A new soil matric potential sensor based on time domain reflectometry

We developed and tested a new sensor based on time domain reflectometry (TD R) to measure soil matric potential (h). The TDR-matric (TM) sensor is cons tructed of porous disks having different known maximum pore sizes and stack ed within a coaxial cage. The constant relationship between water content ( theta) and h of the TM porous matrix is initially calibrated and subsequent ly used to infer matric potential of the surrounding soil, similar to exist ing porous heat dissipation and electrical resistance sensors. The theta of the sensor's porous matrix in hydraulic equilibrium with the surrounding s oil is measured by TDR travel time analysis. Prototype sensors were constru cted using porous ceramic and plastic disks having maximum pore diameters b etween 120 mu m (2.5 kPa) and about 0.6 mu m (0.5 MPa). Calibration tests t o evaluate sensor theta-h relationships were completed in a pressure chambe r apparatus using four soils. These results and those from sensors installe d in soil lysimeters in the presence of growing plants showed consistent th eta-h relationships and synchronized responses of soils and TM sensors to c hanging water status. Pairing standard TDR probes with the new TM sensors f acilitates in situ determination of soil theta(h) relationships, using conv entional TDR instrumentation. The sensor design accommodates construction o f 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. Add itionally, a mismatch between the pore size distributions of the TM sensor and the soil (mostly relevant to coarse-textured soils) can lead to hydraul ic decoupling of these and other porous sensors.