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Generating discrete data points for retention
and conductivity functions
HYPROP measures the water tensions
suring levels of the sample at certain points of time
HYPROP-VIEW configuration. In the multi balance mode also the total mass
of the sample is weighed (in g) at the same time. In the single balance mode
the sample mass is weighed manually – usually twice a day. The software cal-
culates the medial water content of the sample based on the sample mass
minus all tare components (sensor unit, sample ring, dry mass of the soil). The
dry mass of the soil can be determined after the measurement by drying the
sample in an oven at 105 °C.
The measuring data are evaluated with the software HYPROP-FIT according
to Schindler's method (1980). The precise calculation of the water content
needs the input of the soil's dry mass. As long as the dry mass has not been
entered HYPROP estimates the water content upfront. HYPROP-FIT calcula-
tes based on linearization assumptions discrete points of the retention and
the conductivity curves. For this in a first step the raw data are interpolated
by Hermitian splines. This has the advantage that differing measuring times
of tensions and water contents can be adjusted and the number of time
points that are used for calculating data are fixed a priori. The default set in
HYPROP-FIT is a calculation at 100 time points that are taken from the splines.
At every calculation time point a medial water content
viding the mass of the soil water by the volume of the soil body. Each of these
points is assigned to a tension that is calculated from the measured and avera-
θ
ged tensions
h
and
1
curve
(h
). In order to calculate the conductivity function it is assumed that
i
i
the water flow through a horizontal plane that lies exactly in the middle of the
two tensio shafts (and thus in the symmetry plane of the column) between two
time points
t
and
i-1
cm³) over the mass change,
points and
A
the cross-sectional area (in cm²) of the column. The data points for
the hydraulic conductivity function are calculated by inverting Darcy's equati-
on:
K
(h
)= -q
/{(∆h
i
i
i
∆h
the difference of the two tensions at the two measuring levels, and
i
distance of the measuring levels (i. e. the height difference of the tensio shafts).
h
. Finally this procedure results in 100 points of the retention
2
t
is
q
=
/
(∆V
/∆t
1
i
i
2
i
i
∆t
the time interval between two calculation time
i
/∆z)-1}. With
h
being the time- and space-averaged tension,
i
i
80 | Theory
h
and
h
(in hPa or kPa) in two mea-
1
2
)/A. With
∆V
being the water reduction (in
i
t
being defined in the
θ
is calculated by di-
i
∆z
the
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