整个流域地表面的平均降水量。气象站所观测的降水量只能代表站区很小范围的降水情况，是点状观测资料。
IRI也没有预测阿玛逊河流域会出现低于平均降水量，而该地区去年夏天遭受了百年不遇的严重旱情。
IRI did highlight the Amazon as a region likely to have below-average summer rains, though, and the Amazon ended up having a 100-year drought last summer.
对于森林流域和灌木流域，其多年平均径流系数主要受多年平均降水量、多年平均蒸散量和平均坡度共同影响；
For forest watershed and shrub watershed, the annual mean runoff coefficient was affected by mean annual precipitation, mean annual evapotranspiration and mean slope grade.
对流域历年平均降水量序列最大熵谱分析表明存在3.6年的显著周期和5.7年的次周期。
Spectrum Analysis and Maximum Entropy Spectrum Analysis of precipitation show 3.6, 5.7 years period and the evaporation has no significant period.
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