| چکیده انگلیسی مقاله |
The trend of atmospheric water cycle components (precipitation and precipitable water) in catchments of Iran
Introduction
Climate change and global warming have led to widespread and persistent droughts. These changes caused uneven distribution of rainfall and adverse effects on water resources in many parts of the planet. Iran, where is located in an arid and semi-arid climate zone with less than one third of the average rainfall in the world, is no exception to these effects. Such areas require proper management of water resources. We have to plan for water exploitation in the coming years, since water resources are scarce and it has seasonal and periodic fluctuations. Therefore, one of the most important challenges in the present age is competition for access to water resources. For this purpose, it is necessary first to assess the trend of precipitation and related components that is unknown in study area. Therefore, the aim of this study is to investigate the trend of two important components of precipitation in Iran, so that a clearer picture of climate change and its effects can be presented.
Methods
In this study, we investigated the trend of simultaneous changes in precipitable water (atmospheric humidity) and precipitation to identify the relationship between the changes of these two variables in catchments of Iran. For this purpose, monthly precipitation and precipitable water data (ERA5) of the European Centre for Medium-Range Weather Forecasts (ECMWF) were obtained in the period 1979-2019. ERA5 is the latest generation of ECMWF Global Analysis produced by ECMWF since 1979. Man-Kendall test has been used in monthly, seasonal and annual time scales to examine the trends and changes of these variables.
Results
The results showed in the spring, the trend of precipitable water has increased significantly only in small areas in the southeast, south and southwest (in the southern Balochistan, Bandar Abbas, Hamoon Jazmourian, Kol-Mehran, Karun Bozorg basins) and also in the north of the Iran (in the Sefidrood Bozorg and Haraz basins). It has been observed no significant trend in other areas. In the summer, a significant increase has been recorded in the southwest and to some extent in the south and north of Iran (in the western border areas, Karkheh, Karun, Hindijan, Hilla, Mendel-Mehran, Baluchistan and Sefid Bozorg), while in other parts of Iran, there was no significant trend. In autumn, the studied variable has shown a significant increasing trend in almost two thirds of the country (most catchments except Lake Urmia, Qarasu, Gorganrood and Atrak). In winter, precipitable water has shown a significant decreasing trend in the southeastern and eastern regions of Iran (Bandar Abbas, Balochistan, Hamoon Jazmourian and Hamoon Meshkil, Central Desert, Lut, Daranjir, Namakzar Khaf, Hamoon Hirmand) and increasing trend in small areas in northern and southwestern of Iran (Talesh Basins, Sefidrood Bozorg, Sefidrood and Hezar, Karun Bozorg and Gavkhoni).
In general, the results indicated a decreasing trend in both variables in the eastern catchments and central regions of the country in winter and also an increasing trend in most watersheds in autumn during the studied period. Therefore, this increase does not necessarily mean an increase in precipitation or its potential in the region; According to the results, the lack of sufficient precipitation, especially in winter and spring, is not due to insufficient humidity throughout the country. On the other hand, precipitable water has increased in the warm seasons, which is due to high temperatures, lack of saturation and stable atmospheric conditions that provide dry conditions. Because precipitable water has increased with increasing air humidity capacity as a result of an increase in temperature, and therefore the distance to saturation has not changed despite the increase in precipitable water. Therefore, it cannot be expected that with its increase, the precipitation will also increase in the region. So that in the spring, it has not been observed any significant trend in the precipitation despite of the precipitable water increasing trend in the northern, southern and southwestern catchments of the country. Therefore, for the occurrence of precipitation, not only the presence of water vapor and moisture in the air is necessary, but also a mechanism for cooling the air until it reaches saturation condition. Thus, precipitable water is one of the important climatic indicators that its temporal-spatial distribution study shows both humidity and temperature conditions well.
Also, the annual trend of variables showed that precipitable water has increased in the western, southern, southwestern and northern basins, while the precipitation variable has increased in the northern basins but has decreased in the eastern basins of the country. The study found that the potential for heavier rainfall was exacerbated by rising precipitable water. Also, the results of precipitation changes in the form of irregularities that provide severe droughts and water shortages in various parts of the countrychr('39')s water resources.
Keywords: precipitation, precipitable water, humidity, variability, Man-Kendall test, ERA5
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