Time Domain Reflectometry Field Calibration in the Little Washita River Experimental Watershed

Accurate measurement of profile soil water content is essential to many areas of environmental and agricultural research. In this study five methods were evaluated for determining volumetric profile soil water content from time domain reflectometry (TDR) data at nine locations within the Little Washita River Experimental Watershed (LWREW) in south central Oklahoma. Soil compositions for the sites ranged between 24.6 to 86.4% for sand and 5.2 to 29.2% for clay. Comparisons were made between gravimetric soil sample data and soil water content as determined by the TDR factory instrument calibration, two methods of site-specific calibration, a regional calibration technique, and an empirically derived universal approach. Method 1 is the factory calibration, which uses average values for model coefficients that were derived from extensive laboratory work and theoretical analysis. Method 2 fits a site-specific linear regression of TDR time delay on measured soil-core water content. Method 3 uses the factory calibration equation and site-specific values for the ratio of TDR time delay in dry soil, to that in air (Tₛ/Tₐ). 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 (Kₐ) measured by TDR. Statistical analysis of the data showed that of the five methods, only the mean root mean square error (RMSE) value for Method 2 was significantly different from all other methods (α = 0.05). Method 2 also had the smallest standard deviation and standard error, and the narrowest range of RMSE values. All field calibration methods show that it is necessary to include very low water content data in determining absolute water content. When compared with 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.