اثر تغییر اقلیم بر جریان حوضه آبریز رودخانه‌ قره‌سو کرمانشاه

The increase of greenhouse gasses, the so called reason of climate change, may cause the global average temperature to be surged up and consequently, patterns of regional precipitation, evaporation, land use, soil moisture and river flow rates will be altered. Based on climate change, the earth’s temperature continues to rise, it is possible to expect a significant impact on water resources. As temperatures increase, some parameters like evaporation increases, sometimes resulting in rain and droughts. Rainfall and runoff variability are important phenomenon and causes severely impacts on the water resources planning and management. It is the most studied hydro-climate variables because of its significance for sustainable water resources management, water supply, environment, agricultural activities and ecological management. In this research, the outflow of Gharesu River basin, a river in the west of Iran, has been predicted in current and future period. The Gharesou River basin is one of the sub-basins of Karkheh basin with an area of 5354 km2 and is contain of three major streams. In this study, future hydrologic variables and outflow of the basin have been simulated and predicted using climate models and under different emission scenarios. This study has tried to discover the effects of climate effects on hydrologic variables, rainfall and runoff variability of a river basin using a methodology combining climate models, rainfall-runoff model, GIS and predictors.
In this study, the impact of climate change on the stream flow of the Gharesou River for the future period of the years 2046 to 2065 is analyzed, utilizing two climate models, HADCM3 and MPEH5, under three emission scenarios- A1B, A2, and B1. Moreover, MapShed, a rainfall- runoff model that requires GIS layers of surface water, topography, soil type, regional climatological stations, accompanying with precipitation and maximum and minimum data produced by LARS-WG model, was used to simulate the stream flow in a given upcoming period. LARS-WG model produced future hydrological data based upon present precipitation. Major predicted data are maximum and minimum temperature and sunny hours of a day. Then, the MapShed model was calibrated and validated based on the observed stream flow, provided from the Kermanshah regional water authority, for the years 1990 to 1995 and 1996 to 1998, respectively. In order to predict the future condition, the upcoming parameters of the rainfall and the temperature were predicted by downscaling the HADCM3 and MPEH5 models’ output by means of Lars-WG statistical downscaling model. For accomplishing this goal, the past climatic parameters of the period of 1960 to 1995 are used. The output of these climate models (for the period of 2046 to 2065) associated with GIS layers of the basin’s land use, topography, surface runoff and soil texture were introduced to MapShed, a GIS-based watershed simulation model, to produce the future flow of the river basin. The goodness of fit criteria, for flow rate produced by MapShed model, including RMSE, R2 and minimum cumulative errors (CE) indicate good performance in calibration and validation processes for the periods of 1990 to 1995 and 1996 to 1998, respectively. In all model-scenarios except for MPEH5-A2, the future, like long-term observation period, peak flow happens in March. Under MPEH5-A2 scenario, the peak flow was transferred to April. Additionally, model MPEH5, under the A1B scenario and in all months, predicts flow rate less than the observed values, while this model forecasts flow rates more than the observed values for all months of the years. Moreover, most of the model-scenarios indicate a decline of flow rate in the months of February, March, April, September and October and an increase in the months of June, July, August and January.
The results show that the 20-year average of future flow in the Gharesou River under four model-scenarios of MPEH5-A1B, HADCM3-B1, HADCM3-A1B, and MPEH5-B1 will increase and under two model-scenarios of MPEH5-A2 and HADCM3-A2 will decrease, in comparison with the long-term mean of the observed annual flow. It is worth mentioning that most of the scenarios predict the increase of flow rate, with respect to observation period, in the late spring and during summer and the decline of flow rate in the early spring. Like observed period, in the most of the scenarios, peak discharges of outflow of the basin have been estimated in March, and just in MPEH5 scenario, peak discharge happened in April.