Remember learning the water cycle in school? It goes something like this:
The sun shines on lakes and rivers, causing water to evaporate and form clouds. Clouds grow heavy and begin to rain down. Rain runs off the ground, back into lakes and rivers, where the cycle repeats.
But what you learned is not completely true in urbanized areas like CRWD.
Natural areas are landscapes covered by grasses, trees, shrubs, and other plants. They are “soft” and act like a sponge soaking up water. In natural areas, only 10% of rain and snow actually runs off the land. Of the rest, 50% soaks into the ground and 40% evaporates. Instead of being taught that rain runs off the ground, we should have been taught that much more rain soaks into the ground.
As landscapes develop into cities, they become “harder,” with the ground covered by roads, parking lots and buildings. In highly urbanized areas where almost all the ground is covered by hard surfaces, 55% of rain and snow runs off the ground, and only 10-15% soaks in — five times more water runs off than in natural areas.
So, as landscapes are covered with hard, (impervious) surfaces the amount of water that soaks in (infiltrates) decreases and the amount that runs off increases. The runoff in city landscapes is directed to storm drains that lead to lakes, rivers, and wetlands. On its way to the storm drains, however, the runoff picks up all types of pollution that degrade water quality.