Determination of regional scale evapotranspiration from NOAA-AVHRR images: application to the Afyon-Akarcay Basin, Turkey

Reliable methods for estimating evapotranspiration (ET) are critical for accurately assessing the water balance at basin scale. An operational model that computes the daily latent heat flux as a residual term of the energy balance to obtain ET from available ground truth meteorological and remotely sensed data is presented. A simplified algorithm, which uses a remotely sensed surface temperature in combination with air temperature in a resistance form equation has been used to calculate the sensible heat flux. Surface parameters required in the algorithm have been determined for reference crop type (alfalfa) by using the approach of FAO-56. Afyon-Akarcay Basin, a semi-arid land of 7337 km², was selected as the application field. Radiometric, geometric, and atmospheric corrections have been applied to cloud-free NOAA-AVHRR noon images of the water year 1998-1999 to retrieve NDVI, albedo, emissivity, and land surface temperature maps of the basin. Results of the model are in good agreement with those of conventional Penman, Penman-Monteith, and Makkink equations, in which ground truth data from the meteorological stations in the basin are used. Model derived daily average free water surface evaporation values, which were calculated for the Eber (125 km²) and Aksehir (338 km²) lakes in the basin, are also in reasonable agreement with those of based on local Class A pan measurements.