| Summary: | Tritium present in deep vadose zones is a useful tracer for estimating groundwater recharge, but its full utility is constrained by not knowing where and for how long the tritium tracing method remains applicable. We obtained 44 tritium profiles from 17 globally distributed sites with vadose zone thicknesses of 13−624 m and used transport models to estimate the number of years that tritium may still be useful. Results show that the method may still be usable for 26 of 44 soil profiles surveyed, mainly in China, Australia, USA, South Africa, and Senegal, with a remaining useful period of between 6 and 83 years. We also developed a statistical model that uses outputs from a hydrological model to predict the applicability of the tritium tracing method. Global implementation of the statistical model showed that the method remains usable at 20% of Earth's land mass (excluding Antarctica and Greenland) over the next few decades. Funding provided by: National Natural Science Foundation of China Crossref Funder Registry ID: https://ror.org/01h0zpd94 Award Number: 51179161 Funding provided by: National Natural Science Foundation of China Crossref Funder Registry ID: https://ror.org/01h0zpd94 Award Number: 2018JZ4001 The dataset – soil water tritium profiles (n = 40) – was compiled from the peer-reviewed literature (see Text S2 in manuscript). This was supplemented with 4 profiles from study sites in Changwu, Shaanxi Province, China. The dataset was compiled by co-authors Yanan Huang ( huangynbox@nwafu.edu.cn ) and Zhi Li ( lizhibox@nwafu.edu.cn ) of Northwest A&F University.
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