Investigating Changes in the Intensity of Short-Term Rainfall in the Last Half Century in the Context of Data Scarcity (Case Study: Rasht City)

With the increase in the production of greenhouse gases due to the industrialization of the countries of the world, we are witnessing the occurrence of global warming, which has caused climate changes all over the planet in recent years. One of the effects of this phenomenon is the change in the behavior of weather parameters such as temperature and precipitation. The effects of climate change phenomenon on precipitation have led to changes in the intensity of precipitation and finally changes in intensity-duration-frequency (IDF) curves of precipitation for different regions of the world and country. Therefore, updating the intensity-duration-frequency (IDF) curves is necessary because of their importance in the design of hydraulic structures used in urban flood management. One of the problems of producing intensity-duration-frequency (IDF) curves is the lack of access to rainfall data with different continuities. The purpose of this research is to use the fractal method to obtain precipitation with different continuities and then evaluate the effect of climate change on the intensity of precipitation in Rasht City. .

In this research, the accuracy of the fractal method for generating intensity-duration-frequency curves is first evaluated. Then intensity-duration-abundance curves are produced by using fractal theory and by determining the effect of climate change on rainfall intensity by TREND software, two periods before and after climate change are evaluated and compared.

In this research, the validation of the Fractal method shows that the IDF curve production for Rasht city using the Fractal method compared to the IDF curve production using observational data has about a three percent difference. Therefore, this method was used to generate IDF curves for the years when rainfall data with less than three hours of duration was not available. The evaluation of IDF curves with TREND software on the rainfall intensity for different durations demonstrates that the rainfall intensity jump occurred in 2003 towards becoming more intense due to the effect of climate change. For example, for 10 minutes with a return period of 100 years before the effect of climate change, the intensity of rainfall is 158 (mm/h) and after the effect of climate change, the intensity of rainfall is 225 (mm/h). Also, the results showed that the period of short returns has changed more than the period of large returns, that is, the 2-year return period has increased by about 70% and the 100-year return period has increased by about 40%.

In this research, by evaluating the fractal method, it was determined that if there is no access to rainfall data with different durations, the fractal method is a suitable method for generating different rainfall durations and generating IDF curves with acceptable accuracy. be Also, the results showed that the jump of intensity-duration-frequency curves occurred under the effect of climate change in 2003, and in the period of climate change, compared to the period before climate change, the intensity of rainfall moved towards more intense rainfall, and this event occurred in the period Short-term returns show a greater increase.