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Estimating interannual to decadal variability of terrestrial evapotranspiration (ET) requires use of standard meteorological data complemented with some high-resolution satellite data. A semiempirical expression for this purpose is developed and validated with data from 2000 to 2007. These data were collected at 64 globally distributed sites, including the continuous measurements collected by the Atmospheric Radiation Measurement (ARM) and FLUXNET projects, and are the longest available, with continuous worldwide multisite measurements of ET, and a total of 274 site years. The sites are mainly located in North America and Asia, with the exception of three sites in Australia, two in Europe, and one in Africa. The climates of the sites vary from tropical to subarctic and from arid to humid. The land cover types of the sites vary from desert, croplands, grasslands, and shrub land to forests. On average, the 16 day average daily ET can be estimated with an error (standard deviation) of 17 W m(-2) (25% in relative value), and with an average correlation coefficient of 0.94. The standard deviation of the comparison between measured and predicted site-averaged daily ET is 9 W m(-2) (14%), with a correlation coefficient of 0.93. The model is also satisfactory in reproducing the interannual variability at sites with 5 years of data in both humid and arid regions. The correlation coefficient between measured and predicted annual ET anomalies is 0.85. This simple but accurate method permits us to investigate decadal variation in global ET over the land as will be demonstrated in part two of this paper series.