A diagram showing flash flooding in the Jucár basin in Spain.
Source: Gabaldo Sancho et al.
The intensity of rainfall is a measure of the amount of rain that falls over time. The intensity of rain is measured in the height of the water layer covering the ground in a period of time. It means that if the rain stays where it falls, it would form a layer of a certain height. We say things like: 30 millimeter of rain fell today or it rained 20 millimeter in two hours. Sometimes people speak of the volume of water that falls on a square meter in a period of time: ten liter per square meter per day for instance. A millimeter of water equals a liter of water on a square meter.
What a high or low intensity is, is hard to say. It depends on the local circumstances. Generally speaking a relatively low intensity is for instance 2 millimeter of rain a day and relatively high may be 30 millimeter an hour. High intensity of rainfall on steep slopes, may lead to flash floods. On flat areas it may lead to ponding, or urban floods when the drainage capacity is insufficient for the intensity of the falling rain.
To the right is a diagram from the river basin Jucár in Spain. This diagram shows the typical characteristics of a flash flood. Above in blue you see the rainfall over time. Below you see the amount of water flowing through the river, in red, at the same time. At first, there’s hardly any water in the river. Then there’s rain with a high intensity. Shortly afterwards there’s a huge peak of water flowing through the river.
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