Advances in Estimations of Vadose Zone Properties with the Heat Pulse Probe

Accurate measurements of water flow, nutrient/chemical and heat transport processes are essential for understanding hydrological mechanisms in the complex heterogeneous vadose zone. Although improvements in techniques have lead to a wide range of reliable in situ sensors, most sensors measure only one specific property. The heat pulse probe (HPP), however, provides for a single sensor that measures temperature, water content, nutrient/chemical concentration, and water and heat fluxes, in concept. With its small dimension, the HPP measures multiple parameters, minimizing the effects of sampling volume and soil heterogeneity, evaluating the different processes and properties at the same spatial and temporal scales.

In this work, we focus on water flow and heat transport, and propose alternative HPP designs, improving both probe robustness for field applications and measurement capabilities. Although the HPP has been considerably developed in the last decade, it still has significant limitations. First, the lowest possible measured water flux with the current HPP is approximately 5 cm/day, whereas vadose zone fluxes are generally below 1 cm/day. Second, the probe needles can be easily deflected during installation, thereby drastically influencing measurement accuracy. Using numerical and analytical solutions, coupling water and heat, for sensitivity analysis, we suggest specific changes that improve HPP performance. By changing (1) heater needle diameter; (2) heat pulse pattern and intensity; and (3) the needle spacing, water flux density is measured to near 1 cm/day. Additionally, a robust needle-less HPP design, with a ring heat source, shows accurate water content estimations.