by Adil Yakubov (Uzbekistan)
This study attempts to map the evapotranspiration (ET) for agricultural fields in Tashkent province with Remote Sensing models and GIS tools. Agriculture is a large sector in the economy of Tashkent with cotton as the main crop. Fresh water resources in Uzbekistan are very limited and the amount of irrigation water must be managed in a very sustainable way to avoid fast depletion of fresh water resources. ET is one of the basic variables to be known in order to estimate the amount of irrigation water that is needed for any agricultural field. It encompasses the total amount of water that is lost from the soil to the air as water evaporates and transpirates through plants. This study first applied the Remote Sensing model 'Simplified Surface Energy Balance Index' (S-SEBI) to estimate the ET of Tashkent province. To validate the estimated ET the external dataset ERA-Interim (a global reanalysis program with easy access to different meteorological data) was used.
The main data that were used were downloaded LANDSAT images, the NASA POWERSET data for the surface temperature and weather data. A total of 15 observation images from LANDSAT were used in this study of the period 2009-2013. Only cloud-less images could be used for S-SEBI.
A correlation coefficient of 0.78 was found for the ET estimated by ERA-Interim and S-SEBI. This is a strong relationship between the ET from ERA-Interim and S-SEBI and indicates that the ET estimation for Uzbekistan with S-SEBI is relatively accurate and a reasonable option for further use of this model for Uzbekistan.
The temporal variability seems to follow the pattern of the growing and harvesting seasons of crops. During the harvesting seasons, the latent heat flux decreases and after the sowing season the latent heat flux increases again to reach the maximum values in the summer months, just before harvesting. This pattern for the latent heat flux is different for different types of crops.
The spatial variability of the ET data shows that there are locations all over Tashkent province with a much higher latent heat flux (and thus a high ET) compared to their surroundings. These areas match with the areas with low temperatures and high NDVI. This shows that the crop type can influence the spatial variability between different districts significantly.
Keywords: evapotranspiration; S-SEBI; surface energy balance; latent heat flux; Tashkent province; Uzbekistan.