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Title: TDR FIELD CALIBRATION IN THE LITTLE WASHITA RIVER EXPERIMENTAL WATERSHED

Author
item Heathman, Gary
item Starks, Patrick
item Brown, Michael

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/24/2002
Publication Date: 1/1/2003
Citation: HEATHMAN, G.C., STARKS, P.J., BROWN, M.A. TDR FIELD CALIBRATION IN THE LITTLE WASHITA RIVER EXPERIMENTAL WATERSHED. SOIL SCIENCE SOCIETY OF AMERICA JOURNAL. 2003. v. 67. no. 1.

Interpretive Summary: Accurate measurement of soil water content in the root zone is essential to many areas of environmental and agricultural research. It is a key component to many practical considerations regarding agricultural and water resource management systems. Time domain reflectometry (TDR) is a measurement technique that makes use of the electrical prperties of the water molecule to estimate water content of the soil. In this study five methods were evaluated for determining volumetric profile soil water content from domain TDR data at nine locations within the Little Washita River Experimental Watershed (LWREW) in south central Oklahoma. Comparisons were made between soil water content as determined by the TDR factory instrument calibration, two methods of site-specific calibration, a regional calibration, and a universal equation. Values of soil water content determined by each method were compared to the gravimetric soil- core water content data taken at the time of calibration, as well as to an independent collection of soil-core samples. All field calibration methods indicate that it is necessary to include very low water content data in determining absolute water content. When compared to the factory calibration, all other methods improved the measurement of soil water content, with a linear regression site-specific method providing the most accurate results. This study is significant in terms of demonstrating how site-specific calibration improves the accuracy of TDR measurements of volumetric profile soil water content.

Technical Abstract: Accurate measurement of profile soil water content is a key component to many practical considerations regarding agricultural and water resource management systems. Time domain reflectometry (TDR) is a measurement technique that makes use of the electrical properties of the water molecule to estimate water content of the soil. In this study five methods were evaluated for determining volumetric profile soil water content from TDR data at nine locations within the Little Washita River Experimental Watershed (LWREW) in south central Oklahoma. Values of soil water content determined by each method were compared to the gravimetric soil-core water content data taken at the time of calibration, as well as to an independent collection of soil-core samples. Method 1 is the factory calibration which uses average values for calibration coefficients. Method 2 fits a site- specific linear regression of TDR data on measured soil-core water content. .Method 3 uses the factory calibration equation and site-specific values fo the ratio of TDR time delay in dry soil, to that in air. In Method 4 a regional linear regression equation was developed from an analysis combining data from all study sites. Method 5 applies a universal equation based on the linear relationship between soil water content and the apparent dielectric constant of soil measured by TDR. All field calibration methods show that it is necessary to include very low water content data in determining absolute water content. When compared to the factory calibration, all other methods improved the measurement of soil water content, with Method 2 providing the most accurate results at the site- specific level of analysis. Method 5 also performed better than the factory calibration, and may be a better alternative for general applications.