Investigation of Temporal and Spatial Variation of Evapotranspiration by Remote Sensing in Semi-Arid Regions

Accurate estimation of evapotranspiration plays an important role in quantification of the water balance at regional scale for better planning and managing water resources. Evapotranspiration can be obtained from either estimation of potential ET using data of meteorological stations or، directly، from field measurements. ET is subject to rapid changes in time and space، attributable to the wide spatial variability of precipitation، hydraulic characteristics of soils، and vegetation types and densities. Therefore، it is nearly impossible to determine its spatial and temporal distributions over large areas only from lysimeter and precise measuring instruments. Thus، researchers have used remote sensing data to estimate areal actual ET. In this study، actual evapotranspiration variations of Neyshabour plain was investigated by algorithm of energy balance of the earth since 2000 to 2013 by using MODIS images and meteorological data. Also، the determined evapotranspiration was compared and evaluated with Penman-Monteith and Hargreaves-Samani models. Low amount of coefficients of the error between Penman-Monteith model and SEBAL algorithm showed the accuracy of SEBAL model in the estimation of evapotranspiration and its parameters. The results derived from comparison of evapotranspiration and NDVI vegetation index indicated a good correlation between vegetation and evapotranspiration (R2=0. 908). Also، the variations of NDVI index، land surface temperature، and evapotranspiration in the studied fields showed that evapotranspiration increased by lower land surface temperature and higher densities of vegetation.