A gas-phase partitioning tracer method for the in situ measurement of soil-water content

The purpose of this paper is to describe a gas-phase partitioning tracer method for the in situ measurement of soil-water content and to illustrate the application and performance of the tracer method in well-defined systems. The method is based on the use of a tracer test with nonpartitioning and partitioning tracers introduced into the system in the gas phase. Partitioning tracers dissolve into the water, which retards their gas-phase transport relative to that of the nonpartitioning tracers. Retardation of the partitioning tracers is a function of the amount of water present. The method provides an integrated field-scale value that complements smaller-scale methods, such as neutron moderation, and regional-scale methods based on remote sensing. Experiments were conducted in the laboratory and in a large weighing lysimeter to test the performance of the gas-phase partitioning tracer method. Soil-water contents estimated from the trader tests were reasonably close to values obtained using gravimetric and time domain reflectometry measurements, indicating the tracer method has the potential to provide accurate measurements of soil-water content at the field scale.