请教水文方面precipitation exceedance probabilities的意思

Precipitation exceedance probabilities : 降水超越机率

(Precipitationshould not be limited to the rain. In meteorology, precipitation is any productof the condensation of atmospheric water vapour that falls under gravity. The main forms of precipitation includedrizzle, rain, sleet, snow, graupel and hail. See http://en.wikipedia.org/wiki/Precipitation)

Exceedance probability (PE, 超越机率) : Probability that an event of specified magnitude will be equalled or exceeded in any defined period of time, on average. (See http://www.ewater.com.au/glossary/QID.ljb63f2c.1/QID.ljb63f2c.1.qy5pxe.271.html )

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