Lecture26-29(2008)

Lecture26-29(2008) - SOIL WATER FLOW Water flow in soils...

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Unformatted text preview: 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 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. 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. Poiseuilles Law is as follows: Q = B R 4 ) P/8 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; = viscosity of water. 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 In 1856, H. Darcy noted the discharge rate (Q) through a sand bed was proportional to the bed cross-sectional area (A) times the hydraulic potential decrease and inversely proportional to the length of the sand bed: Darcy Q % A ) H/L Q % Ai or Q/A % i (i = gradient) Q/A is the volume of water flowing through a unit cross-sectional area per unit time, t. V/tA = Q/A = q (THE FLUX) q % i The proportionality factor is K , the hydraulic conductivity. q = Ki q = K( ) H/L) q = Ki q = K( ) H/L) The K is an important property of soil. It is not constant for all soils. Values of K depend on the size of void spaces, which in turn depends on the size, shape, and density of packing of the soil grains and soil structural complexity. Thus K is not readily obtainable. Estimatin g/measurin g, K ! Laboratory measurement: measured with constant or falling head permeameters....
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Lecture26-29(2008) - SOIL WATER FLOW Water flow in soils...

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