1.2.1 Soil moisture measurements

Soil moisture measurements are used to determine ET through the water balance. The soil moisture measurements allow for the determination of ΔS, and with measurements of P and I, the water balance can be solved to yield ET.

Time-domain reflectometry (TDR) is a method to determine the soil moisture content and can be used to calculate change in soil water content as a surrogate for ET using the water balance. A TDR instrument consists of multiple probes (typically three) connected to a cable tester. The instrument works on the theory that changes in soil water content change the apparent permittivity of the soil as determined by the probes [10]. The soil moisture status can be calculated by the velocity of an electrical pulse through the probes.

A neutron soil moisture meter, or neutron probe, is an instrument that contains a radiation source that emits high energy neutrons. The high energy neutrons collide with water molecules in soil, and the reflected slower (lower energy)

neutrons are counted by the probe counter. The neutron count is related to the soil moisture by a calibration. The amount of lower energy neutrons that is reflected back to the sensor provides an accurate indication to the soil moisture status [12]. In addition, the sphere of influence of the neutron meter is correlated to the soil moisture content with lower moisture contents having larger contributing values.
