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Temperature Effects on Soil Dielectric Properties Measured at 50 MHz

^a USDA-ARS-NWRC, 800 Park Blvd., Boise, ID 93712
^b 3310 Bren School of Environmental Science and Management, Univ. of California, Santa Barbara, CA 93106-5131. Mention of manufactures is for the convenience of the reader only and implies no endorsement on the part of the authors or the USDA

View larger version (10K): [in this window] [in a new window]   FIG. 1. Instrument temperature effect, defined here as the apparent change in soil water content () due to Hydra Probe electrical components when subjected to a 40°C temperature change, plotted as a function of the actual water content. For example, if soil at 0.30 m3 m–3 experiences a temperature change of 5 to 45°C, the instrument effect will cause an apparent water content rise of about 0.007 to 0.307 m3 m–3.

View larger version (17K): [in this window] [in a new window]   FIG. 2. Oven-dry (OD) soil corrected for instrument effects. The instrument correction equation (Eq. [7]) was fitted to the air data. After correcting for instrument effects to 25°C, r', the real component of relative complex permittivity, is essentially independent of temperature and slightly greater than 2.5.

View larger version (20K): [in this window] [in a new window]   FIG. 3. Effect of temperature on r' (real component of relative complex permittivity) for eight representative soils (Tif5 = Little River, 5 cm; Ames =Ames, 5 cm; Breaks = Breaks, 30 cm; Wat50 = Watkinsonville, 50 cm; LW = Little Washita, 50 cm; WG5 = Walnut Gulch, 5 cm; On50 = Onward, 50 cm; and FR50 = Fort Reno, 50 cm; see Table 1). Each point is the average of data from three individual sensors. Twenty measurements were made at each temperature. Lines represent the linear temperature effect for each soil.

View larger version (21K): [in this window] [in a new window]   FIG. 4. Effect of temperature on r'' (imaginary component of relative complex permittivity) for eight representative soils (Tif5 = Little River, 5 cm; Ames =Ames, 5 cm; Breaks = Breaks, 30 cm; Wat50 = Watkinsonville, 50 cm; LW = Little Washita, 50 cm; WG5 = Walnut Gulch, 5 cm; On50 = Onward, 50 cm; and FR50 = Fort Reno, 50 cm; see Table 1). Each temperature–r'' combination is represented by 12 to 15 measurements.

View larger version (18K): [in this window] [in a new window]   FIG. 5. Comparison of the measured r' (real component of relative complex permittivity) and r'' (imaginary component of relative complex permittivity) temperature (T) response with apparent permittivity, a, calculated using Eq. [3]. The Ames soil is representative of high r'' soils, and the Tif5 soil is representative of low r'' soils. Where r'' is low, there is little impact on a, and sensor type is not critical.

View larger version (10K): [in this window] [in a new window]   FIG. 6. The temperature (T) response of d (dielectric conductivity) as a function of r'' (imaginary component of relative complex permittivity) measured at 25° (r25''). The soils with relatively high r'' respond as if all losses were from . Even at lower values, the difference between the dd/dT slope and 0.019 is small.

View larger version (13K): [in this window] [in a new window]   FIG. 7. The effect of temperature (T) on estimated soil water content (), quantified by d/dT, as a function of r25'' (imaginary component of relative complex permittivity measured at 25°C). Although there is considerable scatter about this relationship, especially for relatively low values of r25'', it is clear that positive temperature effects are associated with high r25'' values.