Estimation of Evaporation from the Surface of the Caspian Sea and its Temporal and Spatial Analysis

Introduction
Increase in the population of the Caspian Sea, particularly in its coastal zones, will force the governments to use water in the years ahead to gain water from the sea. Therefore, the study of seawater fluctuations influencing the sea ecosystem and the changes resulting from these perceptions will be necessary to prevent serious damage to this environment. In order to investigate the fluctuations in lake water, we need to calculate their water balance, to estimate inputs and outputs, and to determine the level of seawater. One of the most important outguns is evaporation. Calculating evaporation from the lakes is carried out in a variety of ways, with different results. The most accurate estimation of evaporation shows the highest accuracy in estimating the sea level. Thus, the aim of this study is to identify the most accurate and easiest method for estimating evaporation from the Caspian Sea water surface based on available data.
Materials and methods
The data used in this study were collected from three data centers on a daily basis: the Russian Academy of Sciences, the Princeton University of Hydrology, and the NCEP / NCAR database center in 1982-2010. The data were initially reviewed, verified and synchronized. To do this, the data from 1982 to 1998 were selected and the evaporation rate of water was estimated by balance method, Hefner, SHahtin, Meyer, US Bureau of Civil Engineering, Marciano and Ivanov. It should be noted that as in the method, discharge the output water into the Gulf of Kara Bogaz is extracted from the equation of the water balance, in order to compare the evaporation rate with the selected methods, the accuracy of the sampling is decreased. In all these methods, the Gulf of Kara Bogaz is reduced from the Caspian Sea level. Assuming that the water balance method is the most accurate estimation for evaporation, the other methods were compared with. The method that had the least difference in estimation with this method was selected as the optimal method. Then, with the selected method, the evaporation rate was estimated in 1982-2010. The evaporation zonation of the sea level was calculated and plotted on monthly, seasonal and yearly maps.
Results and discussion
The findings of the research showed that Meyer's method in comparison with other selected methods in this study has the closest distance and the highest correlation with the balance method. The amount of annual evaporation by Meyer method in different parts of the Caspian Sea is not the same and varies from 688 mm to 1769 mm from north to south. However, the average of the total body weight is from the body. In the Caspian Sea, during the cold season, the least evaporation rate occurs in the northern part and the most in the southern part of the Gorgan Bay.  
Conclusion
According to the results of the base method (balance method), the results showed that Meyer's method has more potential than the other selected methods for estimating evaporation from the Caspian Sea. The methods main relying on climatology element, such as temperature (USBR method), we cannot accurately estimate the evaporation, as a complex and multi-dimensional phenomenon, in the Caspian Sea. On the other hand, among the selected methods, when an influential element such as wind speed can be studied only at one level (the Hefner and Marciano method) alone, we cannot provide an accurate estimate of evaporation changes from the Caspian Sea. Finally, it can be said that among different methods, any method that can check the relationship between at least three factors of wind speed, temperature and water vapor pressure on two levels can provide a better and more realistic picture of evaporation changes from the Caspian Sea level.