Measuring Soil Water Content

Major advances in the measurement of soil water content have arisen from electromagnetic (EM) methods that have developed rapidly in the last 20 years. Estimates of water content from EM measurements make use of the large relative permittivity of water compared to other soil components. Time domain reflectometry (TDR) and capacitance approaches use “probes” that convey signal into the soil and thus can measure principally the upper one-meter depth. Ground penetrating radar (GPR) using noninvasive, transmitting, and receiving antennae possesses the capability to measure to even greater depths without causing soil disturbance. Remote radar and passive microwave methods, operating generally above 1 GHz, derive their information from within a few centimeters of the ground surface. Thermogravimetric and neutron moderation continue as viable long-standing methods but are being used less as these methods are not amenable to data-logging. The variety of instruments has increased the surface and near-surface soil water measurement capabilities. Now it is possible for hydrologists to make informed choices among methods, and it is important to do so to optimize their study results.