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3.1. SOIL PHYSICS 95
Main methods
Multi-Step Outflow Measurement Hydraulic material properties are estimated from experiments
where the boundary condition is changed in discrete steps, the resulting outflow is measured,
and the whole setup is inverted numerically.
Hele-Shaw Cell A quasi-twodimensional porous medium is constructed by filling the 3. . . 6 mm gap
between two glass plates with sand. Transmitted light is a good proxy for volumetric water
content and dye tracers facilitated the measurement of flow fields. Such Hele-Shaw cells allow
monitoring of flow and transport processes with very high spatial and temporal resolutions.
Ground-Penetrating Radar (GPR) Propagation of electromagnetic energy through porous media
depends strongly on their dielectric properties which in turn, at frequencies between some
50 MHz and 2 GHz, depends strongly on liquid water content. This in turn depends on soil
texture which often changes discontinuously in natural porous media, thereby leading to strong
reflectors. We have demonstrated that multi-channel GPR-measurements yield both the depth
of the reflectors and the average water content between them. In the meantime, we use this
method operationally in simple settings and work on improving it for more complicated ones.
Electrical Resistivity Tomography (ERT) The electrical potential at the soil surface that results
from injecting a very-low frequency, mHz to Hz, electrical current into the subsurface reflects the
spatial distribution of the electrical conductivity. Combining the results from a large number
of injection points leads to an estimate of that distribution which in turn is determined by soil
texture, soil water content, and solute concentration.
Soil-Weather Monitoring Station Automatic logging of profiles of soil temperature and liquid
water content together with atmospheric variables (temperature, precipitation, wind velocity,
humidity, net radiation or even its components, pressure, snow height) allow the microclimatic
characterization of a particular site. We operate such a station at the Grenzhof test site and
several of them on the Tibetan plateau.
Grenzhof Test Site This site near Heidelberg is our testbed for developing and demonstrating measuring
and monitoring techniques as well as for exploring the emerging field of hydrogeophysical
approaches where hydraulic experiments and and geophysical observations are inverted jointly
in order to arrive at more robust estimates of large-scale material properties.
Main activities
1. study flow instabilities and the dynamics of the capillary fringe in Hele-Shaw cells
2. improve multi-step outflow methods, in particular the evaporation methods we developed
3. develop and implement improved analysis tools for multi-channel GPR
4. monitor and analyze thermal and hydraulic dynamics at three permafrost sites on the Tibetan
plateau
5. explore synthetic aperture radar (SAR) satellite images from estimating soil surface water contents
Funding
1. DFG RO 1080/8 “Vorhersage einfacher Transportphnomene in Bden auf der Feldskala”
2. DFG RO 1080/9 “Hochauflsende experimentelle Untersuchung von Fluss und Transport in
porsen Medien”
3. DFG RO 1080/10 “Dynamics of Active Layer on Qinghai-Xizang Plateau”