你的问题似乎是一个概念,所以我回答概念上。为了论证假设平面与不透水地面面积,喜欢摇滚,一个不透水墙。After it has rained for some time with a certain intensity over that area — everywhere the same ‐, there will be a certain height of water on the ground, which will be the same height all over this flat area. This height is measured in millimeters and reported. You can now go and divide this area up in square meters, or soccer fields, or square milimeters; the height of the water would obviously not change. That's the beauty of this measurement: It is independent of any particular area. Now of course these are all unrealistic assumptions. The intensity or duration differed from place to place, the ground is not level, water seeps away etc. So the statement actually says "this station measured a rain event which would cover any area with x mm of rain if it had rained everywhere the same amount as it did here." The duration is irrelevant; rain is rain. (It *is* relevant for the consequences; 100 mm in a month is OK, while 100mm in a day are catastrophic).