Climate Change Assessment for a Multipurpose Dam

Samoa has recently been heavily impacted by severe tropical storms that caused significant flooding of lowland areas. Studies indicate that these large rainfall events are expected to become more frequent and intense. As part of a program to address the issue of increased flooding, the Government of Samoa proposed building a multipurpose dam for flood prevention, seasonal water supply and additional hydropower. Our work assesses the possible climate change impacts as part of the feasibility study for the Dam.

Previous studies on climate change in Samoa included investigations of trends in historic rainfalls, and evaluations based on coarse scale Global Climate Model (GCM) results. A major study on Climate Change in the Pacific was undertaken by Pacific-Australia Climate Change Science and Adaptation Planning Program in 2014. The PACCSAP study included downscaling GCM outputs to 60km resolution over Samoa. For our investigation, we extracted the downscaled daily rainfall outputs over the Vaisigano River catchment for five GCMs and two representative concentration pathways, 4.5 and 8.5. We derived quantile-quantile scaling factors for daily rainfalls between current climate and climate at 2090 and applied these factors to the historic rainfall series at a gauge close to the Vaisigano catchment. These adjusted rainfall series, and an increased evaporation, were used as input to a continuous rainfall-runoff model of the catchment and a model of the Aaloa Reservoir. In all future climate scenarios, yields to the proposed Aaloa Reservoir were reduced when compared to the current climate. With the proposed operating rules, water supply reliability was not greatly impacted under any future climate scenario. The hydropower generation was reduced under all future climate scenarios. We derived factors to apply to design rainfalls based on the PACCSP report and Australian Rainfall and Runoff (2016) methodology, and the factored design rainfalls were run through a rainfall runoff event model of the catchment. We found that the flood mitigation potential of the dam was maintained under future climate.

This is the first catchment-scale application of downscaled GCM data to derive yields in Samoa, and the study will provide information that can be used to determine appropriate adaptation options to allow for future climate in the dam design. Adaptation options for the dam include varying operating rules, changing storage size and dam arrangement.