Ground-penetrating radar (GPR) has been used extensively in the borehole, multi-offset subsurface reflection and multi-offset ground wave configurations to measure soil water content (Davis and Annan, 2002; Huisman et al., 2003). These GPR methods determine soil water content from a measurement of the time required for an EM wave to travel a known distance within the subsurface. This case study discusses the GPR surface reflectivity method used for the determination of soil water content from measurements of the electromagnetic (EM) reflection coefficient at the air-ground interface, using a GPR elevated ~1 m above the surface.

The time domain reflectometry (TDR) method is, at present, one of the most widely used methods for measuring soil water content. Unfortunately, the TDR method cannot be used to cover large areas efficiently and provides only local point measurements covering a relatively small surface area (~0.005 m2). For many agricultural applications, the ability to efficiently map soil water content over large areas at a resolution of ~1 to 10 m2 would improve irrigation practices and reduce water requirements. The surface reflectivity method could provide this capability by providing rapid coverage of relatively large areas, because the GPR can be vehicle mounted and is not in contact with the ground. The method also provides better surface coverage by giving average water content measurements over typical surface areas of ~1 m2.

Previous field studies have demonstrated that the method has good potential (Chanzy et al., 1996; Redman et al., 2002; Serbin and Or, 2002). It has been shown that the water content measured with the GPR surface reflectivity method was more variable than and, in some cases, was substantially different from the TDR-determined water content (Redman et al., 2002). These effects were attributed to surface scattering, vertical stratification of the water content, and other spatial

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