The Atmosphere-Land Exchange Inverse (ALEXI) a two-source energy balance model was developed to estimate ET. The Visible Infrared Imaging Radiometer Suite (VIIRS) a polar satellite used in this research to provide 375-m resolution compared to other geostationary satellite data which have more than 1 km resolution. VIIRS acquires images of the Globe on daily basis; day/night images. The ALEXI model takes advantage of day/night thermal infrared imaging to produce daily regional ET estimates using a LST differential to retrieve energy balance components between midmorning after sunrise and before noon local time. Daily Evapotranspiration maps were produced with 15o X 15o grid size (Tile). We ran ALEXI for Tile 153, over Brazil for the years 2013-2018. We created a website called Global Daily Evapo-Transpiration (GloDET) where we publish these maps at (https://glodet.nebraska.edu). The ALEXI estimated ET values were compared with ground data from eddy covariance flux towers. ALEXI ET results were extracted at the towers locations for 2013-2016, to serve as comparison for each tower with energy balance closure. The linear correlation was excellent for all sites with R2 between 0.78 - 0.88, for different types of vegetation.
The regular monitoring of the evapotranspiration rates and their links with vegetation conditions and soil moisture may support management and hydrological planning leading to reduce the economic and environmental vulnerability of complex water-controlled Mediterranean ecosystems. In this work, the monitoring of water use over a basin with a predominant oak savanna (known in Spain as dehesa) was conducted for two years, 2013 and 2014, monitoring ET at both fine spatial and temporal resolution in different seasons.
A global 5 km daily ET product, developed with the ALEXI model and MODIS day-night temperature difference, was used as starting point. Flux estimations with higher spatial resolutions were obtained with the associated flux disaggregation scheme, DisALEXI, using surface temperature data from the polar orbiting satellites MODIS (1 Km, daily) and Landsat 7/8 (60-120m and sharpened to 30m, 16 days) and the previously estimated coarse resolution fluxes. The results achieved supported the ability of this scheme to accurately estimate daytime-integrated energy fluxes over this system, using input data with different spatio-temporal resolution and without the need for ground observations. Daily ET series at 30 m spatial resolution, generated using STARFM fusion technique, has provided a significant improvement in spatial heterogeneity assessment of the ET series, with RMSE values of 0.56 and 0.68 mm/day for each year, representing an enhancement with respect to interpolated Landsat series. In summary, this approach was demostrated to be robust and operative to map ET at watershed scale with a suitable spatial and temporal resolution for applications over the dehesa ecosystem.
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