High-Precision Calculation of the Proportions of Water with delta H-2 and delta O-18, the Cumulative Effect of Evaporation in the Vertical Direction and Depleted delta H-2 and delta O-18 of the Shallow Soil Water Caused by Evaporation

Exploring the water sources taken up by plants is necessary for ecological protection. The purpose of this study was to determine the exact proportions of different water sources absorbed by herbaceous plant species in the wetland of Poyang Lake in an inland humid region. This identified the water sources patterns in wetlands and provide Poyang Lake managers information about the lake water level needed to sustain vegetative life. We analysed the deuterium isotope composition (delta H-2) and oxygen isotope composition (delta O-18) values in the stem water of dominant herbaceous plant during its different growth stages to explore the proportions of water sources in different growth stages by using the Phillips equation, and the results supported the accuracy. The results indicate that the groundwater should not be lower than 0.13 m, otherwise the Carex cinerascens may not be able to absorb it. In previous studies, the lower slopes and intercepts of delta H-2-delta O-18 were attributed to the secondary evaporation under the cloud, but we found that there is a cumulative evaporation effect in rainwater, soil water, and groundwater, which makes the slopes and the intercepts of delta H-2-delta O-1(8) relationship lines become lower from top to bottom. In this study, the final effect of evaporation on the delta H-2 and delta O-18 values of shallow soil water is depleting the delta H-2 and delta O-18 values of shallow soil water, which is different from previous studies. The delta H-2 and delta O-18 values of groundwater varied little with changes of seasons and rainfalls. The delta H-2-delta O-18 relationship lines established by various substances can also reflect the regulation of d-excess by large lakes through secondary sources.