download pdf - Institut für Umweltphysik - Ruprecht-Karls-Universität ...

3.1. SOIL PHYSICS 99
3.1.3 Physical processes in the capillary fringe
Moritz Mie
Abstract In order to understand the impact of different factors on size and shape of the capillary
fringe, we use high-resolution measurements of the water saturation obtained from imaging techniques.
The most important part of this work is to explain the nonlinear behavior of the capillary fringe by
changing the water table. Moreover we try to understand the connection between this non-linear
behavior and the hysteresis of the soil-water characteristic.
motor control Hele−Shaw cell
digital camera
Figure 3.3: Sketch of the experimental setup: A transparent sand-filled Hele-Shaw cell is placed
between a diffusive light source and a digital camera. The water table in the cell is controlled by a
motor connected to a computer
Background In the last century most of the
research efforts by soil scientists and hydrologists
have focused on the vadose zone and on the
groundwater region, respectively. Mostly the impact
of the transition zone, the capillary fringe,
has been ignored. The thickness of this zone depends
on the properties of the respective soil, particularly
on the pore space and on the surface
roughness of the sand grains. With this experiment
we investigate the influence of a periodically
changing water table on size and shape of the capillary
fringe.
Methods and results To measure the water
content that is needed to determine the height of
the capillary fringe we use a Hele-Shaw cell that
consists of two parallel 50 × 30 cm 2 glass plates
with a 3 mm gap filled with homogeneous sand.
At the bottom, the cell is connected to a movable
water reservoir. We use the Light Transmission
Method (LTM) that is based on the fact that
the intensity of transmitted light can be used as a
proxy of water content. The calibration of LTM
with respect to water saturation is done by simultaneous
measurements of X-ray and light transmission.
A digital camera takes interval photos
during the whole experiment. By subtracting the
dry sand image from all images we get a normalized
image time series which represents the water
content in our sand column. In the first experimental
series we measure the dynamics of water
saturation for different water table. We chose different
amplitudes and frequencies for the water table
change. By analysing the first experiments we
found that in the experiments with high frequencies
(period of one hour) the system never reaches
equilibrium. In experiments with lower frequencies
(period of 24 hours) the state of equilibrium
during the first imbibition process is reached after
some hours. Subsequent cycles show a much
faster equilibration.
Outlook Interpreting the results of this experiment
is one part of the future work. This encompasses
the study of limiting cycles as well as the
nonlinear damping characteristics of the capillary
fringe. Moreover we are going to do some similar
experiments with changing parameters. For instance
we will use different velocities of the water
table change by slow or fast drainage and imbibition.
Another parameter to change is the surface
roughness of the sand grains. Different kinds of
sand might have different impact on the shape of
the capillary fringe.