The National Soil Erosion Research Laboratory

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Hydrologic Cycle


Rainfall is the driving force for the hydrologic cycle, that group of physical phenomena which control our water supplies. The soil lost due to rainfall is directly related to the duration and intensity of a storm. As the intensity increases so does the diameter of each raindrop. And as the size of each raindrop increases, so does the amount of energy transferred to the soil when it hits the soil surface. Thus, the more energy a particular raindrop possesses, the greater erosive capabilities it has.


According to the Report of the Governor's Soil Resource Study Commission (November 1984), "Each square mile of land in Central Indiana is bombarded by several quadrillion raindrops annually (assuming 40 inches annual rainfall). Their total energy impact on this square mile amounts to approximately 40 billion foot - pounds or the equivalent of 13,000 tons of TNT!"

The picture here was taken after a simulated rainfall on a sandy soil. Small pillars were formed because cover (pebbles, leaves, etc.) protected the soil from raindrops. A penny was included to show the size of the pillars.

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