Investigating The Effect of Climate Change on Temperature, Rainfall and Intensity-Duration-Frequency Curves In Dry Areas (A Case Study: Kashan watershed)

Introduction:

The Intensity-Duration-Frequency (IDF) curves are a suitable tool for estimating the threshold values of precipitation in different return periods, which are prepared based on the long-term statistics of each region and can change under the influence of climate change (Mohammed Lu et al., 2014). These curves are one of the tools for planning and managing water resources, which indicate the frequency and maximum intensity of annual rainfall for different continuities. These curves have been used for different purposes, especially for the design of flood control and diversion structures in cities, and they are used in different standards for urban flood management.

The purpose of this research is to investigate the effect of climate change on changes in temperature, precipitation and frequency intensity curves in the period of different returns in the study area.

materials and methods:

In this research, the effect of climate change on changes in intensity-duration-frequency curves in Kashan city as a dry region was investigated. Kashan city with an area of 9647 square kilometers is located in the northern part of Isfahan province and is bordered by Aran and Bidgol cities from the east, Natanz, Barkhar and Mimeh cities from the south, Dilijan (from the central province) and Qom cities from the west and north.

In order to carry out this study, the results of the fifth assessment report (AR5) of climate change of the Intergovernmental Panel on Climate Change (IPCC), which was published in 2014, were used to predict the impact of climate change on temperature and precipitation changes. In this report, the release scenarios have three trajectories named RCP8.5, RCP4.5, and RCP2.6, which are introduced as optimistic, intermediate and pessimistic scenarios. Then, intensity curves were calculated and evaluated and compared for the basic and future periods.

In order to examine the changes in rainfall intensity patterns during different return periods, it is necessary to extract intensity-duration-frequency curves in different conditions. One of the usual methods for regional studies of the intensity-duration-frequency of rainfall is Bell's method (Relation 5), which was promoted by Gharehman and his colleagues in Iran. In this study, the latest relationships developed by Gharam and Abkhizr were used to extract intensity-duration-frequency (IDF) curves. Considering that the presentation of the mentioned relations is based on the climatic division, therefore, in this study, from the relation that for the desert and southern regions (Nizar-Salfachgan, Kuhpayeh, Herat and Jiroft basins in the central provinces, Qom, Isfahan) , Kerman and Yazd) has been presented and used.

In this study, in order to investigate the effect of climate change on frequency intensity curves, first the base period curves were drawn based on the values measured at the Kashan synoptic station. Then, using the new climate scenarios and the output data of the Lars exponential microscale model, we will try to extract the intensity curves of the duration of the floods under different climate scenarios in the near future (2011-2030), the future (2050-2031), and the distant future (2070-2051) and then the results of these curves were compared with the base period.

Discussion

The results of analyzing the efficiency coefficients of the Lares model in predicting temperature and precipitation parameters showed that this model has simulated the precipitation and temperature data with acceptable accuracy for the base period. The accuracy of the model in estimating the minimum and maximum temperature parameters has been higher, so that the Nash and explanation coefficients for the maximum and minimum temperatures have been obtained as 0.99 and 0.99, respectively. The explanatory and Nash coefficients for precipitation data were obtained as 0.95 and 0.93, respectively.

In order to investigate the effect of temperature and precipitation changes on the climatic changes of the region, the climate view of Amberje was used. For this purpose, the Ambergris humidity index was calculated. In this regard, Q2 is the Ambergris humidity coefficient, P is the average annual precipitation (mm), M is the average maximum temperature in the hottest month of the year (Kelvin), and m is the average minimum temperature in the coldest month of the year (Kelvin). In the ambergris climate, the horizontal axis is the average lowest temperature in the coldest month of the year in degrees Celsius. The climate of the region was determined under different scenarios by using the climate curve of the Ambergate view. Netabaj showed that under all the examined scenarios, climate change will occur in the region, and the climate of the region will change from dry cold to dry temperate according to scenario 4.5 (2050-2031) and scenario 6.2 (2070-2051) and also according to other Scenarios in the periods of (2011-2030), (2031-2050) and (2051-2070) the climate of the region will change from cold dry to moderate desert.

Based on the results, in long-term rains, the rainfall intensity will have less changes compared to the base period. But for short-term rains (less than 4 hours), the intensity of rain changes more dramatically, and the most changes are related to rains with a duration of less than one hour, but the least changes are predicted for 24-hour rains. . Changes in rainfall intensity in short-term time bases compared to the base time will increase by 4.5 in scenario and decrease by 2.6 in scenario, but in all scenarios there is a significant difference between the average rainfall intensity of 0.17 to 24 hours in the region. can be seen (Table 8). So that in the time of 0.17 hours under the 2.6 scenario in the two-year return period, with an increase of 1.84 percent (from the value of 10.75 to the value of 30.54) under the 2.6 scenario in the hundred-year return period, it will be reached. Also, by comparing the intensity of precipitation under the scenario of 4.5 in one hour during the two-year and hundred-year return period, the upward trend will reach the value of 14.41 with an increase of 1.84 times from the value 0.05 ( P< 0.05).