| dc.description.abstract | The annual rainfall in Taiwan is about 2500 mm/yr, but its spatial and temporal distribution has great discrepancy and is mainly contributed by typhoon events and Plum rains season. The Plum rainfall is the second important source of fresh water except typhoon rainfalls in the island. In addition to typhoon and Plum rainfalls, the amount of spring rainfall is also critical to the water resources management of the Shihmen Reservoir in northern Taiwan. The 2015 drought in northern Taiwan was mainly caused by both typhoon rainfalls in 2014 and spring rainfalls in 2015 were significantly lower than climatology. Restriction on water supply with no water for 2 days in a week was applied in April and back to normal until the hit of Plum rainfall in May.
In this study, observed data of 1982-2011 was analyzed to understand the importance of Plum rainfall, including amount, variation, duration, and onset, to the Shihmen Reservoir Watershed. Then the 2015 drought occurred in the Shihmen Reservoir Watershed was investigated with seasonal climate forecast products of the Central Weather Bureau (CWB). The seasonal climate forecasts, including dynamic regional downscaling forecast and statistical downscaling forecast, by the CWB were used to provide outlooks of monthly precipitation and temperature with 1-3 months of leadtime. A Weather Generator (WGEN) was applied to generate multimembers of daily precipitation and temperature to drive the Generalized Watershed Loading Functions (GWLF) for reservoir inflow predictions. Finally a System Dynamics Model was employed to simulate water allocations of the regional water supply system in association with the Shihmen Reservoir Watershed.
The results show that precipitation forecasts have better forecasting skills in May, but overestimate in June causing larger estimation of inflows. Since drought is caused by persistent low rainfall lasting for several months, like the case in 2015, overall improvement of seasonal climate forecast skills are required to build confidence to water sectors. On the water resource management perspective, it is too conservative to rely on current reservoir capacity and climatology information to the decision making of reservoir operations. Climate outlooks certainly should be involved in reservoir operations subject to improved skill can be demonstrated.
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