Lecture 11

rule.gif (1629 bytes)

Chapter 4

Soil Water Measurement


Environmental monitoring has become an integral component of land management. Soil water properties are among the commonly monitored environmental characteristics. Specific reasons for monitoring soil water properties include optimizing benefits from irrigation, and protecting groundwater. This lecture discusses methods for monitoring soil water.

  1. Water content. Water content can be measured in various ways. A few of the standard methods are briefly outlined below.

    Gravimetric. This involves collecting a sample, weighing it, drying it, and then reweighing it. With these weights one can calculate

    Porous resistance blocks. These can be calibrated to measure either content or potential. Their performance is only acceptable in relatively dry soil where the
    q - y relation is more or less linear (see Figure 4-9 in the textbook). These are easy to use once calibrated, but are not particularly accurate.

    Neutron probes. These provide high accuracy and non-destructive testing, by measuring water content surrounding an access tube installed in the soil. Because of the health risk and legal reporting requirements, their use is declining.

    Time domain reflectometry. This relatively new method requires expensive instrumentation, but provides good accuracy.

    Also, some minor methods in use include:
            frequency domain reflectometry
            capacitance probes

    Lysimeters are often used for "balance-sheet" studies in which one monitors water in and water out of a system.

  2. Water potential. Various instruments, a few of which are described below, can measure water potential.

    Pressure plate. This is a lab apparatus used to determine water content for a wide range of matric potentials with reasonable accuracy. Tests are slow and laborious.

    Tensiometer. This is an instrument for field use. These are commonly used in irrigation applications (see Figure 4-12 in the textbook). They only work on the wet end of the water release curve, and only measure matric potential.

    Psychrometer. These provide the most scientifically rigorous readings, yet provide rather poor precision; however, precision improves drastically as soil wetness decreases. Measuring total water potential by psychrometer is possible because of the following physical chemistry relation:

            Ytotal = RT ln RH

    where R, T, and V are the ideal gas law constant, temperature, and volume; and RH is relative humidity.

    As mentioned previously resistance blocks can be used to monitor water potential.

    Piezometer. These are access tubes inserted in the soil. The tubes typically have a porous cup on the lower end to allow water in. They are useful for measuring pressure potential due to a water table.

    Filter paper. Filter paper can be exposed to soil to measure the tendency for the paper to attract water from the soil. This is a low-tech method with accuracy similar to that of resistance blocks.

  3. Water quality parameters. Water quality depends on the specific use intended. For plant growth, the factors of greatest concern are:

            specific ions likely to be toxic to plants: sodium, chloride, and boron

    These factors are of great importance in evaluating water quality for irrigation. The needs and tolerances of the specific plant to be irrigated would set the quality standards.

  4. Hydraulic conductivity. Hydraulic conductivity is another name for the k value explained in the Lecture 10 discussion of Darcy's Law. This value is of great concern in monitoring the likelihood of run-off, the tendency of soil to become ponded or flooded, and the tendency of soil to protect groundwater from surface contamination. Many field methods of measuring hydraulic conductivity are in use. Two examples include measuring flow rates in concentric rings installed in the soil, and measuring the rate of flow into a shallow well dug into the soil surface. For simple evaluations of a soil for such purposes as a septic tank drain field, engineers often measure the rate of water level drop in an excavated cylindrical hole. This method is called the "Perc test". Laboratory tests are also used to obtain precise measurement of hydraulic conductivity, but normally give results that do not agree closely with field readings.

  5. Evaporation studies. Ecosystem studies and agricultural applications often require knowledge of evaporation or evapotranspiration rates. This information can be obtained using lysimeters, Class A Evaporation Pans, or by using any one of various mathematical models. The Penman model and the Bowen Ratio model are examples. One can also measure indicators of plant stress. This is done by measuring canopy temperature with an infrared heat sensor, or by measuring leaf water potential with a psychrometer.


Students are encouraged to look up the following vocabulary words in the textbook glossary or elsewhere and to browse the following web site.


Resistance Blocks
Hydraulic Conductivity

Web site

Many companies that manufacture or sell soil moisture instrumentation have useful web sites. Without implying endorsement of any particular company, a few web sites are listed here.

URL: www.soilmoisture.com

URL: www.esica.com/support/index.html

URL: www.decagon.com


left3.gif (1296 bytes) Return to Lectures