Time domain reflectometers (TDRs) are in-situ monitoring probes that record soil moisture content when calibrated to a particular soil. Typically TDRs are used in agricultural settings, but this technology may also be applied to urban soils. The Urban Watershed Management Branch located at the Edison Environmental Center (EEC) has been exploring the use of TDRs as a way to monitor stormwater infiltration practices. TDRs are installed in pilot- and full-scale bioretention units (e.g., rain gardens). The TDRs are capable of measuring soil moisture and sensing the wetting front as the stormwater infiltrates through the planting media and into the native soil.

Recently, a new permeable pavement parking lot was constructed at the EEC. This parking lot was designed for long-term monitoring and included the installation of TDRs, both in the crushed concrete storage layer and in the underlying soil. Early indications suggest that while this novel application of TDRs in crushed concrete may not provide calibrated moisture content, wetting fronts correspond to those of the TDRs in soils. Extensive bench-scale testing was performed in crushed concrete prior to the permanent installation below the paved surfaces as a proof of concept test.

Application of this technology may be suitable to urban areas that are interested in modifying tree pit design, controlling stormwater through rain gardens and porous pavements, or using manufactured or engineered soils, and seek or require supporting data that water is infiltrating and available to plants.