Temperature Effects on Soil Dielectric PropertiesMeasured at 50 MHz

759 Soil water content (θ, m3 m−3) is a basic hydrologic parameter that affects groundwater recharge, surface water fl ow, transpiration, and carbon fi xation. Electronic monitoring of θ facilitates the study of those processes by providing detailed temporal data. Most instruments currently in use measure soil dielectric permittivity, directly or indirectly, and then calculate θ with an equation relating permittivity to θ (e.g., Topp et al., 1980). Unfortunately, the relationship between permittivity and θ is not unique but is affected by the measurement frequency, soil properties (e.g., electrical conductivity, clay mineralogy, clay content), and temperature (Or and Wraith, 1999; Jones and Friedman, 2000; Friedman and Robinson, 2002). We focus on temperature because, although temperature effects on time domain refl ectometry (TDR) have received some attention (e.g., Persson and Berndtsson, 1998; Or and Wraith, 1999; Evett et al., 2005; Logsdon, 2005b), temperature effects on other commercial instruments are relatively unknown even though these instruments are widely used and evidence suggests that they may experience dramatic temperature effects (Baumhardt et al., 2000; Seyfried and Murdock, 2001; Blonquist et al., 2005). Issues related to the use of soil dielectric properties to determine θ result, at least in part, from to the fact that permittivity (ε*) is a complex number. Thus,