Deuterium‐excess determination of evaporation to inflow ratios of an alpine lake: Implications for water balance and modeling

Abstract

The numerous lakes on the Tibetan Plateau play an important role in the regional hydrological cycle and water resources, but systematic observations of the lake water balance are scarce on the Tibetan Plateau. Here, we present a detailed study on the water cycle of Cona Lake, at the headwaters of the Nujiang‐Salween River, based on 3 years (2011–2013) of observations of δ18O and δ2H, including samples from precipitation, lake water, and outlet surface water. Short‐term atmospheric water vapor was also sampled for isotope analyses. The δ2H–δ18O relationship in lake water (δ2H = 6.67δ18O − 20.37) differed from that of local precipitation (δ2H = 8.29δ18O + 12.50), and the deuterium excess (d‐excess) in the lake water (−7.5‰) was significantly lower than in local precipitation (10.7‰), indicating an evaporative isotope enrichment in lake water. The ratio of evaporation to inflow (E/I) of the lake water was calculated using both d‐excess and δ18O. The E/I ratios of Cona lake ranged from 0.24 to 0.27 during the 3 years. Observations of atmospheric water vapor isotopic composition (δA) improved the accuracy in E/I ratio estimate over a simple precipitation equilibrium model, though a correction factor method provided nearly identical estimates of E/I ratio. The work demonstrates the feasibility of d‐excess in the study of the water cycle for lakes in other regions of the world and provides recommendations on sampling strategies for accurate calculations of E/I ratio.

When to conduct an isotopic survey for lake water balance evaluation in highly seasonal climates

“…Figure presents the calculated E/I as a comparison with the estimated annual mean E/I values (dashed lines). Annual mean E/I calculated from both δ 18 O and δ 2 H in the 3 years were 0.23, 0.25, and 0.27, close to the results from deuterium excess‐based estimation (Cui et al, ). δ 18 O‐ and δ 2 H‐based E/I were very similar with a mean difference of 0.01.…”

“…A sampling location on the steep west bank was selected for ease of access and to avoid potential for incomplete mixing related to surface stream inlets. The isotopic composition of surface lake water in this study was found to be consistent with that of outlet water and central lake water, indicating that the west bank location was fairly representative of lake‐average isotopic composition under well‐mixed conditions (Cui et al, ). In total, there were 110 surface lake samples collected on the west bank from June 2011 to December 2013.…”

“…For hydrogen, we used the ε K -h relationship presented in Zuber (1983) to calculate ε K , which produced a better match between E/I from δ 18 O and δD than the ε K -h relationship in Gonfiantini (1986) in regions with low humidity. The ε K -h relationship of Zuber (1983) has been successfully used in previous studies (Biggs et al, 2015;Cui et al, 2017). Temperature, h, and δ A were weighted by monthly evaporation flux according to Gibson, Birks, and Edwards (2008).…”

“…Measured isotopic values were corrected for humidity dependence and instrument drift before calibrating to Vienna Standard Mean Ocean Water, following Steen-Larsen et al (2013) and Cui et al (2017). The precision of vapour measurements was estimated to be 0.2‰ for δ 18 O and 0.5‰ for δ 2 H in vapour isotope measurement.…”

“…Atmospheric water vapour isotopes were measured at Amdo station (Figure ) and Nagqu station (70 km in southeast) using a high‐frequency (1 Hz) Picarro L2130‐i analyser, together with a standard delivery module and a vaporizer module for both δ 18 O and δ 2 H. Measured isotopic values were corrected for humidity dependence and instrument drift before calibrating to Vienna Standard Mean Ocean Water, following Steen‐Larsen et al () and Cui et al (). The precision of vapour measurements was estimated to be 0.2‰ for δ 18 O and 0.5‰ for δ 2 H in vapour isotope measurement.…”