National Aeronautics and Space Administration

Stage 3 validation has been achieved for the MODIS Collection 6 evapotranspiration product (MOD16).

The mean absolute errors are 24% of the ET measurements, within the 10-30% range of the accuracy of ET observations (Mu et al, 2011). The correlation coefficients of the tower-specific improved ET estimates with the ET measurements averaged over all the available days is 0.86 when driven by NASA Goddard's Global Modeling and Assimilation Office (GMAO) meteorological data.

The MODIS evapotranspiration (ET) algorithm is based on the logic of the Penman-Monteith equation, which uses daily meteorological reanalysis data and 8-day remotely sensed vegetation property dynamics from MODIS as input. MOD16 Collection 6 products include terrestrial ecosystem evapotranspiration (ET), latent heat flux (LE), potential ET (PET), and potential LE (PLE) data layers and are produced at 500m spatial resolution.

To validate the MOD16 algorithm, we used the observed latent heat flux for 46 field-based eddy covariance flux towers over the 11 years (2000 to 2010), driven by two sets of meteorological data: tower-based meteorological data and GMAO meteorological data (Mu et al, 2011).

Micrometeorological tower sites use the eddy covariance technique to measure exchanges of carbon dioxide, water vapor, and energy between terrestrial ecosystems and the atmosphere. Their data have been used extensively for direct validation and optimization of many global water and carbon models. Figure 1 shows an example the validation carried out of the MOD16 ET algorithm for two selected sites, while Figure 2 compares average ET estimates by MOD16 and the 46 micrometeorological tower sites.

Figure 1. The ET measurements (black dots, OBS), the ET estimates driven by flux tower measured meteorological data (red lines) and GMAO meteorological data (blue lines) over 2000-2006 at seven tower sites, Donaldson (a) and LBA Tapajos KM67 Mature Forest (b). From Mu et al (2011).

Figure 2. Comparisons of the average ET observations to the average daily ET estimates across all the available days at the 46 flux tower sites. These data were created using (1) tower-specific meteorology (left) and (2) the global GMAO meteorology (right) with the MOD16 ET algorithm. The solid grey lines represent that the ratio of ET estimates to ET measurements is 1.0 and the solid black lines are the regression of the ET estimates to measurements. (from Mu et al 2011)