Extension of IDF curves under different scenarios of climate change and estimation of maximum flood discharge in Darkeh basin in Tehran (Case study: Shemiran, Geophysic and Mehrabad synoptic stations

Intensity-duration-frequency (IDF) curve is one of the most common tools used in water resources management (Bernard 1932). In this regard, obtaining IDF curves plays an important role in designing of hydraulic structures such as dams and water transfer canals. Intensity, duration and frequency of precipitation are changed according to the change in the hydrological cycle and the increase of greenhouse gases. The optimal designs of surface runoff systems extremely rely on IDF rainfall curves (Liew, et al. 2014). Since rainfall characteristics are often used for designing hydraulic structures, it is necessary to review and update rainfall characteristics such as the IDF curve for the future climate scenarios (De Paola, et al. 2014). Climate change affects the intensity and frequency of rainfall as well as runoff in future periods. According to the studies conducted in the western reversible flood basin, the amount of runoff and maximum flows and the probability of flooding will increase significantly in the future time horizon, (Binesh, et al. 2018). As well, it is necessary to define the flood peak thresholds in order to determine the discharge values for different return periods, for which it is necessary to determine the maximum precipitation values with a certain intensity and continuity for future periods (Van, et al. 2020). According to the researches, it seems that the effect of climate change on marginal currents in urban basins has been less considered. Therefore, in the present study, using the SDSM model outputs according to the fifth report, intensity- duration- frequency (IDF) curves under the influence of climate change scenarios were extracted for Shemiran, Geophysic and Mehrabad synoptic stations located in Tehran province with different return periods in order to better evaluate the role of climate change on the intensity of rainfall and floods in the basin and to consider the necessary measures in accordance with the conditions ahead.

The data of historical storms of meteorological stations is used to obtain the intensity-duration-frequency curves. In the present study, IDF curves of Shemiran, Geophysic and Mehrabad synoptic stations located in Tehran province influenced by climate change of the historical period (1991-2015) with 2, 5, 10, 25, 50 and 100 year return periods were extracted. SDSM software version 5.3 has been used for downscaling. Based on the following downscaling, precipitation forecasting was performed under three scenarios: RCP2.6, RCP4.5 and RCP8.5 for 5 periods of 15 years and a period of 10 years from 2016 to 2100. The SCS method has been used to calculate the concentration time in this study. Three criteria of correlation (r), Nash Sutcliffe coefficient (NSE) and Bias were used to evaluate the efficiency of SDSM model. The proposed method of the US Soil Conservation Organization is based on the proposal of the World Meteorological Organization (WMO) about the temporal distribution pattern of storms. The amount and intensity of precipitation in each return period is calculated by means of this temporal distribution pattern. 6-hour precipitation was obtained under RCP scenarios for different return periods using the hero relationship.

In this study, forecasts for the next 50 years indicate that the rainfall will decrease by 16.55% and 14.36% at Shemiran station according to the RCP2.6 and RCP8.5 scenarios. Compared to the observational state; however, precipitation will increase by 45.24% based on the RCP4.5 scenarios. At the Geophysic station, the annual rainfall will decrease by 25.12 and 16.8 percent based on the scenarios of RCP2.6 and RCP8.5, but it will increase by 20.61 percent based on the RCP45. The precipitation at Mehrabad station will decrease in all scenarios. The precipitation at Mehrabad station will decrease by 11.038, 10.6 and 5.75, respectively under the scenarios of RCP2.6, RCP4.5 and RCP8.5.

In the present study, it is found that the rainfall intensity by comparing the obtained IDF curves in Shemiran, Geophysic and Mehrabad synoptic stations under RCP2.6 scenario has dramatically increased by 45.66, 54.49 and 31.74 percent in comparison to the base period for the return period of 50 years. RCP4.5 scenario contains 40.89, 61.5 and 43.9% and RCP8.5 scenario contains 65.77, 66.12 and 48.6 percent. Finally, using the extracted IDF curves for Shemiran, Geophysic and Mehrabad synoptic stations, the maximum flow influenced by climate change was compared with the base flow. The results show that the maximum discharge is increasing. According to the results, the maximum flow rate based on the rainfall intensity obtained at Shemiran station increased by at least 40.89% and at most 65.77% under the influence of RCP4.5 and RCP8.5 scenarios for the return period of 50 years compared to the maximum of base flow. According to the intensity of precipitation calculated at the geophysic station, the maximum flood discharge increased by about 54% with the RCP2.6 scenario and about 66% with the RCP8.5 scenario compared to the base period. Finally, by comparing the maximum discharge under the influence of climatic scenarios and the maximum of base flow (based on calculated by rainfall intensity) at Mehrabad station, it was determined that the minimum increase of the maximum of base flow is related to RCP2.6 scenario was 31.74% and the maximum increase was related to RCP8.5 scenario which is 48.61%