Modular subsurface borehole-tower for vadose zone monitoring

Motivation: Most urgent contemporary societal challenges ranging from climate change to ecosystem services and food security are strongly linked to processes taking place in the vadose zone. Long term and highly resolved monitoring of key variables such as soil moisture, pressure, temperature, gas fluxes, and more are essential to the growing interest in this critical zone.

In the absence of off-the-shelf sensor systems capable of vertically resolved acquisition of these variables, we developed a prototype of a modular borehole-based tower for simultaneous monitoring of water content, temperature, oxygen and CO2 gas concentrations, and potentially other variables (relative humidity, capillary pressure).

Design: The modular tower is based on a 1.5 m sections of 75 mm diameter PVC tubing with TDR waveguides mounted on outer walls. Each paired waveguides (0.15 m in length) were installed on two opposing sides of inflatable sections along the modular unit to ensure contact with the borehole walls.

Innovations:

• Only a small amount of roots will be cut while drilling the borehole in heavily vegetative sites
• Long term monitoring of the vadose zone with a replicate of the temperature and the water content measurement for each depth. Two opposing sides are equipped with sensors
• Area of perturbation is very smallcompared to standard sensor setups
• Deep monitoring, making use of themodularity of the tower. Four modules can be connected.