August 214 
14 May 134 17 August 229 
30 May 150 29 August 241 
15 June 166 11 September 254 
2 July 183   
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