Lecture54-56(2008)

Lecture54-56(2008) - WATER EROSION AND SOIL LOSS PREDICTION...

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WATER EROSION AND SOIL LOSS PREDICTION Chapter 15 in Introduction to Environmental Soil Ph ysics Erosion of soil by water and wind destroys good farm and forest land. Eroded soil/sediment is often a source of pollution to surface water. Erosion = removal of the nutrient-enriched topsoil, resulting in reduced soil production potential (productivity). Soil erosion by water may be divided into three modes: (1) detachment (2) trans port (3) de position - reduced overland flow velocity areas, zones: i.e., toe slope positions, stream channels, lakes, and reservoirs.

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Factors Affectin g Detachment Raindro pim pact : Soil erosion has been correlated to the kinetic energy of raindrops. Kinetic energy (KE) of a raindrop, KE = 1/2 mv 2 , where m = mass of drop, and v = velocity of drop. Terminal, v, is proportional to the square of the radius of raindrop The KE of a given raindrop of mass, m is proportional to the 4th power of the radius of the drop--- ---if the radius doubles, the KE is increased 2 4 = 16 times. As rainfall intensity increases , drop size increase ,---thus the kinetic energy increases significantly because both mass of water and velocity of the drops increase. Raindrop impact is the major cause of particle detachment and suspension ---leading to erosion.

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K.E. = ½ mv 2 K.E. - Kinetic energy of raindrop m - mass of raindrop v - velocity of raindrop Kinetic Energy of Raindrops F down F up F down = mg = D w 4/3 B r 3 g F up = Cf B r 2 ( D a v 2 / 2) D w 4/3 B r 3 g = C f B r 2 ( D a v 2 / 2) v 2 = 8/3 D w g r / C f D a C = 8/3 D w g (constant) C f D a v 2 = C r v " s r=r 1/2
K.E. = ½ mv

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Lecture54-56(2008) - WATER EROSION AND SOIL LOSS PREDICTION...

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