Soil Water Movement
More water falls on land than originates from land.
Yet, the land is not becoming increasingly wet. Therefore, to keep the land in a more or
less steady state with respect to water, water must flow off the land. Water flows
downward to the sea as liquid water, and upward to the atmosphere as water vapor. The
properties of water flow have an enormous impact on our planet.
- A simple downward flow model.
Assume that water flows when soil is wetter than FC.
Remember, depth of soil x qv = depth of water
or, ds qv = dw
and, ds = dw/qv
One can introduce change: Dds qv = Ddw or, Dds = dw/Dqv
The soil is uniformly
dry with qv
The soil receives
3" of rain
FC for the soil is 0.28
How deep will the rain
wet the soil?
A wetting front will advance through the soil such that qv is .28 behind the front
Dds = dw/Dqv
3"/(0.28 - 0.06)
So a 3" rain wets 13.6" of soil
- Saturated flow.
Saturated flow is assumed in the above simple model. In saturated flow the pores are
full of water and because at least some of the water is a long distance from solid
surfaces, matric potential is considered to be negligible. Under these conditions, flow
Rapid became it is
through large pores
Driven by gravity and
Downward flow into soil from the top is called infiltration. Infiltration
slows as the soil wets (see Figure 4-13 in the textbook). Infiltration is usually
limited by subsoil properties. Typical rates may be about:
1-2 inches/hr for sand
0.5 for loam
0.2 for clay
Water flow through the wetted soil is called percolation. Transporting
soluble material in percolating water is leaching. Leaching is important in
environmental issues because potentially harmful nitrates and organic pesticides can leach
into an aquifer used for drinking water.
Saturated flow is described by Darcy's law
Q = k DY
Q represents quantity of water, such as cubic centimeters. The A represents area, such as
square centimeters. The t represents t, such as seconds. The L term represents length or
distance such as centimeters. If the potential is also expressed in length units, k has
the units of cm/sec. Soils have different k values for use in the above equation. The
greater the k value, the better the soil is at conducting water under saturated
- Unsaturated flow. Unsaturated flow occurs along soil
surfaces, not through large pores. Unsaturated flow is driven by matric forces that are
much stronger than gravity. Gravity is not sufficiently strong to exert a significant
influence on unsaturated flow because when unsaturated, much of the soil water is affixed
to solid surfaces. Unsaturated flow is slow. Even though the driving force is usually
greater than for saturated flow, the resistance to flow is enormous. Water will flow
toward a lower (more negative) potential regardless of direction. In other words it will
- Evaporation. Most of the solar energy reaching the
earth is spent evaporating water. For convenience, evaporation & transpiration are
often lumped together and called evapotranspiration (ET). As soil dries evapotranspiration
slows (see Figure 4-18 in the textbook). ET + water retained in plants is called consumptive
use. Consumptive use varies greatly by time of the season. It may be as much as ~ ½
inch/day at the annual peak, such as on a warm day when the soil is moist and plants are
large and active. Plants differ in water use efficiency (WUE). WUE is the water required
per unit of material produced.
- Why does water flow through a plant? Water flows
through a plant because Y drops all along the flow pathway. In other words, a water potential gradient
exists, whereby water can obtain a lower potential by flowing from one location to
another. Curiously, this often means flowing upwards, as through the stem of a plant. The
ultimate driving force is vapor pressure deficit. When the atmosphere has a
relative humidity (RH) less than 100%, the atmosphere has a negative water potential, and
can attract water existing at a higher potential. The magnitude of the negative water
potential in the atmosphere can be very large compared to soil values. For example, at
~50% RH, the Ytotal
in the atmosphere is -1000 bars. This is much lower than the soil water potential which
might be on the order of -5 bars.
Students are encouraged to look up the following vocabulary
words in the textbook glossary or elsewhere and to browse the following web site.
Soda Bottle Hydrology is instructional material
provided by the Department of Energy containing useful, practical demonstrations. URL: www.em.doe.gov/soda/index.html