پژوهشنامه مدیریت حوزه آبخیز (Jul 2024)
River Discharge Changes in Eastern Watersheds of Mazandaran Province under the Impact of Climate Change
Abstract
Extended Abstract Background: Climate change directly impacts hydrological components, water resources, and watershed outflow. Calculating the amount of possible changes in rainfall and runoff will play an important role in the policy-making and planning of water resources under climate change conditions. In this research, runoff simulation in the eastern watersheds of Mazandaran province (Talar, Tajan, Nekarood, and Babolrood) was investigated under the impact of climate change in the future period (2021-2040). Methods: In this research, one of the atmospheric-ocean general circulation models, named HadGEM2, was used under climate scenarios (RCP 2.6 and RCP 8.5) to investigate climate change. The Lars-WG model was used for the downscaling of climate data during three stages of data calibration, data evaluation, and meteorological data generation for the future period. This purpose was achieved using the daily data of rainfall, maximum and minimum temperatures, and sunshine hours of the selected stations of four watersheds in the current period (1997-2016), and the model was implemented for the base period. After ensuring the ability of the Lars-WG model to simulate meteorological data by comparing the data produced by the HadGEM2 model and the observational data of the base period, this model was implemented to generate future data using climate scenarios (RCP 2.6 and RCP 8.5). The daily values of the climate data of rainfall, maximum and minimum temperatures and sunshine hours were produced and simulated for the next 40 years (2021- 2060). Runoff simulation in the future period and the effect of climate change on runoff were investigated with the IHACRES hydrological model, which is a continuous conceptual-dynamic model for simulating rainfall-runoff at the watershed scale, using climate scenarios. For this purpose, the daily data of rainfall, mean temperature, and discharge of the current period (1997-2016) in four watersheds were introduced into the IHACRES software. Then, the 1997-2008 and 2009-2016 periods were selected for calibration and validation, respectively, to determine the ability of the IHACRES model to simulate future runoff. After calibration and validation, the climate data produced by the HadGEM2 model were introduced into IHACRES, and the daily series of discharge was predicted for the future period (2021-2040). Then, the annual average of the observed discharges in the current period and the simulated discharges in the future period under the RCP 2.6 and RCP 8.5 scenarios, as well as the 20-year average runoff in the stations in the future period, were compared and evaluated with the observation period. Results: The results of the climate change investigation showed that mean monthly minimum and maximum temperatures in all months of the future period would increase in all four watersheds in both scenarios RCP 2.6 and RCP 8.5 compared to the current period (1997-2016). Compared to the current period, the mean monthly rainfall in both scenarios does not have a constant trend and is different in various months. The results of the runoff simulation show that the changes in the discharge of the watersheds under the effect of climate change are not constant compared to the discharge of the current period. This could be attributed to the inconsistency of the trend of changes in the precipitation of the future period compared to the current period. Despite the increase in the average rainfall, the discharge will decrease in some years in the future period, which can probably be related to the increase in temperature in the future period due to climate change. Despite the decrease in the average rainfall of that year, the increase in discharge could be caused by, for example, the possibility of heavy rains or showers in that year, thereby increasing the amount of discharge. Compared to the current period, the 20-year mean of the predicted discharge has decreased in the selected stations (Talar, Tajan, and Nekarood watersheds) in the future period for both scenarios. In the station considered for the Babolrood watershed, however, this value has increased in the RCP 2.6 scenario compared to the current period. In the Polsefid station of the Talar watershed, the simulated discharge under the RCP 2.6 and RCP 8.5 scenarios was equal to 6.18 and 6.12 m3/s, respectively, which shows a decrease compared to the observed discharge of 7.41 m3/s. In the Soleimantange station of the Tajan watershed, the predicted discharge under the RCP 2.6 and RCP 8.5 scenarios was equal to 7.33 and 7.29 m3/s, respectively, which has decreased compared to the observed discharge of 9.82 m3/s. In the Sefidchah station of the Nekarood watershed, the simulated discharge under the RCP 2.6 and RCP 8.5 scenarios was equal to 3.85 and 3.72 m3/s, which reveals a decline compared to the observed discharge of 4.10 m3/s. In the Babol station of the Babolrood watershed, however, the discharge under the RCP 2.6 scenario was predicted to be equal to 11.40 m3/s, which has increased compared to the discharge (11.34 m3/s) during the observation period. Under the RCP 8.5 scenario, it was simulated with a value of 11.22 m3/s, less than the observed discharge. Conclusion: In general, the impact of climate change on water resources is one of the major challenges facing water resources planners and can have serious consequences for ecosystems and communities. Based on the findings of the research and the importance of climate change effects on the hydrological conditions of the watersheds in the study area, it is necessary to apply appropriate policies for the sustainable management of water resources.
