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Measuring Soil Water Content with the Ground Penetrating Radar Surface Reflectivity Method: Effects of Spatial Variability

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Paper number  032276,  2003 ASAE Annual Meeting . (doi: 10.13031/2013.13823) @2003
Authors:   David Redman, Lakshman Galagedara, Gary Parkin
Keywords:   GPR, ground penetrating radar, soil moisture, reflectivity, water content

A ground penetrating radar (GPR) system elevated ~1 m above the surface can be used to determine the near surface (<0.5 m) water content by measuring the surface reflectivity at the air/ground interface. A vehicle mounted GPR surface reflectivity system could efficiently map water content over large areas compared to the limited coverage possible with time domain reflectometry (TDR), currently the most widely used and accepted technique in agricultural and hydrological applications.

Field studies performed at local test sites have shown that the GPR surface reflectivity method was able to map the water content distribution but there were substantial differences observed between these measurements and those acquired with TDR. These discrepancies are attributed to the different sampling volumes for the two methods and to inaccuracies in the surface reflectivity method, resulting from surface scattering and spatial variability in water content.

Numerical modeling studies have been performed to investigate the effects of a horizontally stratified water content distribution on water content estimates obtained using the surface reflectivity method. Modeling of a two-layer case has demonstrated that the measured water content could, depending on the thickness of the upper layer, be overestimated for a wet layer over dry layer and underestimated for a dry layer over a wet layer. The field and modeling results have provided insight into the limitations and potential problems that will need to be addressed when using the GPR surface reflectivity method for estimating soil water content.

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