The impact of climate change on the streamflow and sediment yield in the Derbendkhan and Hemrin Watersheds is an important challenge facing the water resources of the Diyala River in Iraq. The Soil and Water Assessment Tool (SWAT) was used to project this impact on streamflow and sediment yield until year 2050 by applying five climate models for scenario A1B involving medium emissions. The models were calibrated and validated based on daily observed streamflow and sediment recorded for the periods from 1984 to 2013 and 1984 to 1985, respectively. The Nash-Sutcliffe efficiency and coefficient of determination values for the calibration (validation) were 0.61 (0.53) and 0.6 (0.62) for Derbendkhan and Hemrin, respectively. In addition, the average of the future predictions for the five climate models indicated that the streamflow (sediment yield) for the Derbendkhan and Hemrin Watersheds would decrease to 49% (43.7%) and 20% (30%), respectively, until 2050, compared with the observed flow of the base period from 1984 to 2013. The spatial analysis showed that 10.4% and 68% of the streamflow comes from Iraqi parts of the Derbendkhan and Hemrin Watersheds, respectively, while 10% and 60% of the sediment comes from the Iraqi parts of the Derbendkhan and Hemrin Watersheds, respectively. Deforestation of the northern part of the Hemrin Watershed is the best method to decrease the amount of sediment entering the Hemrin Reservoir.Hydrology 2019, 6, 63 2 of 21 poorer, water resources will decrease, and temperatures will be higher. The Diyala River is one of the most important tributaries of the Tigris River, but its water resources suffer from scarcity and an increase in the sediment rate. The main purposes of water use in the Diyala River are agricultural, drinking, and industrial. In the Tigris River Basin and the Diyala River, water resource deterioration is indicated, and severe shortages in all water resources are expected, especially in the streamflow, for the future [7,8]. However, a comprehensive understanding of the impact of climate change on the water resources of the Diyala River is needed [9]. Considering the rapid decrease in the Dokan Dam's reservoir capacity (near the Derbendkhan Reservoir), it was concluded that the sediment is the main problem facing this reservoir [10]. An investigation of the negative effects of sedimentation in the Mosul Dam's reservoir showed that sediment plays an important role in the reservoir capacity and usually blocks the intakes of the pump stations [11]. The spatial and temporal distributions of streamflow and sediment yield depend on characteristics of rainfall such as the intensity and duration, topography, land cover, and soil type of the watershed [12]. A lack of effective land cover to catch the surface soil allows surface runoff to erode the surface soil, which is considered to be an unacceptable phenomenon for reservoir operation. Land cover management is a traditional method to reduce the sediment yield of a watershed [13].The Soil and Water Assessment Tool (SWAT) is a s...