Heat pulse measurement techniques for soil water flux, soil water content, and soil volumetric heat capacity

Improving stewardship of soil and water resources often requires forming greater understanding of soil processes. To form such understanding, advances in techniques for measuring soil properties and processes are needed. The purpose of this research was to evaluate and improve heat pulse techniques for measuring soil water flux (J_w), soil water content (&thetas;), and soil volumetric heat capacity (C). A new mathematical analysis was developed revealing a simple linear relationship between J_w and the natural logarithm of the ratio of the temperature increases downstream and upstream from a line heat source. In laboratory experiments the standard deviation of repeated measures of J_w was 46% lower using the new analysis procedure versus a previous more cumbersome procedure. Linear relationships (r2 > 0.99) existed between J_w measured using the heat pulse technique and J_w measured at the outlet of the soil columns for J_w ranging from 0.1 to 40 cm h−1. The heat pulse sensors were found to be 25 to 75% less sensitive to J_w than predicted by the standard heat transfer model. A reduced convection model that accurately accounted for the measured data was proposed. The emerging dual-probe heatpulse (DPHP) technique for measuring &thetas; was evaluated under field conditions. Soil water content measured by the DPHP sensors (&thetas;_DPHP ) was on average 0.040 m3 m−3 larger than &thetas; measured by soil sampling (&thetas;_SS). Linear regressions of &thetas;_DPHP versus &thetas; _SS yielded r2 values >0.84 and slopes of 0.75. Errors in −_SS were a possible cause of these low slopes. A simple matching point procedure was introduced which reduced the average difference between &thetas;_DPHP and &thetas; _SS and reduced the average standard deviation of &thetas; _DPHP from 0.063 to 0.026 m3 m −3. The heat pulse technique for determining C was also evaluated under field conditions. Heat pulse sensors permitted C measurements with a frequency capable of fully describing the temporal variations in C. Heat pulse measurements of C and independent estimates agreed to within 8% on average. The results of this research indicate that heat pulse sensors are versatile tools for scientists to use in obtaining measurements of important soil properties and processes.