Deep soil water ¹⁸O and ²H measurements preserve long term evaporation rates on China's Loess Plateau

Abstract. Knowledge about the long-term average soil evaporation, especially the ratio of evaporation to precipitation (f), is important for assessing the total available water resources. However, determining the long-term f remains technically challenging because soil evaporation is highly dynamic. Here we hypothesize that the stable isotopes (²H and ¹⁸O) of deep soil water preserve the long-term evaporation effects on precipitation and can be used to estimate long-term f. Our results showed that the deep soil water (2–10 m) had a mean line-conditioned excess (lc-excess) less than zero (−13.1 ‰ to −3.8 ‰) at the 15 sites across China's Loess Plateau, suggesting that evaporation effects are preserved in the isotopic compositions of the deep soil water. We then estimated f by the new lc-excess method that combines lc-excess and the Rayleigh fractionation theory, because it does not require the initial source isotopic values of soil water, which has a distinct advantage over traditional isotope-based methods (e.g. Craig-Gordon model) that require such information a priori. The estimated f of the 15 sites varied from 11 % to 30 %, and over 60 % of the variability of f was explained by the well-known Budyko dryness index. These data are also comparable with available annual estimates under similar climate regions of the world. Furthermore, these data represent a long-term average value because soil water tritium profile shows that deep soil water has a long residence time on the order of years to decades. Our work suggests that isotopic compositions of deep soil water can be used to calculate long-term average f where water flow within the unsaturated zone is piston-like flow predominantly, and the new lc-excess method provides an effective tool to estimate f.