تحلیلی بر تغییرات بارش درایران

1. Introduction: Fluctuation and change are the inherent characteristic of air and phenomena related to the atmosphere. This anomaly in the air in the long term or in the prevailing climate (climate), becomes the norm pattern. So the climate does not change in the short and fast, unlike air, But it is possible in the long term and under the influence of natural processes. Today, the behavior of climatic components such as temperature, precipitation, etc. has changed and their anomalies have increased over the past few decades. Researchers have described these anomalies in climate as anthropogenic climate change. They have revealed that misconduct has negatively impacted the Earth's system for the past several centuries, in particular causing system pollution and global warming. Atmospheric models predict an increase in the Earth's temperature and subsequently anomalies in different climates (IPCC, 2007). Therefore, climate of Iranian plateau is fragile to fluctuations of heat and dry periods or cold and humid periods in south of temperate climatic region and northern region of subtropical. Because in the arid and semi-arid climate of Iran, precipitations has been of particular interest in the past and the methods of obtaining water are their genius (qanat). On the other hand, the rainfall in these areas is irregular in time and location (Alijani, 2017), because where alluvial plains are located, Lack of water and water resources, and elsewhere, water is abundant in the northern and west of Iran mountainous regions. In other words, where there is water there is no land and where there is no water there is land. (Velayati, 2019). On the other hand, time of rainfalls with growing time does not match and most of the precipitation falls in the cold half of the year. Now has the behavior of these types of precipitation changed in the long term, and is it consistent with the prediction of anthropogenic climate change (IPCC) that precipitation anomalies is matches?
 2. Data and methods:to this end, daily rainfall data from 33 synoptic stations (fig 1) in Iran over the past 60 years from 1959 to 2018 were studied. Precipitation values of one millimeter and more were introduced as precipitation days and were extracted at each station separately for 6 decades (10 years) to investigate changes in precipitation in Iran. Indicators is for changes in precipitation, including number of days of precipitation (Rd), number of days of severe rainfall (Rd> 0.95), maximum rainfall (RR Maximum) and amount of precipitation (mm) per year and decades (10 years) at each station in the study period. So each index calculated based on differences and changes with the last decade and changes of decades(trend or fluctuation) was determined at each station, In the next step, the spatial averages for the whole of Iran were calculated for 6 decades and the changes in each index were determined for Iran. Also, in order to investigate changes in the whole of Iran, the average annual time series of the above indices were determined during the study period. These series show the behavior of these indicators over the past 60 years in Iran.
3. Results and discussion:
3.1. decades changes of precipitation in station: As Figure (2) shows, Except for the 1970s, most of the regions in Iran had increased rainfall days compared to the previous decade, In the last 4 decades, most of Iran's stations have had a decrease in precipitation, And in the 2000s, all stations except Tehran, Khoy, and Ramsar, the other 30 stations in Iran, experienced a decrease in rainfall over the previous decade. Most precipitation falls are in western, southwestern and northern stations of Iran such as Rasht and Gorgan. Also precipitation in the station of Zahedan, and Yazd in of Iran's southern and central regions has decreased.
On the other hand, the number of days of heavy rainfall increased in the four decades ending 2010 (Figure 3), but the decade of 2010 was recorded in most regions of Iran with days of heavy rainfall. In the 90s, most of the heavy rainfall was experienced at Iranian stations and western stations of Iran had the highest rainfall. But in the 2010s, except for northwestern stations of Iran, Ramsar, Tehran and Kerman, other parts of Iran had less severe rainfall than in the previous decade (2000), and Most of the reduction was experienced by Shahrekord and Hamedan stations and Caspian stations including Rasht and Babolsar.
 Also, changes of the maximum daily precipitation amounts in Iran over the past 6 decades indicate (Figure 4), during the period of study, the maximum precipitation increased in all parts of Iran, especially in the 80s and 90s most of the stations in Iran maximum rainfall increase experienced. In the 2010s, some areas of Iran, including west and southwestern Iran and Babolsar station in northern Iran, recorded the highest rainfall decrease over the previous decade.
 On the other hand, changes in amount of precipitation over the study period showed (Figure 5), In the three decades leading up to the 2000s (70s, 80s and 90s), the amount of precipitation in most parts of Iran has increased compared to the previous decade, but in the last two decades, the amount of precipitation has decreased in most parts of Iran. As Sanandaj and Uremia stations in the last three decades and Yazd, Torbat Heydariyeh, Shiraz, Rasht, Sabzevar, Khorramabad, Birjand and Bandar Abbas have experienced decreasing trend in precipitation in the last two decades. The most decline in the last decade occurred at Babolsar and Tehran stations.
 3.2. Decades changes of Rainfall in Iran: Generally, decades changes of precipitation indices in Iran show (Table 1), the number of rainy days in Iran has been negative and during the whole study period(the last 60 years) there has been decreased an average of 7 rainy days in Iran. On the other hand, the days associated with heavy rainfall increased during this period except 2010, and the number of days of heavy rainfall in Iran increased by 3 days in the whole period studied. The maximum precipitation amount in Iran during the study period is increasing and positive trend, as the entire period of study the places average 7 mm increased. In terms of the amount of precipitation, severe downward trend in the spatial averages of the index is also evident in recent decades, especially in the last two decades. But throughout the period, given the increase in precipitation in the 1970s and 1980s, the average spatial value of this index across Iran indicates a 15 mm increase.
 Table 1: Decadal spatial variations of precipitation indices in Iran





change


2010


2000


1990


1980


1970


1960


decade


index




-7


37


39


43


45


45


43


average


Rain day




-2


-4


-2


-1


2


-


change




+3


21


27


26


23


20


19


average


Extreme rain day




-6


1


4


2


2


-


change




+7


34


34


33


31


28


27


average


Rain maximum




0


1


2


3


1


-


change




+15


318


329


342


340


316


302


average


Rain amount




-12


-12


1


24


14


-


change





 3.3. Iran Rainfall Time Series: The time series of rainfall characteristics over the past 60 years shows (Figure 6), The number of rainy days in Iran is decreasing, As before the 90s, the average rainfall in Iran was between 40 and 60 days per year and after that decade the average rainfall in recent decades reaches 30 days per year (graph a). So the rainfall in Iran has declined between 10 and 20 days in the long term. While days of heavy rainfall in Iran show a significant positive trend (graph b), and an average of 60 days of rain in the first decades of the study period to an average of 80 days has changed in recent decades. In this regard, the amount of annual maximum precipitation in Iran during the study period has a significant positive trend (graph d). Therefore, the average of maximum precipitation has increased over the past 60 years and has increased from at least 20-30 mm prior to the 1990s to more than 35 mm after this decade. Thus, during the period of maximum precipitation has increased. On the other hand, the amount of precipitation in Iran fluctuated during the period under study, not the directional change (graph c), and has moved from a minimum of 250 mm to a maximum of 420 mm around the average, Therefore, on average 330 mm of rainfall per year is averaged over 1 mm in Iran. So the amount of precipitation in Iran has not changed in the past 60 years.
  4. Conclusion:
Rainfall in Iran has changed and is abnormal to predict climate models and increase precipitation anomalies. This change is evident both on a decade scale and in time series, as the number of rainy days has decreased in most parts of Iran for the past 4 decades, this trend is more obvious in the west and southwest of Iran. While the amount of rainfall in Iran has not changed over the past six decades (the last 60 years), the average of 320 mm of precipitation from days with precipitation of 1 mm or more is almost constant. But Iran extreme rainfall days have significant increase. That is, the average annual precipitation is on a station that has already been created with 30 days of rain, today, with 20 days of rainfall, the same amount of rain falls, which should naturally be severe. This is also confirmed by the increasing trend of maximum rainfall amount in Iran. But given the variations in topography and the position of the stations on the Iranian plateau, the change in precipitation indices in all locations over the past 60 years has not been uniform; rather, the spatial heterogeneity of the indices is evident over time, that indicate the diversity of Iran's rainfall climate.