Estimation of evapotranspiration and its partition based on an extended three-temperature model and MODIS products

Evapotranspiration (ET) is an energy balance component and a key component of water budget; thus, accurate estimates of ET are critical for understanding hydrological processes and water resources management. Despite growing concerns, challenges remain in estimating ET by remote sensing technology for regional applications, due to the difficulty in determination of aerodynamic, canopy and soil resistance. Instead of using resistance, ET and their partition of evaporation (Es) and transpiration (Ec) can be evaluated based on an extended three temperature model (3T model) and Moderate Resolution Imaging Spectroradiometer (MODIS) products for the period of 2001–2009. A case study was conducted in the Heihe River Basin in northwestern China. Validation results indicated that the mean absolute error was 0.08 mm/d, with a maximum and minimum absolute error of 1.28 mm/d and 0.02 mm/d, respectively, which presented a simple extended 3T model in estimating ET with adequate accuracy and could satisfy regional research requirement at large scales. Results showed that: (1) seasonally, ET was highest (varied from 0.80 mm/d to 1.27 mm/d with a mean value of 1.05 mm/d) in summer and lowest in winter (varied from 0.35 mm/d to 0.44 mm/d with a mean value of 0.37 mm/d); (2) spatially, it rendered a decrease from the Qilian Mountain of the upper reaches (515 mm/a) to the Gobi desert of the lower reaches (82 mm/a), which was closely related to land covers and climate conditions; (3) the comparison of different ecosystems indicated that forest has the highest ET (588 mm), followed by grassland (308 mm), farmland (225 mm), and desert land (160 mm); (4) ET was separated into Es and Ec, and about 38–72% soil water returned to the atmosphere in the form of Es. This information will prove to be useful for water-use efficiency of the Ecological Water Conveyances Project (EWCP) conducted in the basin.