Thermo-TDR probes combine the technology of the TDR probe and heat pulse probe into one single probe. Thermo-TDR probes are capable of making in-situ measurements of the soil dielectric constant, bulk electrical conductivity, and soil thermal properties. Three stainless steel needles connected to a coaxial cable allow for the measurement of the dielectric constant and bulk electrical conductivity using time domain reflectometry. The dielectric constant is used with Topp’s equation to determine the soil volumetric water content. A resistance heater is embedded in the center needle and two thermocouples are embedded in the two outer needles. Soil thermal properties are determined by applying a heat pulse to the center needle and measuring the change in temperature at the two outer needles in response to the heat pulse from the center needle.
11 needle heat pulse probe is a sequence of heat pulse probes designed to measure subsurface evaporation in the field. Measurement of soil temperature and soil thermal properties at each needle allows us to determine indirectly the evaporation rate at each thin subsurface soil layer.