Lecture 10

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Chapter 4

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.

  1. 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 = 0.06
                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/D
                        = 3"/(0.28 - 0.06)
                        = 3"/0.22
                        = 13.6"
    So a 3" rain wets 13.6" of soil

  2. 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 is:
                Rapid became it is through large pores
                Driven by gravity and sometimes pressure

    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 D
                At          L

    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 conditions.

  3. 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 flow towards:
                drier medium
                small pores
                finer texture

  4. 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 ~ 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.

  5. 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.


Saturated Flow
Unsaturated Flow

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


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