Environmental Engineering Reference
In-Depth Information
sensitive to soil disturbance and air gaps between electrodes and the soil matrix.
Therefore, probes need to be inserted carefully to minimize sensor error. Detailed
discussion on the capacitance method and practical procedures are found in Starr
and Paltineanu ( 2002 ).
If a large block of undisturbed soil representative of field conditions can be
removed intact, capacitance probes may be calibrated in the laboratory. Briefly,
the sample is placed in a sealed container, a probe is placed in the sample, and the
sample is brought to saturation. The total weight is measured and the output of the
probe is recorded. The sample is then drained and dried in several stages with each
stage given enough time to establish uniform water content in the container, and
total weight and probe output are recorded. At the end of drying, subsamples are
collected from the container to determine the dry bulk density of the soil, from
which the weight of the soil is converted to volumetric water content. Starr and
Paltineanu ( 2002 ) describe detailed procedures for laboratory calibration using
disturbed and repacked soil, which is suitable for agricultural mineral soils but
may not be applicable to organic soils. If it is not feasible to conduct laboratory
calibrations, soil samples can be collected from the probe depth at the time of
installation, and
θ v determined with the thermo-gravimetric method is then com-
pared to the initial probe data for a single-point calibration.
3.9.4 Specific Yield
Fluctuations of the water table represent changes in subsurface storage. Since it is
easy to measure the water-table elevation in monitoring wells, attempts have been
made to estimate changes in subsurface storage (
Δ
S sub ) from changes in water-table
elevation (
h WT ) using a concept called specific yield ( S y ), also known as drainable
porosity. When
Δ
Δ
S sub is expressed as depth of water, S y is the ratio of
Δ
S sub to
Δ
h WT , or more precisely, it is the volume of water released from or taken into
storage per unit cross sectional area following a unit change in water-table elevation
(Freeze and Cherry 1979 :61).
Despite being conceptually simple, S y is somewhat complex because soils can
retain a sizable and variable amount of water above the water table that is related to
the size of void spaces in the soil matrix. The relation between water content and the
magnitude of tension force holding water in pores is called the soil water charac-
teristic (SWC) curve. Under static conditions in the absence of vertical flow, the
magnitude of tension force is proportional to distance above the water table.
Therefore, SWC is commonly shown as a vertical profile of
θ v , which represents
the theoretical distribution of water content after complete gravitational drainage of
the vadose zone following complete saturation (Fig. 3.31 ). Suppose that a certain
amount of groundwater is extracted, causing the water table to drop. This extraction
induces drainage of water from the vadose zone until the new static condition is
reached (Fig. 3.31 ). The amount of extracted water should be equal to the difference
between the pre- and post-extraction profiles (Fig. 3.31 ). In other words,
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