Evaluation and Calibration of Empirical Relationships for Estimating Evaporation from Free Water Levels in Urmia Lake Basin

Evaporation is one of the main reasons for water loss and pressure on water resources, which is important in many situations. This study was carried out in order to evaluate the accuracy of different methods for estimating evaporation of free water surfaces using pan evaporation monthly data in the Lake Urmia basin. For this purpose several experimental equation coefficients were calibrated using the Newton-Raphson method. According to the results, the largest and least amounts of evaporation were occurred in the eastern parts, and south and west of the lake basin, respectively. The obtained results indicated that in general, Meyer and Ivanov methods were upper estimated and their predicted values were more than actual amounts, versus Jensen Haise and Blaney–Criddle relationships estimated values were most similar to pan evaporation data. The latest tow methods are arranged in the category of temperature - radiation and temperature - sunshine hour relationships, respectively, and have the highest accuracy in the Lake Urmia Basin. In average, Jensen Haise and Papadakis methods were introduced as the best and worst methods with RMSE of 1.07 and 2.71 mm/day in the Urmia Lake basin, respectively. Also, the most and least corrective correlation variations were obtained as 54.12 and 9.80 percentage for Stephens Stewart and Jensen Haies method coefficients.