Climate change could affect the water resources system globally in common and particularly at the catchment level. Future changes in climate would affect streamflow and subsequent water resources. This study aims to use the Water Evaluation and Planning (WEAP) model for assessment of climate change impacts on hydrological processes and water resources in Phuoc Long catchment, which plays a vital role in the life of inhabitants at the downstream of the Thac Mo reservoir. WEAP is a powerful tool in scenario building for predicting climate change impacts, which has been implemented in many catchments around the world. For this study, data collection and processes are conducted to input the required data into the WEAP model. Streamflow data (1978–1993) were used to calibrate and validate the WEAP hydrological model, where the period of 1978–1988 for calibration and 1989–1993 for validation, respectively. By using WEAP and General Circulation Model (GCM) outputs, both streamflow and subsequent water shortage during the baseline period (1994 – 2003) and midterm period (2046–2064) were simulated and compared to indicate the effect of climate change. Downscaled monthly rainfalls and temperatures under B1 and A2 emission scenarios from different GCMs were used to generate streamflow through WEAP model and subsequent water shortage. The results show that mean streamflow tends to increase around 10.1% in the wet season due to an increase in rainfall, but also tends to decrease in December (-4.4%), January (-1.0%), and February (-0.85%) of the dry season relative to the baseline scenario. Consequently, the downstream of the Thac Mo reservoir may face a big challenge in the dry season in terms of water use. In addition, water shortage at the downstream of the catchment increases around triple compared with the baseline scenario for consideration of a combined assessment of climate change impacts and socioeconomic development in this region.

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