## 2 Operational Retrievals of ET from EO
### Operational Capability of EO Technology in ET Estimation
#### 2.2.3 MODerate Resolution Imaging Spectroradiometer (MODIS)  
Observations from the polar-orbiting MODerate Resolution Imaging Spectroradiometer (MODIS) sensor are used to derive operationally global evapotranspiration (ET)/latent heat flux (LE)/potential ET (PET)/potential LE (PLE) from a product named MOD16-ET. This product has a spatial resolution of 1 km and is available on an 8-day, monthly and yearly basis. The MOD16 ET product is available from the University of Montana's Numerical Terradynamic Simulation group.  
In the product, ET estimation is based on the Penman-Monteith equation, following a technique proposed by [PERSON] et al. [40; 41]. Terrestrial ET includes evaporation from wet and moist soil, from rain water intercepted by the canopy before it reaches the ground and the transpiration through stomata on plant leaves and stems. Vertically, ET is the sum of water vapour fluxes from soil evaporation, wet canopy evaporation and plant transpiration at dry canopy surface. Canopy conductance for plant transpiration is calculated by using LAI to scale stomatal conductance up to canopy level. Daily minimum air temperature (Tmin) is used to control dormant and active growing seasons for evergreen biomes. For a given biome type, two threshold values for Tmin and vapour pressure deficit (VPD) are listed in the Biome-Property-Look-Up-Table (BPLUT) to control stomatal conductance [40; 41]. MODIS 8-day fraction of Photosynthetically Active Radiation (fPAR) is used as vegetation cover fraction to quantify how much surface net radiation is allocated between soil and vegetation; MODIS 8-day albedo and daily surface downward solar radiation and air temperature from daily meteorological reanalysis data are used to calculate surface net radiation and soil heat flux; daily air temperature, Vapour Pressure Deficit (VPD) and relative humidity data and 8-day MODIS LAI are used to estimate surface stomatal conductance, aerodynamic resistance, wet canopy, soil heat flux and other key environmental variables. MODIS land cover is used to specify the biome type for each pixel and the biome-dependent constant parameters for the algorithm are saved in a Biome-Property-Lookup-Table (BPLUT). Except for minimum daily air temperature and VPD, which are adopted directly from the MODIS global terrestrial gross and net primary production (MODIS GPP/NPP), the BPLUT is tuned largely based on a set of targeted annual ET for each biome derived from MODIS GPP and water use efficiency calculated from eddy flux towers.  
A number of studies have been performed evaluating the ET retrieval accuracy by the product at different ecosystem types (e.g., [42]). Overall results from those studies have shown that the algorithm is able to provide ET at a resolution which is useful in a number of applications related to global terrestrial water and energy cycles and environmental changes [42; 43]. For example, [PERSON] et al. [44] evaluated spatiotemporal MODIS ET products in the Hai river basin. Inaccuracies, such as over- or under-estimation and no relationship were observed between the flux tower and MOD16 ET estimates in the above publications. Errors or uncertainties are assumed to be caused by misclassification of land cover types from the global MODIS land cover product, scaling from flux tower to landscape and algorithm limitations. [PERSON] et al. [45] evaluated the MOD16 ET product and found that the MOD16 method underestimated ET. Authors also pointed out the need to evaluate global remote sensing products again at long term monitoring sites in South Africa.