Monitoring soil hydrologic processes above shallow aquifers using spatial TDR and FDR

The spatial distribution of soil water content within the entire range of the unsaturated zone is imperative for several hydrologic, agricultural and geotechnical applications. Monitoring of soil water content within the unsaturated zone is conventionally conducted with TDR and FDR sensors at discrete points within the soil matrix. Although both TDR and FDR sensors provide reliable measurements of soil water content for a representative volume around the placement area, their installation at various depths is laborious and time-consuming work, causing significant disturbance to the soil matrix modifying its soil structure. Furthermore, commercial profile sensors measuring soil water content at discrete points, are typically limited to lengths of 1 m., preventing the monitoring of critical hydrologic processes below this depth. To address this limitation, custom-length waveguides made from flexible insulated flat copper wires have been developed. Unlike non-insulated rod waveguides, these custom waveguides maintain signal integrity over lengths exceeding 1 meter. They enable soil water content monitoring along their entire length by applying inverse modeling techniques to reflected signals reconstructed through profile reconstruction algorithms. The installation of custom waveguides in the field is achieved through portable drilling equipment that ensures minimal soil disturbance.

This study evaluates the performance of TDR and NanoVNA devices, connected to custom waveguides, within the extent of an experimental field. The use of low-cost NanoVNA devices offers a portable and affordable alternative to traditional TDR systems, which are often cost-prohibitive even for research purposes. NanoVNA devices function as virtual TDR instruments, making them suitable for field installations. The development of supportive Python code for waveform acquiring and inverse modeling automation, facilitates high-resolution, in-field monitoring of soil water content, providing valuable insights into soil hydrological processes throughout the length of the transmission lines. Soil hydrologic processes above shallow aquifers are dominated by capillary flow phenomena, emerging from capillary rise over the groundwater table. Spatial TDR and virtual TDR monitoring, from NanoVNA devices, with custom waveguides results to high time and spatial resolution of capillary action above shallow aquifers.

Acknowledgments

This research is part of the Project “e-Pyrros: Development of an integrated monitoring network for hydro-environmental parameters within the hydro-systems of Louros-Arachthos-Amvrakikos for the optimal management and improvement of agricultural production” (MIS 5047059) and received financial funding from the Operational Program “Competitiveness, Entrepreneurship and Innovation 2014–2020 (EPAnEK)”.