ABSTRACT Accurate determination of transient water flow density ( J w ) within the vadose zone is essential for elucidating hydrological processes in the Earth's Critical Zone (CZ), with key applications in irrigation scheduling, soil erosion mitigation, and sustainable land management. This study evaluates the thermo‐time domain reflectometry (thermo‐TDR) method for real‐time J w monitoring in unsaturated soils, addressing limitations of traditional heat pulse techniques and advancing instrumental development for Critical Zone Observatories (CZOs). A theoretical error analysis was conducted to quantify the sensitivity of J w estimates to inaccuracies in the input parameters of the heat‐pulse method. The results revealed high sensitivity at low flow rates (< 10 mm h −1 ), where probe‐spacing errors alone can produce relative errors up to 160%. An in situ probe spacing calibration method was developed to enhance accuracy without requiring complex sensor designs. A total of 33 soil column experiments across five sub‐tropical soils (one sandy soil, two silt loams and two silty clay loams) under varying unsaturated conditions were conducted to evaluate the accuracy of the improved thermo‐TDR method. The results demonstrated that thermo‐TDR measurements closely matched independent J w values in sandy soils but showed greater variability in finer‐textured soils, primarily due to time lags in heat and water transport and flow heterogeneity. Statistical analysis indicated reasonable correlations ( R 2 ≈ 0.62) between thermo‐TDR and independent J w values, with RMSE values of 10.6 and 6.24 mm h −1 for the upper sensor‐derived infiltration rates and lower sensor‐derived drainage rates, respectively. Despite challenges at low J w , the thermo‐TDR method shows promise for in situ J w monitoring, offering potential for broader application in CZOs to quantify anthropogenic impacts on water resources and enhance interdisciplinary CZ research.
Qin et al. (Wed,) studied this question.