chapter  2
70 Pages

Physical Laws of Water Flow in Soils

II. Gas-Phase Configuration in Unsaturated Soils.......... 30 A. Free and Entrapped Air in Soils .......................... 30 B. Forced-Closed System........................................... 33 C. Water Flow in Open Systems ............................... 34 D. Significance of Gas-Phase Configurations

in Fields................................................................. 38 III. Infiltration .................................................................... 38

A. Physical Properties of Infiltration........................ 38 1. Infiltration into Dry Soils .............................. 38 2. Infiltration into Wet Soils .............................. 40 3. Factors Affecting Infiltration......................... 43

B. Infiltration Capacity ............................................. 45 C. Infiltration Rate in Fields..................................... 46 D. Mathematical Formulation of Infiltration........... 49

1. Empirical Equations ...................................... 49 2. Green-Ampt Equation ................................... 50 3. Philip’s Method............................................... 54 4. Inverse Analysis of Infiltration by

Using Philip’s Solution................................... 58 IV. Steady Water Flow in Soils........................................... 59

A. Steady Flows in Fields.......................................... 59 B. Steady Upward Flow ............................................ 61 C. Steady Downward Flow........................................ 65

V. Transient Water Flow in Soils ...................................... 66 A. Evaporation........................................................... 66

1. Fundamental Pattern of Evaporation ........... 66 2. Transient Water Flow in the

Constant-Rate Stage ...................................... 69 3. Transient Water Flow in the

Falling-Rate Stage.......................................... 72 B. Redistribution ....................................................... 73 C. Water Flow in Deep Soils ..................................... 75

1. Gravity-Predominant Flow (GPF) ................. 75 2. Wetting and Drainage in Deep Soils ............. 77 3. Moisture Profile Velocity and Mean

Pore Water Velocity in Deep Soils ................. 81 4. Limitation of the Gravity-Predominant

Flow Concept .................................................. 84 Appendix: Method of Characteristics ................................... 84 References.............................................................................. 85

I. DARCY’S LAW

A. Darcy’s Equation for Water Flux

Liquid water in a soil tends to move from a location where the potential energy is high to a location where the potential energy is low. The potential energy of water in a soil is termed total potential and is composed mainly of matric potential, gravitational potential, and osmotic potential. The former two potentials contribute to the driving forces of water in all soils. The osmotic potential contributes as a driving force, especially in clayey soils. Since almost all natural soils contain

a clay component, the osmotic potential has a greater or lesser influence on the flow of water in soils. In moderately moistened natural soils, it is assumed that the contribution of the osmotic potential is negligible and that gravitational and matric potentials predominate in the flow of water. Flow affected by the osmotic potential is discussed in Chapter 6.