بررسی روند تغییرات بارش‌های منطقه جنوب غرب آسیا

Introduction: Despite our scientific development and awareness of the consequences of regional and global climate change little attention has been paid to the effects of the changes in the Middle East and Central Asia yet. In the Middle East, climate change is a big challenge, especially if successive droughts and persistent increase as well as growing demand for water and water shortages attention, the challenge take on a large scale. Iran is a vast country with a different climate Rainfall distribution. Also precipitation is influenced by air mass entering the country from the neighboring countries, so rainfall in different regions of Iran is heavily influenced by the situation in neighboring countries. The aim of this study is evaluation of the trend of annual and monthly precipitations of the South-West of Asia with modified Mann-Kendall test by considering the effect of autocorrelation.
Materials and Methods: In this study monthly and annual precipitation data of 4152 rain gauge stations in Iran and its 15 neighboring in a period of 1970-2014 was used and been downscaled to evaluate the trend of monthly and annual precipitations. In this study the monthly and annual precipitations time series of Afghanistan, Azerbaijan, India, Iraq, Kuwait, Oman, Pakistan, Saudi Arabia, Syria, Tajikistan, Turkey, Turkmenistan, Qatar, Yemen and Iran were used. The purpose of the trend test is to specify the presence or absence of ascending or descending order in the data series. Since there are assumptions in the parametric methods such as the normality, stationary and independent variables and this assumption is often not valid for hydrological variables, the nonparametric Mann-Kendall method that is applicable to the hydrological and meteorological studies can be used.
Results and Discussion: The results of evaluating the trend of annual precipitation of study stations in the period of 1971-2014 using the Mann-Kendall modified by omitting the effect of autocorrelation indicated that all of the regions of Iran has decreasing trend in annual precipitations and there are significant decreasing trend in the western regions of Iran and western areas of Caspian sea, some central and eastern regions of Iran in five percentage significantly. The rest of the decreasing trend in annual rainfall amounts included in the country has experienced. In annual terms in countries, that border the study area is faced with an increasing trend in annual rainfall amounts so that the country at the center of the crisis (lack of rain) is located. The southern part of India, southwestern Saudi Arabia, the northern region of Turkmenistan and the eastern regions of Afghanistan and Pakistan with the increasing trend in annual rainfall amounts over the 1970-2014 statistical has faced. The trend of monthly rainfall amounts for the month of January (second month) showed that the amount of rainfall during the month trend of central and eastern regions of the study area is decreasing. In February (second month of the year) rainfall conditions in the study area as well as in the country in terms of changes time has improved and areas of Iran is faced with increasing precipitation. Changes decreasing the amount of monthly precipitation in March moved to the West study area and focus a significant decline in rainfall in the western regions of Iraq and Syria and Iran. However, in May (fifth month) most regions of Iran, Turkmenistan, northwestern Turkey and the West areas of India has been facing a decreasing trend in rainfall amounts. Other areas showed an increase in precipitation. In July (the seventh month), India (regions Northeast and East), Pakistan, Qatar, Saudi Arabia, the South East of United Arab Emirates has significant decreasing trend in rainfall amounts. Focus of decreasing monthly precipitation for the August moved to India and much of the country is included. Unlike other months of the study, in the eighth month (September) process to reduce the amount of monthly precipitation moved to south western parts of the study area (South West Asian countries) and Saudi Arabia in this month is central of decreasing.
Conclusion: The results of the annual trend of precipitation in Iran indicated that in an annual scale the North West of Iran is faced with the significant decrease trend in rainfall. The annual rainfall across eastern and northern part Iran also has significant decreasing trend and Central regions had a decreasing trend of precipitation in the period of studied. Iranian medium-scale review of the annual and monthly precipitation showed that the annual precipitation is reduced about 1.06 mm per year that the average amount of it’s in the study area (South-West of Asia) equal to the reduction of 0.33 mm per year which represents more than three times decreasing precipitation of Iran's regional in a year as South West Asia. Also the results of evaluating the slope of trend line in different months indicated that in December, March, January, the Iran’s precipitations is most decreasing as average of annual precipitation in studiing regions about 5, 3 and 5 times respectively