Assessment of climate change impacts on low flows in Karkheh River Basin in 2070 to 2100

Increasing the greenhouse gases not only has impacts on the weather parameters, but also, has impact on water resources, agriculture, environment, health and economy. Climate change has significant effects on water resources by changing the hydrological cycle. There are several simulation methods for investigating the effects of climate change on different systems in the future, like climate models. The AOGCM model is able to simulate global climate in an area of some ten dousant km2, but, while they are not suitable for regional scale. For this reason, downscaling methods such as dynamic methods are used. These methods are based on high resolution and analysis of climate models. In this research, the impact of climate change was investigated on the low flow of Karkheh Basin as one the most important basins due to its water product and agricultural point of view. For this purpose, the PRECIS which is exponential dynamics and downscaling model, was used to estimate the temperature and precipitation in the period of 2070 to 2100 under A2 and B2 scenarios. The SWAT model, a comprehensive and continuous hydrological model was also used to estimate the flow discharge for the watershed. After calibration and validation of the SWAT model, the amount of rainfall and temperature used as input for PRECIS model under different climate scenarios and finally the daily flow rate was estimated for sub-basins. Then the indices of low flow rates (Q75, Q90 and Q95) and low flow series frequency analysis of 10 and 30 days were assessed. Results showed that rainfall and flow rate have negative and temperature have positive trend. In general, the results of PRECIS model indicated that this model has a good estimate of temperature and precipitation in the region, but, it is not strong for rainfall in autumn and spring, due to the local nature of the precipitation. The climate change assessment under scenario A2, indicated the rise of low flow rate by 70 percent, and this increase of low flow was more in the northern parts of basin, while under the scenario B2 the low flow rate droped by 50 percent for the period of 2080. But, low flow distribution had no change compared to base period.Thus, sever droughts would happen in the central and some northern parts of basin. It can be concluded that under different scenarios, climate change has different impact on the low flow rate. Therefore, due to the different impacts of climate change on low flow rate under different scenarios, uncertainties of scenarios as well as regional economic and social status should be considered in the management plans.