Comparison between the vegetation index and the relative evapotranspiration from the Energy Balance Monitoring System, in north eastern China.

Left image: Difference in vegetation index between 1st decade of August 2001 and the 1st decade of August 2000. Results from the polar Fen Yun 1 satellite. Red and brown = vegetation index in 2001 lower than in 2000. Yellow = vegetation index in 2001slightly lower than in 2000. Green = vegetation index in 2001 higher than in 2000. Black = no vegetation or area covered by clouds.

Right image: Difference in relative evapotranspiration between 2nd decade of July 2001 and the 2nd decade of July 2000 from the Energy Balance Monitoring System. Red = relative evapotranspiration in 2001 lower than in 2000. Green =  relative evapotranspiration in 2001 higher than in 2000. Yellow = no change

Very comparable results, but Fen Yun-1 vegetation index is 20 days behind EBMS relative evapotranspiration

The vegetation index lags behind the relative evapotranspiration. In the case of a dry spell the spectral behavior of vegetation in the visible and near infrared part of the spectrum (use in the vegetation index) will not change immediately, but the actual evapotranspiration will do so, because when experiencing water stress the stomata of leaves respond immediately and will close partly or fully. Therefore, the relative evapotranspiration represents the real actual crop growth conditions.