Lecture26-29(2008) - Chapter 7 in textbook SOIL WATER FLOW...

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SOIL WATER FLOW Water flow in soils and many other porous media is hard, if not impossible to see directly. It is not easily characterized and in many cases to measure; therefore, I want to introduce water flow in capillary tubes first. Chapter 7 in textbook
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Flow in a capillary tube is described as follows: 1. Surface layer of water molecules along the walls of the capillary are practically immobile. 2. Water layers near the surface retain some structure and have higher than normal viscosity. 3. Flow velocity increases with distance from surface — therefore, it is easy to understand why flow velocity is greater in large tubes than in small tubes.
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4. This relationship is expressed by the Poiseuille equation, which states that the rate of flow through a tube is proportional to the 4th power of the radius of the tube. Poiseuille’s Law is as follows: Q = B R 4 ) P/8 0 L where: Q = quantity of flow per unit time (L 3 /T); R = radius of capillary tube (L); ) P = change in pressure potential ( R p ) (L) over a distance L; 0 = viscosity of water.
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Flow of H 2 O in soil involves : Movement from one position to another; this movement occurs with respect to some reference. Change in water content (increase/decrease) over time at a specified position. Flow occurs as a result of a hydraulic potential gradient : The hydraulic potential gradient is defined as the decrease in total potential (head) divided by the distance in which the drop (decrease) occurs. Flow in Saturated Soil and Other Porous Material i = ( )R t )/L = ( ) H)/L
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