Modeling analysis of 21st century climate change projections and their impacts on the hydrology of the Karaj-Jajrood basin, Iran

Two main rivers, Karaj and Jajrood which provide metropolitan Tehrans’ water, flow in Alborz and Tehran province, respectively. In the last decades, the impact of climate change on temperature and precipitation is tangible in these two provinces. For the Karaj-Jajrood basin, during the period 2003-2012, the minimum temperature increased by 1°C, the maximum temperature by 0.6°C and precipitation decreased by -7%, relative to the average of 1972-2012. To assess climate change impacts in the region, the Statistical Downscaling Model (SDSM) is used to downscale projected temperature – and precipitation- predictors of the CanESM2 Global Circulation Model (GCM) under RCP2.6, RCP4.5 and RCP8.5 scenarios. Mann-Kendall trend tests on projected temperature indicate an upward trend for Tmin and Tmax by the end of the 21st century under RCP4.5 and RCP8.5. More specifically, Tmin and Tmax each will increase by 2°C, 3°C and 5°C under RCP2.6, RCP4.5 and RCP8.5, respectively, relative to the historical period. The MK test shows a downward trend in precipitation over the whole period (1972-2100). The annual average precipitation will be decreased by ~ -29% in future under all RCPs. Monthly analyses indicate that precipitation will decrease the most (by -75- -80%) in future summer months (June, July, August) under all RCPs, and the least (-6%) in October and November under RCP2.6 and RCP4.5 and by -14% under RCP8.5. For the winter months, the rainfall will drop by around -35% under all RCPs. Tehran province has faced droughts repeatedly in the last decades. Identifying vulnerable areas is an essential prerequisite to mitigate future drought damage costs in an affected region. For this purpose, the future occurrence, and characteristics of droughts across the 21st century are surveyed over the Jajrood basin. The projected precipitation under RCP2.6 and RCP8.5 is then employed to compute the 12- month-scale Standardized Precipitation Index (SPI), in the Jajrood basin from which four drought characteristics, i.e., frequency, duration, magnitude and severity are extracted. The results reveal no significant changes in the number of droughts in the three 30-year future periods, though drought durations will increase across the 21st century under both RCPs. In addition, the most severe droughts will occur in the near future (2013-2042) under RCP8.5. To be more precise, in this period the middle part of the basin will confront longer droughts with greater magnitudes under both RCPs. In the middle future (2043-2072) both drought magnitude and severity will decrease under both RCPs. However, drought magnitude and severity will rise again in the far future (2073-2100). One unanticipated finding is that the drought magnitudes in the middle and far future periods are larger under RCP 2.6 than under RCP8.5, due to the fact that the latter scenario predicts more future rainfall than the former.