Towards an isotope-based conceptual catchment model of the ecohydrological cycle in the Critical Zone on the Loess Plateau of China

A mechanistic understanding of the interacting processes governing the ecohydrological cycle is of paramount importance for comprehending the soil–plant-atmosphere continuum (SPAC) in the Critical Zone. The analysis of these processes may necessarily consider the different water types that characterize ecohydrological flux exchanges at the catchment scale but, so far, few studies have disentangled functional interactions among various water types within the Critical Zone. This study leveraged three years of isotope data (δ¹⁸O and δ²H) collected from twelve water sources, including precipitation, throughfall, snow, stream water, groundwater, dew water, frost water, mobile and less-mobile soil water, root water, stem water, and leaf water in two catchments with distinct land cover (forestland versus grassland) on the Chinese Loess Plateau (CLP). We infer the main ecohydrological processes controlling water exchange in the Critical Zone under the contrasting vegetation covers. Our results showed new interactions among the several investigated water types, and in particular highlighted that: i) The seasonal isotopic variation in precipitation played a critical role in the seasonal isotopic patterns observed in other water types; ii) Dew water significantly contributed to leaf water uptake, more in forestland (26 ± 6 %) than in grassland (16 ± 11 %). Snow and groundwater were more influential for root water of shrubs and grasses in forestland (59 ± 34 % and 16 ± 8 % for snow and groundwater, respectively) than in grassland (36 ± 26 % and 6 ± 6 %) and they were very important for stem water of trees in forestland (84 ± 14 % and 45 ± 22 % for snow and groundwater, respectively); iii) Isotopic values in mobile and less-mobile soil water differed significantly between forestland and grassland (p < 0.05), but those in plant water (root, stem, and leaf water) did not differ significantly (p > 0.05); and iv) There were dynamic exchanges between mobile and less-mobile soil water, and between groundwater and soil water on the CLP. All these observations allowed us to establish a new isotope-based conceptual model of the ecohydrological cycle in the Critical Zone of the CLP that provides the foundation for future research and sustainable water resource management in this region.