The question demands two things simultaneously: the method must work in situ (in place, without removing a sample) and must yield volumetric moisture content (θᵥ, volume of water per unit bulk volume).
(A) Neutron probe ✔ — fast neutrons emitted into the soil are thermalised by hydrogen atoms; the count of slow neutrons is calibrated directly against volumetric water content, measured in place through an access tube.
(C) Time domain reflectometry (TDR) ✔ — measures the soil’s apparent dielectric constant from the travel time of an electromagnetic pulse along buried waveguides; the dielectric constant is converted to volumetric water content (Topp’s equation), again in place.
(B) Tensiometer ✘ — measures soil-water potential (matric suction) in kPa or bars, not water content. Converting to content requires a separately determined soil-moisture characteristic curve.
(D) Piezometer ✘ — measures groundwater pressure head / water table elevation, a saturated-zone instrument entirely.
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