Decadal Variation of Evaporation in Relation with the Variability of Some Climatic Elements in the Zayandeh Roud Basin

Evaporation is the process of transferring moisture from the earth's surface - the water bodies to the atmosphere. Recognition and evaluation of evaporation is an important part in calculating water balance and proper management of water resources. From a climatic point of view, several factors are involved in the evaporation process, the most important of which are radiation and air temperature, wind speed and relative humidity. Evaporation variability at different time scales, especially during the process of global warming trends, is one of the considerable manifestations of climate change. Investigating the decadal changes in evaporation and the associated effective parameters play an effective role in the medium-term planning and management of water resources. The main purpose of current study is to analysis the decadal changes of evaporation in relation with the effective parameters (temperature, humidity, and wind) on the parameter.

To this end, daily databases of evaporation, temperature, relative humidity and wind speed, with a spatial separation of 4 × 4 km, obtained from stations (synoptic, climatology and rain gauge) data interpolation for the period 1969-2018 were used. In order to check the changes of the studied variables, the spatial average daily distribution of each variable for the whole period and for the five decades studied was estimated and analyzed. In addition, multivariate standard regression was used to track changes in the role and importance of each of the three climatic parameters studied (humidity, wind and temperature) over the decades of evaporation. first, each of the variables is standardized in relation to their mean and standard deviation as follows:Then the following regression model was used for K the independent variable to justify the spatial variation of evaporation ( ):

In Zayandehrood catchment, with increasing altitude, decreasing temperature and consequently reducing annual evapotranspiration can be observed. As altitude decreases from northwest to southeast, climatic elements such as heat and evaporation increase and some others such as humidity and precipitation decrease. And the cold and humid climate is gradually giving way to the hot and dry climate. The lowest average temperature is observed in the northwest of the basin and mountainous areas And the temperature rises to the southeast of the range. Therefore, we see areas with lower temperature, less evaporation and vice versa Which can be deduced from the high correlation (0.9) of these two parameters. This relationship is reversed in the case of moisture. That is, the highest relative humidity is observed in the northwest of the study area; Towards the southeast, the humidity decreases. After temperature, the moisture element has the highest correlation with the evaporation meter (0.79). The relationship between wind speed and evaporation is lower but more direct than other parameters. In the western half of the study area, its amount is less and in the eastern half of the basin, its speed increases.Also, The results showed that the temperature parameter has a more important and effective pattern on the evaporation process than other parameters, but the share of this parameter has been decreasing since the first decade (1978-1969) towards thelast decade. Humidity have ranks second among atmospheric elements in terms of its effect on the evaporation process that converse to temperature plays an increasing role from the early decades to the final decade (2019-2009). The same trend is repeated for the wind and its role is increasing from the first decade (1978-1969) to the last decade (2018-2019).

In this study, an attempt was made to use multivariate regression, Analyze the relationship between these parameters and the evaporation parameter in the period 2018-1969 And determine the most important parameter affecting evaporation in the study area. Then, the changes of decades of the relationship between these three variables and evaporation were discussed. Based on the general characteristics of this basin, it was found that the temperature and wind in the end parts of the basin is higher than the northern parts. Similarly, the rate of evaporation is lower in the northern half and higher in the southern half Conversely, the amount of moisture in the northern parts of the study area is higher than the southern parts. The results also show that The most effective parameter in relation to evaporation is the temperature parameter. This relationship was significant at 95% confidence level in the whole basin. . Moisture of the second share and Wind has the last category in the evaporation process. Research findings also showed The role of temperature is declining from the first decade to the last decade and Conversely, the share of humidity and wind is increasing.