The Hazards of Rainfall Concentration in Iran

Document Type : مقاله پژوهشی

Authors

• Omid Mafakheri
• Mohammad Saligheh
• Bohlool Alijani
• Mehri Akbari

Kharazmi University

10.22067/geo.v6i3.60987

Abstract

1. Introduction
Since the spatial and temporal distribution of rainfall in Iran is influenced by the distribution of global circulation systems, the lowest change in its pattern is resulted in the severe weather abnormalities. Therefore, spatial and temporal abnormalities of rainfall, severe changes in rainfall intensity, and rainfall difference are the main characteristics of Iran's rainfall. Worthy studies have been carried out in relation to the temporal and spatial variations of rainfall, as a result, their conclusion showed that the number of precipitation is lower but the severity is higher. The purpose of this study was to investigate and identify changes in the number of rainy days, average daily precipitation, rainfall variation changes and the resulting hazards in the long-term statistical period.
2. Material and methods
First, daily precipitation data of 53 synoptic stations with a common statistical period from 1984 to 2013 were received from the Meteorological Organization. Then, they were selected to determine the rainy day (precipitation at 0.1mm and above). From the viewpoint of precipitation duration, 7 classes were identified. 1 day precipitation, precipitation with two-day sequence, and precipitation with a sequence of 3 to 7 days of rainfall were extracted. Cluster analysis was used to identify climatic regions and their features. The final result was the division of the country into seven regions with the highest intra-group similarity and the most difference among groups in terms of the number of days. In order to identify hazards and heavy rainfall, the frequency of one day to seven days and more, to 3 parts of short term precipitation include 1 day precipitation, 2 days and 3 days, medium term (4 days and 5 days precipitation) and long-term (6 days and more) was divided. Then, in order to determine the changes in the daily rainfall of each area, first, the average daily rainfall of the short, medium and long run rainfall was calculated and the coefficient of variation of rainfall day and average daily precipitation for each of the areas was obtained. The final map was used in GIS using spatial analysis for zoning. In order to provide the time and spatial distribution pattern of precipitation in each area and to identify the severity of daily precipitation, the ratio of maximum daily rainfall to annual precipitation was calculated and analyzed. In order to compare rainfall variations in the first 15 years of the first 15 years, the coefficient of increasing or decreasing rainfall changes was calculated in two periods.
3. Discussion
In area 1 (south and south east of the country), the number of rainy days, the average rainfall of short-term days (precipitation of one to three days), and the medium-term (precipitation of four and five days) decreased. The average daily rainfall in this area was 17 days a year. In area two, the number of rainy days and the average amount of short-term and medium-term rainfall have been decreasing. The number of rainy days was 27 days a year with a mean precipitation of 3.1mm. In the third zone (Caspian coastlines), it was shown that the short-term rainy days are increasing, and the long-term and long-term periods of decline are decreasing. The number of overnight days in this area has been the highest (113 days) per year. In area 4, short and medium rainfall is increasing in terms of the number of days of precipitation. However, long-term precipitation (precipitation with 6 and 7 days and more) has been decreasing. The number of days in this area is 74 days (with an average rainfall of 4mm) per year. In zone 5, both short-term and medium-term rainfall showed a decrease in both trends, while the shortened rainfall is also higher in this area. The total number of days in the study period showed a decreasing trend. The annual rainfall in this area is 39 days (with an average rainfall of 2.3mm) per year. In area 6, the number of rainy days and the average daily rainfall (1-3 days) is increasing. Finally, in area 7, the number of overnight days (with a coefficient of 101) and the short run average had an increase. The medium-term and long-term rainfall have declined. The annual rainfall in this area was about 5mm in 66 days with an average rainfall.
4. Conclusions
1. Dry and semi-arid regions of Iran include areas 1, 2 and 5 (Bandar Lengeh, Konarak, Chabahar, Zahedan, Bandar Abbas, Bam, Zabol, Iranshahr, Kish, Abadan, Yazd, Tabas, Fasa, Bushehr, Birjand, Kerman, Kashan, Abadeh, Semnan, Isfahan, Ahvaz, Sabzevar, Qom, Shiraz and Torbat Heydarieh). The number of short-, medium-term precipitation days (precipitation with a 1-day, 2-day and 3-day sequence) and medium-term precipitation (precipitation i) has declined over the course of 30 years. Long-term precipitation (more than 5 days) has not occurred in these areas. The average daily rainfall in arid regions of Iran is 27 days a year. The average daily rainfall of both short and medium term has been decreasing. The average daily rainfall was 2.3mm per day.
2. The northern coast of Iran was designated as area 3. The frequency of short-term barge days and the average rainfall are both incremental. An increasing trend indicated that rainfall has been severe. 9% has been added to the amount of daily precipitation. The daily rainfall in the region is 113 days per year and the average daily precipitation is 22.9mm.
3. Mountainous area includes areas 4, area 6 and 7 (Ardebil, Gorgan, Parsabad, Khoy, Bojnourd, Tabriz, Quchan, Shahroud, Dashan Tepeh, Maraghea, Karaj, Qazvin, Shahrekord, Mashhad, Orumieh, Zanjan, Hamedan, Sanandaj, Yasouj, Ilam, Khorram Abad, Arak, Hamedan Nogheh, Kermanshah and Saqqez). In the statistical period of 30 years in the mountainous areas, the number of days and average rainfall has decreased for the medium and long term, and has been increased by the number of short-term overnight days and short-term average rainfall. The number of rainy days in mountainous areas is 68 days per year and the average daily rainfall is 3.74mm.
4. The ratio of maximum daily rainfall to annual rainfall in all areas is increasing. This indicates heavy rainfall and that rainfall in the region of Iran is rising. Most rainstorms occur within just a few days. Such anomalies in the regime of rainfall, long dryness, destruction of vegetation, followed by flood descendants and the destruction of water and soil resources and human facilities.

Keywords

• Rainfall hazards
• Rainfall variations
• Daily Precipitation
• Precipitation sequence
• Showers

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