## 2 Operational Retrievals of ET from EO
### Operational Capability of EO Technology in ET Estimation
#### 2.2.2 Geostationary Operational Environmental Satellite (GOES)  
A product has been developed to provide operational information on ET for North America based on Geostationary Operational Environmental Satellite (GOES) observations acquired in the visible and infrared parts of the electromagnetic spectrum. This product is generated from the Atmosphere-Land Exchange Inversion model (ALEXI) surface energy balance model [38; 39]. This is a the Two-Source Energy Balance (TSEB) approach based on the partitioning of the composite surface radiometric temperature into soil and canopy temperatures, based on the fractional vegetation cover. ALEXI has been specifically designed to minimise the need for ancillary meteorological data while maintaining a physically realistic representation of land-atmosphere exchange over a wide range in vegetation cover conditions. It is also one of few diagnostic land-surface models designed explicitly to exploit the high temporal resolution afforded by geostationary satellites [25]. These advantages make ALEXI capable of routine, long-term mapping of ET and soil moisture stress. A complete ALEXI processing infrastructure has been developed to automatically ingest and pre-process all required input data, to execute the model and to post-process model output which can later be visualised or used in practical applications or research. Because thermal infrared (TIR) retrievals of Land Surface Temperature (LST) are possible only under clear-sky conditions, a gap-filling algorithm has been implemented to estimate ET during cloudy intervals [38; 39]. The gap-filled model currently runs daily on a 10-km resolution grid covering the continental US (CONUS) using data from GOES. ALEXI is constrained to operate on spatial scales of 5-10 km, where atmospheric forcing by uniform land-surface behaviour becomes effective. Also, ALEXI ET is retrieved over clear-sky pixels daily and ALEXI drought product is generated over 1 to 6 month compositing periods each day. The generated ET maps are converted to the required formats (GRIB and others) and sent to OSPO for QC monitoring, to ESPC distribution server for distribution and to NCEI/CLASS for archiving.  
Outputs of the algorithm include the daily ET product and 2, 4, 8 and 12 week composite of the Evaporative Stress Index (ESI) computed from the ET daily estimates over North America at 8 km. The ALEXI ET estimates--consequently the GOES ET product--have been rigorously evaluated against multi-source in-situ ET observations and various existing drought indices, e.g., US Drought Monitor, Standardised Precipitation Index (SPI), etc. Findings of those studies suggest that the GOES  
Figure 2: Examples of the agreement between in-situ and predicted ET from SEVIRI for the different seasons for year 2011 for different sites. In particular, results are shown for: (**a**) FR MAU; (**b**) ES AGU; (**c**) IT MBO; (**d**) UK EBU. Green represents the in-situ ET daily mean, Red is the SEVIRI-predicted ET, Blue is daily standard deviation of the in-situ ET. Figure adopted from [35].  
ET algorithm performs well over a range in climatic and vegetation conditions and is capable of producing an average value of 5.5 mm h\({}^{-1}\) within an uncertainty of \(\pm\) 0.5 mm h\({}^{-1}\), when operated with an effective surface emissivity of 0.96 \(\pm\) 0.02 and a heat flux coefficient \"B\" of 0.20 \(\pm\) 0.05 [38; 39].