TEMPERATURE-DEPENDENT MEASUREMENT ERRORS IN TIME-DOMAIN REFLECTOMETRYDETERMINATIONS OF SOIL-WATER

With the recent development of improved time domain reflectometry (TDR ) probe design, measurement systems, and calibration procedures, it is now possible to detect and quantify the effect of temperature on the soil apparent dielectric constant (K-a). We investigated measurement e rrors in K-a associated with soil temperature variations and compared measured changes in K-a with those predicted by a dielectric mixing mo del. After confirming the accuracy and resolution of our measurement s ystem with a series of measurements on distilled water, we measured ch anges in K-a with temperature for a range of soil types, including san d, loam, and peat, at soil water contents (theta(nu)) ranging from 0.0 9 to 0.81 m(3) m(-3). The measured variation with temperature in the d ielectric constant of distilled water (0.322 degrees C-1) was very clo se to that reported in the literature (0.356 degrees C-1). In soils, c hanges in K-a with temperature were highest at high water contents. Fo r soils near saturation, the overall changes observed in K-a with temp erature were lower than those predicted by the dielectric mixing model by 17% for sand, 24% for loam, and 39% for peat. These results sugges t that the temperature dependence Of the dielectric constant of water in a soil matrix is lower than that of bulk water. Absolute water cont ent errors increased linearly with the size of the water fraction, ran ging from 8.75 x 10(-5) m(3) m(-3)degrees C-1 at 0.05 m(3) m(-3) soil water content to 1.40 x 10(-3) m(3) m(-3)degrees C-1 at 0.80 m(3) m(-3 ) soil water content. To obtain the highest measurement accuracy, part icularly at higher theta(nu), we suggest that a temperature correction of 0.001758(nu)degrees C-1 be employed.