Abstract:

The goal of this study was to study the applicability of the stable isotope mass balance method in the calculation of the water balance of Dali Lake in the Inner Mongolia Plateau. The water bodies of the Dali basin were collected three times in the summer of 2018, and surface water and ice samples were collected from the lakes during the winter. The stable hydrogen and oxygen isotopes (δD, δ¹⁸O) were tested in the different water samples. Then, the stable isotope mass balance method was used to calculate water evaporation and recharge from Dali Lake, and δD was used to analyze the source of water evaporation and recharge. The results showed that the stable isotopic spatial variability of hydrogen and oxygen in the Dali Lake water body was low, and the rate of enrichment of the water body continued to decrease in summer, which was mainly affected by the precipitation process. During winter, stable hydrogen and oxygen isotopes were enriched in the ice, which is mainly affected by the freezing process. The groundwater was hardly affected by the local atmospheric precipitation. The δ ¹⁸O-δD relation point of Dali Lake falls to the lower right of the local evaporation line, indicating that the lake region has the highest evaporation. Part of the δ¹⁸O-δD river-groundwater relationship point falls above the local atmospheric precipitation line, suggesting that the local atmospheric precipitation may not be the primary source of groundwater replenishment. The water of Dali Lake is uniformly mixed, with δD, δ¹⁸O, and the volume of lake water remaining relatively stable, which satisfies the applicable conditions of the stable isotope mass balance method. The stable isotope mass balance method was used to estimate the evaporation and recharge of groundwater from Dali Lake on an annual basis. The evaporation volume was about 269 million m³, and the annual groundwater recharge was about 165 million m³. The groundwater supply accounts for about 61% of the total supply of Dali Lake and is its main supply source. Using the relationship between δD and the residual water mass in the process of stable isotopic fractionation of the water mass, and taking into account the dynamic fraction and the initial isotopic composition of the water mass, the loss by evaporation of Dali Lake was calculated to be about 41%-46% of the initial water body. These results indicate that groundwater is an important source of recharge for river water.

Key words: stable isotope of hydrogen and oxygen, evaporation, replenishment, Dali Lake