Measuring Soil Moisture in Nottingham County Park
In
this week’s field exercise we will be measuring the water availability in the
soils in Nottingham County Park. As we
have seen previously, the diverse environmental systems in the Park are
associated with very different atmospheric conditions (temperatures,
atmospheric humidity, radiation balance) and very different surface conditions
such as the albedo. Always keep in mind
that the environmental systems in the Park are a result of the synergistic
interactions between the underlying geology, the soils, the vegetation and the
atmospheric conditions.
The
amount of water available in the soil is of great importance to both the
natural and cultivated vegetation of a given area. Plants draw soil water into their systems through huge networks
of tiny rootlets. After the water is
drawn in by the plant, it is carried through the trunk and branches into the
leaves, where it is discharged into the atmosphere as water vapor. This process is called plant
transpiration. When large amounts of
water are available in the soil the rate of transpiration is very high and
plants generally thrive. As the soil
water becomes depleted, transpiration slows down as plants use various mechanisms
to reduce the rate that water is used.
This allows a plant to survive longer over an extended dry period. Agricultural droughts such as the dust bowl
of the 1930s in the Great Plains and the extreme agricultural drought of 1999
in the Mid-Atlantic region underscore the importance of moisture availability
to the vegetated landscape.
Measuring Soil Moisture:
As
with many environmentally important variables, soil moisture is not
straightforward to measure. There are
many different methods available to estimate the water available in a
soil. These include the “feel method”,
the gravemetric method, electrical resistance blocks, tensiometers,
and water content reflectometers. We will be using the two most sophisticated methodologies
tensiometers and water content reflectometers in our field exercise.
Tensiometers work on
the principal of directly measuring the “soil suction”, the force that
determines the direction of moisture flow in a soil and the force that plants
must overcome to gain needed water.
When soils are dry, the soil suction is large, while wet soils have very
small soil suction. A tensiometer
consists of a water-filled probe with a ceramic tip that allows the flow of
water into and out of the probe. If the
probe is inserted into dry soils, water moves out of the tip into the
surrounding soils creating a vacuum in the probe that is measured by a vacuum
gauge. The larger the vacuum recorded
by the probe, the drier the soils.
A water content
reflectometer (WCR) measures the volumetric water content of a soil. Two parallel rods are inserted into the soil
to the depth that water content information is desired. An instrument connected to the rods sends
out an electromagnetic pulse (wave) along the rods. The rate at which the electromagnetic pulse is conducted into the
soil and reflected back from it is directly related to the average water
content of the soil.
The Field Exercise:
We will be
measuring the soil moisture at 10 stations throughout Nottingham County Park
with both a tensiometer and a water content reflectometer. These stations are located in areas of the
park with distinctly different environmental systems including forested areas,
savannah areas (associated with the serpentine barren) and human modified
areas. We will “map” these observations
to ascertain whether there is a significant relationship between the
environmental system and the soil moisture.