A new soil metric potential sensor based on time domain reflectometry

We developed and tested a new sensor based on time domain reflectometry (TDR) to measure soil matric potential ( h ). The TDR‐matric (TM) sensor is constructed of porous disks having different known maximum pore sizes and stacked within a coaxial cage. The constant relationship between water content ( θ ) and h of the TM porous matrix is initially calibrated and subsequently used to infer matric potential of the surrounding soil, similar to existing porous heat dissipation and electrical resistance sensors. The θ of the sensor's porous matrix in hydraulic equilibrium with the surrounding soil is measured by TDR travel time analysis. Prototype sensors were constructed using porous ceramic and plastic disks having maximum pore diameters between 120 μm(2.5 kPa) and about 0.6 μm (0.5 MPa). Calibration tests to evaluate sensor θ‐h relationships were completed in a pressure chamber apparatus using four soils. These results and those from sensors installed in soil lysimeters in the presence of growing plants showed consistent θ‐h relationships and synchronized responses of soils and TM sensors to changing water status. Pairing standard TDR probes with the new TM sensors facilitates in situ determination of soil θ(h ) relationships, using conventional TDR instrumentation. The sensor design accommodates construction of media‐ or application‐specific sensors using combinations of disks having different pore sizes. There is a trade‐off between the TM sensor's matric potential range and its sensitivity to changes in the surrounding soil. Additionally, a mismatch between the pore size distributions of the TM sensor and the soil (mostly relevant to coarse‐textured soils) can lead to hydraulic decoupling of these and other porous sensors.