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.
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