June 28, 2007 by Douglas R. Cobos and Colin S. Campbell
In many natural and engineered soils, the output of ECH2O soil moisture sensors is sensitive to variations in the soil temperature. The temperature sensitivity is not caused by the ECH2O sensors themselves which are almost perfectly insensitive to temperature changes, but rather the electrical characteristics of the soil, which can be quite sensitive to temperature changes. The ECH2O sensors measure volumetric water content (VWC) by measuring the dielectric permittivity (ε) of the bulk soil. ε in the soil is a complex quantity with both real (ε’) and imaginary (ε’’) components. ε’ is the real dielectric permittivity of the soil constituents, and has a negative correlation with temperature. ε’’ is related to dielectric losses, and more importantly electrical conduction through the soil. The ability of a soil to conduct electrical current or electrical conductivity (EC) is related to VWC and to the amount of free ions in the soil (solute content). The EC of a soil has a strong positive correlation to temperature. The opposing temperature sensitivities of the real and imaginary components of the dielectric permittivity can be thought of as opposing forces in the soil. In some soils, ε’ dominates, and an increase in temperature causes a decrease in the VWC measured by the ECH2O sensor. In other soils, ε’’ dominates, and an increase in temperature causes an increase in the VWC measured by the ECH2O sensor. In some soils, the two components closely balance each other, and there is no apparent temperature sensitivity in the VWC measurement. Because of these complex interactions, it is impossible to determine a generic correction factor for temperature that can be applied to all soils.
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