Experiments at scales up to a few meters were performed at the partly controllable ASSESS test site near Heidelberg, running until 2018. It consisted of a $20 \times 4 \times 2\, \mathrm{m^3}$ tank filled with partly non-parallel layers of sand and had a controllable water table. Its instrumentation comprised a few TDR- and temperature profiles, a simple weather station, and an automated multichannel GPR-scanner. Main activities on this site included:

  • developing high-precision GPR methods for the non-invasive monitoring of the soil water dynamics at the field-scale,
  • running hydraulic experiments like a changing water table or localized infiltrations to develop and demonstrate methods for estimating field-scale parameters and optimal representations of the entire system,
  • offering regular advanced practical courses for physics students.

A testbed for high-resolution electromagnetic geophysical methods

The ASSESS site is designed to develop hydrogeophysical methods with surface ground penetrating radar (GPR) as a primary instrument for vadose zone observation.

Design

The testbed consists of five uniform layers artificially constructed in a concrete box 20 m long, 4 m wide, and 2 m deep (Fig. 1). The five layers were built from three different sands. The geometry (Fig. 2) of the layers varies along the long axis and is constant along the lateral axis. The top of the box is open to normal atmospheric forcing, all the other boundaries are impermeable. The water level is controlled through a pumping well in one of the corners with rapid transmission throughout the box guaranteed by a gravel layer at the bottom. 32 TDR sensors grouped in 4 profiles are logged periodically to provide information on the distribution of the soil water and its dynamics. Precipitation, air temperature, wind speed, and net radiation are logged right at the box.

Fig. 1

  • Construction of the testbed.
  • Top-view of the completed testbed.
  • On top of the testbed, with the automatic weather station at the left wall and the pumping well next to it.

Fig. 2

  • Layer geometry of the testbed (20 m $\times$ 4 m $\times$ 2 m). The topmost and lowest interfaces indicate the surface and the upper edge of the gravel layer, respectively. Red markers show the positions of the TDR sensors.

In 2015 a fieldscanner was installed for automated GPR measurements.