Document Type : Original Article
Authors
Tarbiat Modares University
Abstract
Abstract
Climate change is likely to intensify hydrological cycles and affect the intensity and frequency of extreme rainfall.An increase in extreme rainfall in recent years has led to the assumption that climate change has made extreme rainfall non-stationary. Nonstationarity is defined as a change in the parameters of the probability distribution function due to the influence of time variables or other physical variables .The purpose of this study was to investigate the effect of temperature on the non-stationarity of extreme rainfall in Iran.The AIC value was calculated to determine whether the extreme rainfall follows the stationary or non-stationary model. The non-stationary GEV distribution was used to analyze the frequency of extreme rainfall. The return period of extreme rainfall was calculated based on the maximum temperature and the average annual temperature. It was observed that the intensity and frequency of extreme rainfall were non-stationarity due to an increase in the temperature and no spatially uniform pattern was observed in non-stationarity across the country. Related to the maximum temperature variable, the intensity of extreme rainfall was significantly decreased in 6 stations, including Hamedan, Shiraz, Shahr-e Kord, Zahedan, Iranshahr, and Shahroud. On the other hand, the trend of nonstationarity in Kerman has been significantly increasing with increasing maximum temperatures. Only two stations, Zahedan and Arak, were found to be significantly non-stationary concerning the annual mean temperature, and both stations showed a decreasing trend.
Keyword: Iran, Nonstationary, Extreme Precipitation,GEV Distribution, Return Level.
Introduction
Climate change will probably change the occurrence of extreme events. According to the sixth report of the IPCC, it is predicted that the intensity of the maximum daily precipitation will probably increase by 7 percentage for every 1-degree increase in the global temperature. The intensification of hydrological cycles due to climate change will most likely affect the intensity and frequency of extreme precipitation. Therefore, the temperature can be considered as an important covariate because the warmer atmosphere is likely to hold more moisture, thereby generating a favorable condition to increase extreme precipitation. The frequency of extremes has been changing and is likely to continue changing in the future. So, Classical stationary frequency analysis techniques used in climate studies are no longer reliable. In the stationary model, the statistical parameters of the probability distribution function do not change over time or other covariates, while in the nonstationary model, the parameters of the probability distribution function are not constant, and can change with time or other covariates. Therefore, models and concepts that can account for non-stationary probability distribution analysis of climatic and hydrologic are needed.
Data and Methods
Present study was conducted using daily precipitation data from 36 synoptic stations from 1960-2021. We employ a non-parametric Mann-Kendall trend test to examine whether the annual extreme rainfall intensity is influenced by climate change and temperature increase. Nonstationary frequency analysis for extreme precipitation was conducted using nonstationary GEV distribution by incorporating the annual average daily temperature and maximum daily temperature recorded at the time the extreme precipitation occurred. We considered the annual average daily temperature and maximum daily temperature as a function of location and scale parameters.
Result
The research findings indicate that rising temperatures has led to a nonstationary trend in the intensity and frequency of extreme rainfall in the country. However, this nonstationarity does not follow a consistent pattern throughout the country. The intensity and frequency of extreme precipitation have generally decreased due to the increase in temperature. The intensity of extreme precipitation was significantly decreased in 6 stations, including Hamedan, Shiraz, Shahr-e Kord, Zahedan, Iranshahr, and Shahroud. On the other hand, the trend of nonstationarity in Kerman has been significantly increasing with increasing maximum temperatures. Only two stations, Zahedan and Arak, were found to be significantly nonstationary concerning the annual mean temperature, both stations showed a decreasing trend. The southern and southeastern regions of the country have shown a more consistent and intense decrease in the intensity and frequency of extreme rainfall. Conversely, the northern regions, including stations along the Caspian Sea, Tehran and Qazvin, and the northeast of the country, have experienced an increase in the intensity of extreme precipitation. The probability of extreme rainfall at the average annual temperature has shown a different pattern, with the greatest increase in the intensity of rainfall observed in the stations of the northern, western, and southwestern regions. Overall, the central regions as well as the eastern, northeastern, and southeastern regions of the country have experienced a higher percentage of decrease in precipitation intensity with an increase in average temperature. However, Kerman station in the east has shown a completely different pattern.
Conclusion
It can be inferred that the increase in temperature has caused nonstationarity and changes in the intensity and frequency of precipitation in Iran. The pattern of change is not uniform across the country.There has been a significant decrease in the intensity and frequency of extreme precipitation due to the increase in temperature.
Keywords
