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Publication #SL39

Movement of Water: Basics of Soil-Water Relationships - Part III1

J. Bouma, R.B. Brown, and P.S.C. Rao2

INTRODUCTION

In Part I , the physical parameters required to describe the soil as a porous medium were examined. In Part II , the significance of these parameters in retention of water by soils was discussed. This fact sheet deals with the fundamental principles of water flow in saturated and unsaturated soils.

ENERGY GRADIENTS

Water in soil moves from points where it has a relatively high energy status to points where its energy status is lower. Two factors determine the energy status of water at any given point in a non-saline soil. The first factor is the elevational position in the soil relative to a reference level. The higher an object is located above the reference level, the higher is its gravitational energy. This is true also of any given quantity of water in soil: the higher the water is located in the soil profile, the higher is its gravitational energy (g). Gravitational energy is expressed as the number of centimeters (or inches) above or below an arbitrarily chosen reference level.

The second factor determining the energy status of water is the water's pressure head (h) , as discussed in Part II . Pressure heads are negative in unsaturated soil where not all the pores are water-filled. Those that are water- filled act in a manner similar to capillaries and exert suction on water. On the other hand, pressure head can be positive, as occurs in the saturated zone below the surface of the water in soil (the "water table"). There, all or nearly all (allowing for some trapped air) pores are filled with water. Pressure head is equal to zero (h =0) at the water table. It becomes progressively higher (more positive) with increasing distance below the water table. Pressure head becomes progressively lower (more negative) with increasing distance above the water table.

The hydraulic head (H) at a given point in the soil is equal to the sum of the gravitational and pressure heads, at that point, as illustrated schematically in Figure 1 for selected conditions.


Figure 1. Schematic diagrams indicating the determination of pressure head (h), graviational head (g), and total head (H = h|g) at different points in the soil. The reference level for determination of g is taken to coincide with the elevation of the lower tensiometer cup. In part A, because H is higher at the higher cup, downward flow of water will occur provided that the soil is permiable. In part B, no flow is indicated because H is the same at both ends.

WATER MOVEMENT

Direction and rate of water flow depend on differences in hydraulic head between different points in the soil. Not only is head difference important between two points, the distance between the points is also important. Expressed in terms of weather phenomena, we may observe a gentle breeze when high and low pressure

areas are far removed from each other. When the same high and low pressure areas are very close to each other, however, much stronger winds prevail. So too with water flow in soils. The hydraulic head difference (delta H)

between two points in the soil must be divided by the distance between those points (L) to obtain the "gradient" (delta H/L) of the hydraulic head between the two points. Direction of flow will be from a zone of higher H to a zone of lower H.

In order for water actually to move from one point to another, however, two conditions must be met. First, there must be a difference in hydraulic head between the two points (that is, delta H must be greater or less than zero). Second, the soil between these two points must be permeable enough to allow the movement of water. "Hydraulic conductivity" (K) is a measure of this ability of a soil to transmit water. The larger the K of a soil, the greater will be the movement of water through it for any given hydraulic gradient.

"Darcy's Law" for liquid movement in porous media states that the rate of water flow (q) through a given soil segment is equal to the hydraulic conductivity of that soil multiplied by the hydraulic gradient that exists in that soil. Darcy's Law is written mathematically as follows: q = -(delta H/L)K , where q = flux, or flow rate in centimeters per hour or day (or volume in cubic centimeters per square centimeter of soil area per hour or day); K = hydraulic conductivity in centimeters per hour or day; delta H = hydraulic head difference between two points in centimeters (which is composed of the gravitational and the pressure heads); and L = distance between the two points in centimeters.

A soil has a maximum K value when saturated (Ksat ) and another, lower, K value for each lower water content and corresponding negative pressure head. Such values are characteristically different for different soils, depending upon soil structure and pore-size distribution. To illustrate this, we use the diagram in Figure 2 , in which "soils" with different pore size distributions are schematically represented by sets of capillaries of varying diameter. The "sand" contains relatively large pores, whereas the pores in the "clay" are finer. At saturation, all pores are filled with water. Large pores conduct much more water than fine pores. When the pore radius is twice as large, for example, sixteen times as much water can be conducted. Correspondingly, in Figure 2 there are much longer arrows from the larger pores than from the smaller ones.


Figure 2. 

Even though the sand is more permeable than the clay at saturation, we see in Figure 2 that the reverse may be true when the soils are unsaturated. The large pores, which resulted in a high hydraulic conductivity for the sand at saturation, become filled with air as the soil becomes unsaturated. More water-filled fine pores remain in the clay. This is further illustrated in Figure 3 , where measured K-h curves from soils of different textures are shown. Unsaturation will occur when the


Figure 3. Four measured hydraulic conductivity curves showing characteristic decreases of hydraulic conductivity (k)upon desaturation of soils having different pore size distributions.

application rate of water is lower than saturated hydraulic conductivity, because only the smaller pores

will be filled with water. In fact, water will only flow into the larger pores when the finer pores cannot

conduct the applied water at the particular rate of application. Soil-water characteristic curves and hydraulic conductivity curves are needed to describe flow of water in soils. Flow rates of water in unsaturated soil cannot, unfortunately, be measured directly with a flow meter. The practical procedure to be followed involves the measurement of the pressure head at several points in the soil to determine the hydraulic gradient (delta H/L). A K-h curve is used to obtain the appropriate K value. The flow rate of the water (q) is found by multiplying K by (delta H/L).

The direction (upward, downward, or lateral) and magnitude of water flow in soils therefore depends upon the direction and magnitude of hydraulic head gradient and the degree of water saturation of the soil. Thus we see that there can be no flow of water (q) in soil without both a hydraulic gradient (delta H/L) and hydraulic conductivity (K). A soil having a very high hydraulic conductivity will experience little water movement if there is a very low hydraulic gradient. On the other hand, a high hydraulic gradient between two points in the soil will not cause water flow if K is essentially zero due to the occurrence of impermeable soil between the two points.

Footnotes

1. This document is SL-39, a fact sheet of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First printed: May 1982. Reviewed: March 1999, September 2003. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.

2. J. Bouma, former visiting professor, R.B. Brown, professor, and P.S.C. Rao, professor, Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.