Calculating net radiation of freshwater reservoir to estimate spatial distribution of evaporation using satellite images

Evaporation over freshwater bodies is one of the main sources of water loss. However, evaporation estimation models using net radiation (Rₙ) over water bodies have not yet provided proper efficiency, the fact which has led to several attempts to improve such models using satellite images. Therefore, the purpose of this study is to introduce a model for estimation of Rₙ through using satellite images, and to utilize such model in evaporation calculation models in order to produce an evaporation map for freshwater bodies. In this regard, two reservoirs of Taleghan and Amirkabir dams in Iran were selected as the areas under study. Thirty-three Landsat satellite images from the years 2010 to 2018 along with the corresponding meteorological data were used in the implementations. A new simplified equation was introduced for calculating Rₙ and the subsequent estimation of potential evaporation over freshwater bodies using remote sensing images. Evaporation was generally estimated using FAO56-PM, Priestley-Taylor, Kimberly-Penman, and Hargreaves-Samani models, and the results were then compared with Pan evaporation data. The obtained results were also compared with the results of two energy balance models, including Surface Energy Balance System (SEBAL) and Mapping Evapotranspiration at high Resolution and with Internalized Calibration (METRIC). The results of this study indicated that Kimberly-Penman model with coefficient of determination (R²) of 0.76, Root Mean Square Error (RMSE) of 1.29 (mm), and Nash-Sutcliffe (NSE) of 0.65, and FAO56-PM model with R² = 0.75, RMSE = 1.76 (mm), and NSE = 0.36 outperformed when compared to other similar methods. Moreover, the proposed model proved higher performance in estimating evaporation over freshwater bodies compared with SEBAL and METRIC models.