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Theoretical basics
9
Theoretical basics
9.1 Evaporation method (overview)
In a soil sampling ring two Tensiometers, comparable to the T5
model, are installed in two depths (z
the sensing tips of the Tensiometers is the centre of the soil sample.
The sample is saturated, closed on the bottom and placed on a
scale. The upper side of the sample is open to atmosphere so the
soil moisture can evaporate. With the soil water tension [kPa] the
average matric potential and the hydraulic gradient is calculated. The
mass difference, measured by the scale, is used to calculate the
volumetric water content and the water's flow rate.
A measuring campaign will last until one of the Tensiometers runs
dry or the mass changes become marginal. Then, the remaining
moisture content is determined by oven drying the sample at 105°C
for 24 hours. With these values the retention curve and the
unsaturated conductivity is extrapolated.
9.2 Discrete data for retention and conductivity
relation
At different points of time
both depths are measured as well as the weight of the sample (in
3
grams cm
). The analytic procedure is based on the assumption
that water tension and water content distribute linear through the
column, and that water tension and sample weight changes are
linear between two evaluation points.
The initial water content is determined from the total loss of water (i.
e. evaporation + water loss by oven drying).
The average water content
loss of weight, and the medial water tension
i
i
of the retention function at any time
(
h
)
For the calculation of the conductivity function it is assumed that
between two time points
and z
1
i
the water tensions
t
i
, derived from initial water content and
1
i
i
and
the water flow through the cross
t
t
71/92
). The middle between
2
i
i
and
(in hPa) of
h
h
1
2
i
give a discrete value
h
i
.
t
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