Abstract: Climate change has significantly impacted the hydrological cycle in the rivers of Pakistan and the streamflow regimes of different rivers have witnessed noticeable flow alteration. These changes in streamflow affect aquatic biodiversity (e.g., freshwater fish species) and the productivity of freshwater ecosystems. This study, therefore, evaluates the streamflow alterations and their impacts on the fish species at the upstream and downstream of the Soan River in Pakistan. The hydrological model HBV light was calibrated and validated for both up- (Chirah) and down- (Dhoke Pathan) stream gauged stations. The model was then forced with an ensemble of NEX-GDDP Global Climate Models (GCMs) to simulate historic and future streamflow. Afterwards, changes in the streamflow characteristics for two future periods against a historic one were assessed. Different ecologically relevant streamflow indices were used. The base flow at upstream and downstream was projected to increase under both the RCP 4.5 and RCP 8.5 emission scenarios for mid and end of century periods. Under the RCP 8.5, base flow was projected to increase with more inter-annual variability at upstream station, and changes depicted less variability for downstream under both emission scenarios. A number of high flow pulses were projected to decrease under both RCP 4.5 and RCP 8.5 in both future periods for upstream and downstream; however, high variability was depicted for upstream than downstream area. The duration of high flow pulses was also projected to increase under the RCP 8.5 for end of century period for both upstream and downstream. These projections of streamflow characteristics may have a marked (positive) influence on the habitat and ecological conditions for the fish species of the Soan River.

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