As a regular listener of car radio, I notice that in recent weather reports of the national radio of China, a new phrase 增水 is used to describe the rainfall, e.g., 台风卡努将会为海南带来100毫米增水, but if it was in the past, the same would be 降水. So what is the difference?
They refer to different things. 降水 means rainfall; but 增水 means 风暴增水(风暴潮) here, from which coastal cities might suffer. e.g. from this news 台风“卡努”：海口今夜至明晨将出现风暴增水,
A storm surge or storm tide is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low pressure weather systems (such as tropical cyclones and strong extratropical cyclones), the severity of which is affected by the shallowness and orientation of the water body relative to storm path, as well as the timing of tides. Most casualties during tropical cyclones occur as the result of storm surges. It is a measure of the rise of water beyond what would be expected by the normal movement related to tides.
The two main meteorological factors contributing to a storm surge are a long fetch of winds spiraling inward toward the storm, and a low-pressure-induced dome of water drawn up under and trailing the storm's center.
From my recollection of secondary Geography classes, 降水／降雨 = precipitation, which means the amount of rainfail. But, this is not equivalent to 增水, how much the dam or the reservoir will accumulate. They're probably two different ways of measuring rainfall.