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.
- 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 qm.
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
Lysimeters are often used for "balance-sheet" studies in which one monitors
water in and water out of a system.
- 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
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
- 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
- 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.
- 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
Students are encouraged to look up the following vocabulary
words in the textbook glossary or elsewhere and to browse the following 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.