High-Precision Calculation of the Proportions of Water with δ2H and δ18O, the Cumulative Effect of Evaporation in the Vertical Direction and Depleted δ2H and δ18O of the Shallow Soil Water Caused by Evaporation

Abstract

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 (δ2H) and oxygen isotope composition (δ18O) 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 δ2H–δ18O 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 δ2H–δ18O relationship lines become lower from top to bottom. In this study, the final effect of evaporation on the δ2H and δ18O values of shallow soil water is depleting the δ2H and δ18O values of shallow soil water, which is different from previous studies. The δ2H and δ18O values of groundwater varied little with changes of seasons and rainfalls. The δ2H–δ18O relationship lines established by various substances can also reflect the regulation of d-excess by large lakes through secondary sources.