水资源 分布随时间变化的波动性，在季节间、多年间、年际间不规则变化。导

水资源 分布随时间变化的波动性，在季节间、多年间、年际间不规则变化。导致的结果是使水电项目的发电能力和财务效益具有很大的不确定性：一方面，有较好财务效益的水电项目在投入运行后正好遇枯水期，发电能力和财务状况可能要比原预期的差很多；另一方面，效益较差的水电项目，如果正好遭遇丰水期，则可 能具有较好的财务效益。 由于水电效益的不确定性，衡量水电效益的指标通常用多年平均值表示。

The fluctuation of water resources distribution with time changes irregularly among seasons, years and years. The result is that the power generation capacity and financial benefits of hydropower projects have great uncertainty: on the one hand, hydropower projects with good financial benefits happen to encounter dry season after being put into operation, and the power generation capacity and financial status may be much worse than originally expected; On the other hand, hydropower projects with poor benefits may have better financial benefits if they happen to encounter wet periods. Due to the uncertainty of hydropower benefits, the indicators to measure hydropower benefits are usually expressed by multi-year average.

The fluctuation of water resources distribution with time varies irregularly among seasons, years and years. As a result, the power generation capacity and financial benefits of hydropower projects have great uncertainty: on the one hand, hydropower projects with good financial benefits just meet the dry season after being put into operation, and the power generation capacity and financial situation may be much worse than originally expected; On the other hand, hydropower projects with poor benefits can have better financial benefits if they happen to meet the flood season. Because of the uncertainty of hydropower benefits, the indicators to measure hydropower benefits are usually expressed by the multi-year average.