Chapter 10
WATER MOVEMENT IN SOIL
Darcy's Law and Hydraulic Conductivity
Water movement in soils is governed by physical laws. As
described in Chapter 9, water movement follows a course of
decreasing energy. In this exercise we will examine water
movement through soil.
Infiltration is the movement of water into soil
from the surface. Infiltration rate is the rate at which water
will enter a soil under specified conditions. Infiltration rate
is usually expressed as centimeters per hour. Water infiltration
is largely governed by the surface properties of the soil. A soil
high in organic matter, having good structure, and of medium to
coarse texture will usually have a rapid infiltration rate. Many
other factors, such as roughness of the ground surface,
vegetative cover, and slope, will also affect infiltration. Water
movement within soil also affects infiltration. Water already in
the soil profile must move downward before more water can enter
at the surface.
The ease with which water moves within a soil is termed
permeability or hydraulic conductivity. Permeable soils conduct
water readily through their mass. Other soils may conduct water
slowly or have restricting layers or horizons which limit or
prevent downward movement of water. Soils or soil layers which do
not conduct water, at all, are termed impermeable. Permeability,
like infiltration, is largely determined by texture, structure,
and organic matter content. Other factors, such as the dominant
ion on the exchange complex, presence of chemical cementing
agents, and type of clay minerals present, also influence
permeability.
Water movement through porous media (including soil) is governed
by Darcy's Law. Below is the expression of
Darcy's Law for saturated conditions.
Q = K ath Where: Q = water flow (or volume of output)
l (cm3)
K = hydraulic conductivity (cm/sec)
a = cross-sectional area of the soil
(cm2)
t = time (s)
h = water height (or 'head') (cm)
l = length of soil column (cm)
The driving force for water movement under saturated
conditions is the height of water in the soil column. Darcy's Law
can also be expressed for unsaturated conditions, but the driving
force for water flow is then a potential gradient. The equation
for saturated conditions shows that water flow decreases with the
length of the soil column, but increases with the cross-sectional
area of the soil, height of water (head pressure), and time. The
proportionality constant (K) in the equation is called the hydraulic
conductivity or soil permeability. Hydraulic
conductivity is a measure of the soils' ability to conduct water.
This factor is dependent on the parameters which affect water
flow through soil, such as texture and structure. Water flow
through a soil increases as the hydraulic conductivity increases.
The equation can be expressed in terms of hydraulic conductivity,
K = Q1/(ath). With Darcy's Law expressed in this form and using
the apparatus shown in Figure 10.1, the permeability of the soil
can be determined. The cross-sectional area (a) and the length of
the soil column (1), the height of the water column (h), and the
time (t) can all be predetermined. The amount of water passing
through the soil column (Q), in the predetermined time, is simply
measured using a graduated cylinder and the hydraulic
conductivity (K) calculated.
Figure 10.1. Permeameter used to determine hydraulic conductivity (K) of saturated soils in the laboratory.
Water Movement in Soil - A Film
The film demonstrates water movement in soils under a variety of
conditions. How water flows, why it flows, and the differences
between saturated and unsaturated water flow is illustrated. The
affects of soil texture, structure, and porosity on water
movement are also demonstrated. Carefully watch and take note of
the following demonstrations as seen in the film.
1. Capillary action
2. Ceramic blocks
3. Glass plates
4. Flow, from furrow to soil
a. gravity vs capillary action
b. influence of sand layer
c. influence of clay layer
5. Flow, through sand vs soil aggregates
6. Flow in channels
a. exposed channel, saturated flow
b. buried channel, unsaturated flow
7. Flow, through moist vs dry sand
8. Sandy loam vs loam vs clay loam
a. rate of water entry
b. water retention
c. erosion control
9. Direction of water movement
a. movement of soluble salt
10. Water movement in potato hills
a. coarse vs fine aggregates
11. Soil management practices
a. buried straw layer
b. straw mixed throughout soil
c. buried root channels
d. vertical mulching
Laboratory Exercise
A. Darcy's Law and Hydraulic conductivity
1. Take measurement of cross-sectional area (a), height of water
column (h), and length of soil column (l) on each of the soil
permeameters.
2. Time (t) the collection of a measurable quantity of water (Q) from each permeameter. The time of collection will vary depending upon the hydraulic conductivity of each soil.
3. Calculate hydraulic conductivity (K) for each soil column.
4. Compare the results among the different soil textures and between the duplicates for each soil texture.
B. Water Movement in Soil - A Film
1. Carefully watch the film and make notes of the demonstrations as they are shown.
2. Review the list of demonstrations shown above.
3. Read the question list and incorporate the answers into the discussion section of the laboratory report.
Question List
1. How is infiltration dependent on permeability?
2. What are the factors which make hydraulic conductivity unique for each soil?
3. What is the driving force for water movement in a saturated soil? an unsaturated soil?
4. What forces account for the retention of soil water?
5. What forces account for the downward movement of soil water?
6. a) In the demonstration in which fine-textured soil overlaid a layer of sand, why didn't the water immediately flow into the sand when the wetting front reach the sand?
b) What were the wetness conditions in the overlying fine-textured soil when the water did enter the sand?
7. a) In the demonstration in which fine-textured soil overlaid a layer of clay, did the clay layer wet up as fast as did the sand layer after the wetting front reached it? Explain.
b) Explain the difference in the rate of water transmission through the sand and the clay layer.
8. In the demonstration with two sand channels, one surrounded by fine-textured soil and one exposed to the water furrow, describe, in each case, where saturated flow and unsaturated flow occurred.
9. Under what conditions will water flow into and through open, but buried, pores such as worm holes and old root channels?
10. What important limitation on the effectiveness of the practice of deep chiselling to increase water intake rate is suggested by the demonstration of saturated flow through sand and straw channels?
11. Under what conditions will water flow into drainage tiles placed in the soil?
DATA SHEET
Chapter 10
A. Darcy's Law and Hydraulic Conductivity
Soil (a) (h) (l) (t) (Q) (K)
--------- cm2 ------ cm ------ cm ------ s ------
cm3 -----
Sand 1
Sand 2
Loam 1
Loam 2
Clay 1
Clay 2
B. Water Movement in Soil - A Film
Notes: