Monitoring 3D soil moisture dynamics at a karst forest site with OhmPi, an open source resistivity meter

Karst aquifers are particularly vulnerable to changes in environmental factors such as climate change or pollutants. In the critical zone, the role of the superficial layer, the soil and the so-called epikarst, is crucial as it can delay water infiltration and host temporary perched water reservoirs, due to high contrasts in hydraulic conductivity with deeper layers. In an effort to better characterise the effect of the plant activity on the water content in the shallow subsurface, we have designed a time-lapse ERT experiment at the Rochefort Cave Observatory (Belgium). We present results from (at least) 6 months of daily 3D ERT measurements on 64 electrodes installed in a 40x60 cm grid covering a 6.0 x 1.8 m surface area centred on a young beech tree. The ERT dataset is supported by data from a vertical profile of soil moisture probes and in-cave water percolation gauges. Our study also includes an artificial drying and sprinkling experiment which main purposes are to replicate extreme weather events and investigate their effect on the soil moisture condition. This experiment also serves as a testbed for using OhmPi as a monitoring tool in the field. OhmPi is an open-source, open-hardware resistivity meter, which runs on a Raspberry Pi. It is designed for enabling flexible data acquisition, and is primarily dedicated to the research community. Relying on low-cost components and devices, and using a low-power injection module (0-50V), OhmPi is particularly suited for small-scale field and laboratory experiments.