Temperature dependence and correction
The two temperature dependent sources of error in CS650 water content measurements are the effect of temperature on the operation of the sensor electronics and the effect of temperature on the dielectric permittivity of the soil.
The effect of temperature on sensor electronics is minimal with period average readings varying by less than 0.5% of the 20 °C reading over the range of 10 to 30 °C and less than 2% of the 20 °C reading over the range of –10 to 70 °C.
The larger error is caused by the change in dielectric permittivity of soil with temperature. This is mostly due to the high temperature dependence of the permittivity of water, which varies from a value of 88 at 0 °C to 64 at 70 °C. Since water is the major contributor to bulk dielectric permittivity of soil, temperature related changes to the permittivity of water lead to overestimation of volumetric water content at temperatures below 20 °C and underestimation of volumetric water content at temperatures above 20 °C.
The Topp equation does not account for soil temperature. The effect of temperature on the soil permittivity is related to soil specific properties such as porosity and the permittivity of the soil solid phase with temperature. Consequently, a general equation that corrects volumetric water content for temperature for all soils is not available.
A temperature correction equation that works well in quartz sand is given by:
θCorr = θ – 0.0044•Tθ3 + 0.0014•Tθ2 + 0.0029•Tθ – 0.0002•T + 2.4•θ3 – 1.6•θ2 + 0.32•θ – 0.046
where θCorr is the temperature corrected volumetric water content, T is soil temperature in °C, and θ is the volumetric water content value at soil temperature T.
The thermistor used for measuring soil temperature is located in the sensor head and is in contact with one of the stainless steel rods. To make an accurate soil temperature measurement, the sensor head should be buried in the soil so that it is insulated from diurnal temperature fluctuations.