Remote Sensing of Evapotranspiration for Global Drought Monitoring

Abstract

Evapotranspiration (ET) is one of the main components of the hydrological or water cycle. The latent heat from evapotranspiration is also one of the most important components of the energy cycle because it is the largest energy source for the atmosphere and thus is significant for weather and climate formation. Local scale evapotranspiration is mostly observed with ground instruments such as a lysimeter, Bowen ratio, or eddy covariance tower. Regional scale evapotranspiration is estimated from surface water balance or atmospheric water balance. Remote sensing approaches have been developed to retrieve regional or global scale evapotranspiration in recent decades. As the lack or reduction of evapotranspiration indicates drought, remote sensing of evapotranspiration has been applied to monitoring regional or global droughts in recent years. In this chapter we briefly review ET remote sensing studies, starting with a historical sketch before introducing the Geostationary Operational Environmental Satellites’ (GOES) ET and Drought (GET-D) product system that is operational at the National Environmental Satellite, Data, and Information Service (NESDIS). The GET-D system implements the Atmosphere–Land Exchange Inversion (ALEXI) model for estimating regional daily ET from observations of the NESDIS Geostationary Operational Environmental Satellites. The Evaporative Stress Index (ESI) based on ALEXI ET is used for monitoring drought currently for North America. An approach to merging the ESI data into microwave soil moisture observations and land-surface model soil-moisture simulations for a blended drought index is presented. The feasibility of using the ALEXI ET estimates from global satellite observations for drought monitoring is discussed.