Impacts of climate change on streamflows under RCP scenarios: A case study in Minab River basin, Hormozgon of IRAN

Researchers often examine hydro-climatological processes via Global Circulation Model (GCM) and hydrological model, which have been shown to benefit water resources management and prediction, especially at the basin scale. Climate change poses water resource challenges for many already water stressed watersheds throughout the world(Ercan et al., 2020:53; Aryal et al., 2018:1). As per the IPCC report, the global temperature may increase by 1 °C to 5 °C by the end of the century (Bajracharya et al., 2018). Climate scenarios for both Global climate model (GCM) or simple analog models are often adapted to examine the effect of climate change on hydrology (Fereidoon and Koch, 2018). Hydrological models or rainfall-runoff models are essential for understanding the hydrological processes of river basins and supporting operational management of water resources characterized with large spatial and temporal variability (Tuo et al., 2016). The Soil and Water Assessment Tool (SWAT) was used for a future projection of changes in the hydrological regime of the basin based on Representative Concentration Pathways Scenarios (RCP) (Bajracharya et al., 2018).

The minab basin is one of the main sources of water supply to the Bandar Abbas. Minab basin is located in the east of  Hormozgan and north of Minab. The basin area is 10605 square kilometers.The basin has a hot desert climate, according to the extended demartern classification system.In this study, daily meteorological data from 1985 to 2018 including maximum temperature, minimum temperature, rainfall, relative humidity of 8 selected stations in the study area were obtained from the National Meteorological Organization (IRIMO) and discharge data 3 hydrometric stations Brentin Minab located at the exit the basin was prepared as a base station from 1985 to 2018 by the Ministry of Energy. Spatial data includes digital elevation model (DEM) data, runoff  network information, soil type, and landuse. In this study, 6 different climate models have been used under RCP2.6, RCP4.5   and RCP8.5  scenarios, including GFDL-ESM2M, HadGEM2-ESM, IPSL-CM5A-MR, MIROC-ESM, NorESM1-ME and CanESM2. The Minab basin simulation uses the SWAT model, which is a distributed and physical hydrological model that is often used to evaluate management decisions and climate change on hydrological processes. The main inputs of the SWAT model include the information layers of the Digital Elevation Model (DEM), the soil layer and the land use layer, and the input microclimate variables of the SWAT model include daily precipitation, maximum temperature ,minimum temperature and discharge. The parameters of sensitivity analysis, calibration and validation with SWAT- tool and using SUFI2 program as one of the algorithms in SWAT-CUP software were determined and their sensitivity and uncertainty were estimated using this program. Statistical indicators have been considered for evaluating rainfall, temperature and discharge outputs.

Results and evaluations of 6 climatic models and in all three scenarios, RCP2.6 RCP4.5 and RCP8.5 to predict the future temperature and precipitation of Minab basin, it is stated that in all models and climatic scenarios, the temperature of Minab basin is constantly increasing until 2099, and the rate of increase in the minimum temperature will occur more than the increase in the maximum temperature in the future; about precipitation, the predictions are complex and uncertain, but in most models, the predicted amount of precipitation will be less than the observed value.The results of the influence of different climatic scenarios on the parameters of the Minab River regime indicate that is calculated the average of discharg under RCP2.6 scenario is equal to 5.2 cubic meters per second and in RCP4.5 scenario is equal to 5.08 and finally in RCP8.5 scenario is equal to 5.07 cubic meters. Therefore, it is inferred that discharge will have a decreasing trend compared to its observation period in the next period, and in the scenarios, RCP2.6, RCP4.5 and RCP8.5, respectively, will be followed by 39.1%, 40.3% and 40.4% of the discharge drop. Due to the future runoff values ​​of the basin, it seems that the Minab River will be affected by climate change and its discharge will drop by about 40%.

The evaluation results of climate models under RCP scenarios show that the maximum and minimum temperatures will continue to increase continuously in all climate models in the future, but the increase in the minimum temperature is greater than the increase in the maximum temperature. In the case of precipitation, complexity is high and uncertainty is expressed, but in all three scenarios, a decrease in future rainfall is indicated. The SWAT model is a hydrological model used to estimate the future discharge of the basin. The results of the numerical index of performance evaluation of the model under RCP climatic scenarios in simulating the runoff of Minab basin in the hydrometric station of Brentin Minab indicate the appropriate efficiency of the model in simulating runoff of Minab basin. The results of the studies show that the average future runoff of the basin has a decreasing trend, which will have a decrease of 39.1, 40.3 and 40.4%, respectively, compared to its observation period. Therefore, under all RCP scenarios, the average discharge of the basin will decrease in the future, and the probability of frequent droughts occurring with a longer duration than the observation period is expected.