Evaluation of the ecohydrological separation of different geomorphological units in a semi-arid area

The ecohydrological separation (ES) phenomenon which states that there are two incompletely mixing subsurface water pools, one of which is supplied to plants for transpiration while the other flows to groundwater and river water, has far-reaching implications for hydrological model simulation and water resources management. Specifically, it is unclear whether it occurs under different geomorphological units (hillside, foothill, and floodplain) within the same climate zone, as well as the reasons for such variations. We collected samples of precipitation, groundwater, river water, soil water, and plant stem water from April to October 2019 and analyzed their stable isotopic composition. Moreover, in order to further confirm whether the ES phenomenon exists in both hillside, foothill, and floodplain, we also calculated the line-conditioned excess and the isotope values of source water. The isotope values of plant stem water were similar to that of soil water rather than groundwater on the hillside. Groundwater on the foothill in April, June, July, and August had considerably different isotope values from plant stem water. The isotopic composition of plant stem water was comparable to that of groundwater in the floodplain, but there were differences in the isotope values of source water and lineconditioned excess. On the hillside, foothill, and floodplain, this study found evidence of the ES phenomenon. As a result of preferential flows through soils that cause soil-water heterogeneity, the ES phenomenon under various geomorphic units differs from one another. This research disproved theories about translational flow across the rooting zone and offered a new perspective on the ES phenomenon.