Comparing Ambient Temperature Effects on Heat Pulse and Time Domain Reflectometry Soil Water Content Measurements

Abstract

Time domain reflectometry (TDR) and the heat pulse method are both used to measure soil water content. Changes in ambient temperature have been shown to affect TDR measurements, but less is known about the behavior of heat pulse sensors in response to changes in temperature. This study directly measured and compared the temperature sensitivity of the TDR and heat pulse methods. Both methods were used to estimate water content in silt loam and sand at two fixed water contents across a wide temperature range. An increase in temperature led to an increase in measured water content in most cases. Across the 40°C temperature range, changes in measured water content were generally 0.04 m3 m−3 or less for both methods. Weighted linear regression showed that in these soils the heat pulse method exhibited greater temperature sensitivity than the TDR method, although the differences were not statistically significant. A previously proposed correction for the temperature sensitivity of the TDR method produced mixed results. The temperature sensitivity of the heat pulse method was attributed to the changes in the density and specific heat of water and specific heat of soil with respect to temperature. When the changes in these parameters were accounted for, the temperature sensitivity was eliminated in three out of four cases.