Agricultural Meteorology
Sakineh Khansalari; Mahmood Omidi; Mozhgan Fallahzadeh
Abstract
Introduction
Due to global warming and climate change, droughts and extreme precipitation events are increasing. Therefore, it is of special importance to know the characteristics of precipitation in the region in order to manage water resources effectively especially during torrential rainfall events. ...
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Introduction
Due to global warming and climate change, droughts and extreme precipitation events are increasing. Therefore, it is of special importance to know the characteristics of precipitation in the region in order to manage water resources effectively especially during torrential rainfall events. This can help to reduce the risk of these events and increase water reserves with proper management. These precipitation characteristics which are the objectives of the present study, include the temporal-spatial distribution of precipitation in different parts of the study area, as well as the number of days with and without precipitation and the maximum precipitation occurring in the region. Also, these precipitation characteristics should give us information about extreme precipitation events.
Materials and Methods
This research analyzed the characteristics of precipitation in Markazi province over a 30-year period (from the crop year 1991-1992 to 2020-2021) using statistical methods and the spatial distribution was drawn and analyzed with ArcGIS software. This province includes the 12 meteorological stations of Arak, Mahalat, Saveh, Tafresh, Ashtiyan, Komeijan, Khondab, Shazand, Khomein, Delijan, Farmahin and Gharqabad, which the precipitation data of these stations were investigated. The trend of precipitation changes in monthly, seasonal, and annual time scales were also examined using the Mann-Kendall test. Moreover, extreme precipitation was assessed using four indices: total extreme precipitation (R95p), number of days with precipitation above the station’s extreme precipitation threshold (R95d), absolute intensity of extreme precipitation (AEPI) and the fraction of total rainfall from events exceeding the extreme threshold (R95pT). The latter index represents the ratio of extreme precipitation to annual precipitation in rainy days (daily rainfall above 1 mm).
Results and Discussion
This study reveals that, on average, 53% of the annual precipitation is accounted for by the maximum index of R95pT, which indicates the percentage of extreme precipitation that occurred at each station relative to its the precipitation of the corresponding year. Knowing the timing of these extreme events can help to manage floods and optimize water resources. More than 20% of these precipitations occurred in March. The spatial distribution of rainfall in Markazi province shows that the south-west regions have the highest average annual and seasonal rainfall, except for the summer season, while the eastern regions have the lowest. The winter season has the highest rainfall on average, followed by spring and autumn. March is the rainiest month with a coefficient of variation of 0.8 and an average monthly rainfall of 55.6 mm during the studied period. Due to most extreme precipitation events occurring in this month, it has the highest importance for water storage and management throughout the year. The average precipitation in March ranges from 32.6 mm (Saveh station) to 91.6 mm (Shazand station) across the stations of the province. The maximum rainfall in this month varies from 124.4 to 254.6 mm among the stations of the Markazi province, which is a considerable amount compared to the provincial average crop year. The standard deviation of precipitation in this month is between 28.7 and 61.3 mm, and the coefficient of variation at the stations of the province is between 0.6 and 0.9. Moreover, in terms of average monthly rainfall 22Nov-21Dec, 20Feb-19Mar, and 23Oct-21Nov are the next priority months for water storage management after 20Mar-19Apr, with average monthly rainfall of 39.3, 38.2, and 36.3 mm, respectively. The Mann-Kendall non-parametric test results did not reveal a consistent trend, but it showed that most of the meteorology stations in Markazi province had a significant decreasing trend in the rainfall in 21Jan-19Feb at a 90% confidence level. The analysis of extreme precipitation indices indicated that Shazand station had the highest extreme precipitation threshold value (28 mm), while Saveh and Delijan stations had the lowest (15 mm). The extreme precipitation threshold average of 30 years in other meteorological stations of Markazi province are 21mm in Arak, 17mm in Tafresh, 21mm in Khomeyn, 19mm in Mahallat, 17mm in Komeijan, 16mm in Farmahin, 21mm in Khondab, 17mm Gharqabad and 18mm in Ashtiyan.
Conclusion
The spatial distribution of rainfall in Markazi Province shows that the southwest regions have the highest average annual and seasonal precipitation, except for summer, while the east regions have the lowest. The average monthly rainfall also indicates that March has the highest rainfall among all months of the year, and that about 20% of the annual extreme precipitation occurs in this month.
