Evapotranspiration Estimates Using ASTER Thermal Infrared Imagery

摘要

The recent availability of multi-band thermal infrared imagery from the Advanced Spaceborne Thermal Emission & Reflection radiometer (ASTER) on NASA's Terra satellite has made feasible the estimation of evapotranspiration at 90 meter resolution. One critical variable in evapotranspiration models is surface temperature. With ASTER the temperature can be reliably determined over a wide range of vegetative conditions. The requirements for accurate temperature measurement include minimization of atmospheric effects, correction for surface emissivity variations and sufficient resolution for the type of vegetative cover. When ASTER imagery are combined with meteorological observations, these requirements are usually met and result in surface temperatures accurate within 1-2 C. ASTER-based evapotranspiration estimates were made during September 2000 over a sub-humid regions at the USDA/ARS Grazinglands research laboratory near El Reno in central Oklahoma. Daily evapotranspiration was estimated by applying instantaneous ASTER surface temperatures, as well as ASTER-based vegetation indices from visible-near infrared bands, to a two-source energy flux model and combining the result with separately acquired hourly solar radiation data. The estimates of surface fluxes show reasonable agreement (within 50-100 W/m~2) with ground-based Bowen Ratio Energy Balance measurements and illustrate how ASTER measurements can be applied to heterogeneous terrain. There are some significant discrepancies, however, and these may in part be due to difficulty quantifying fractional cover of senescent vegetation.