Remotely sensed measurements of apple orchard actual evapotranspiration and plant coefficient using MODIS images and SEBAL algorithm (Case study: Ahar plain, Iran)

Global warming and its consequence, climate change impede crop production in some regions. As Iran mainly consists of arid and semiarid regions and water scarcity is the main feature of such climates, decrease of the water shortage impacts is essential. Therefore, improvement of water resources management and increase of water use efficiency may be one of the main strategies to overcome these shortcomings. Precise evapotranspiration and crop water requirement estimation could beneficially improve cultivation management and water allocation. To precisely estimate actual evapotranspiration (ETa) of apple orchard trees at Ahar plain, East Azerbaijan province, MODIS images and SEBAL algorithm were employed. The energy balance equation was used to make calculations. SEBAL model calculates the energy balance equation parameters by using of surface temperature, surface reflectance and normalized difference vegetation index. Based on achieved data the actual evapotranspiration has an increasing trend toward middle of summer and the highest actual evapotranspiration value (10.1 mm d-1) occurred on 18th July, 2014. The spatial distribution of crop indices and energy balance components revealed that southern part of the plain beside of having the highest crop density it has also the highest amount of evapotranspiration. Highly significant correlation (r = 0.92) was found between SEBAL and Penman-Montieth-FAO estimated ETa values. Meanwhile, t-test showed no significant difference between the two set of data with a RMSE value 0.96 mm d-1. It could be concluded that regional estimation of ET based on remote sensing approach, may be used as an acceptable alternative to point methods for estimation of crop water requirement.