Quantifying groundwater dependence of a sub-polar lake cluster in Finland using an isotope mass balance approach

One‐time or short‐term lake water isotopic surveys are often employed to evaluate regional lake water balance. However, it can be difficult to determine the optimal time‐window for sampling to obtain a representative long‐term perspective of lake water balance in settings influenced by seasonal variations in precipitation, evaporative loss, glacial/snow meltwater, and larger seasonal shifts in isotopic composition of precipitation. This is especially true for areas of the Tibetan Plateau that are influenced by the summer Indian monsoon. Although high‐frequency sampling is always preferred as the most rigorous approach to characterize the water budget of lakes or watersheds, this may be impractical in remote regions and over large spatial scales. To assess the potential sensitivity of isotope balance characterization to seasonal variability, we used a weekly lake water isotope data set acquired over a period of 3 years on the Tibetan Plateau to evaluate the potential inaccuracies that might have arisen from using isotopic data collected during narrower time‐windows. For this assessment, we use weekly isotopic data collected during the study and assume that these sampling events were stand‐alone one‐time surveys. We then demonstrate the sensitivity of the isotope balance method in this setting, particularly for the rainy season that significantly underestimated the evaporation/inflow. In contrast, isotopic composition of the lake water was found to be more representative of long‐term conditions when sampled in October on the Tibetan Plateau. To broaden our evaluation of seasonality effects over a range of climatic zones, published high‐frequency isotopic data were also compiled, and a similar assessment was carried out for selected regions of the world. The synthesized data and model outputs, which confirm pronounced variations in lake water isotopic composition and evaporation/inflow across a range of seasonal climates, were used to determine optimal sampling windows for these specific regions.

Section: Discussionmentioning

confidence: 99%

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

Cui

Tian

Gibson

2018

“…In recent years, several studies have used this tracer to quantify groundwater inflow to lakes but again assume a well‐mixed water column (Arnoux et al . , ; Isokangas, Rozanski, Rossi, Ronkanen, & Kløve, ; Sacks et al . , ).…”

Section: Introductionmentioning

confidence: 99%

Interactions between groundwater and seasonally ice‐covered lakes: Using water stable isotopes and radon‐222 multilayer mass balance models

Arnoux

Gibert-Brunet

Barbecot

et al. 2017

Interactions between lakes and groundwater are of increasing concern for freshwater environmental management but are often poorly characterized. Groundwater inflow to lakes, even at low rates, has proven to be a key in both lake nutrient balances and in determining lake vulnerability to pollution. Although difficult to measure using standard hydrometric methods, significant insight into groundwater–lake interactions has been acquired by studies applying geochemical tracers. However, the use of simple steady‐state, well‐mixed models, and the lack of characterization of lake spatiotemporal variability remain important sources of uncertainty, preventing the characterization of the entire lake hydrological cycle, particularly during ice‐covered periods. In this study, a small groundwater‐connected lake was monitored to determine the annual dynamics of the natural tracers, water stable isotopes and radon‐222, through the implementation of a comprehensive sampling strategy. A multilayer mass balance model was found outperform a well‐mixed, one‐layer model in terms of quantifying groundwater fluxes and their temporal evolution, as well as characterizing vertical differences. Water stable isotopes and radon‐222 were found to provide complementary information on the lake water budget. Radon‐222 has a short response time, and highlights rapid and transient increases in groundwater inflow, but requires a thorough characterization of groundwater radon‐222 activity. Water stable isotopes follow the hydrological cycle of the lake closely and highlight periods when the lake budget is dominated by evaporation versus groundwater inflow, but continuous monitoring of local meteorological parameters is required. Careful compilation of tracer evolution throughout the water column and over the entire year is also very informative. The developed models, which are suitable for detailed, site‐specific studies, allow the quantification of groundwater inflow and internal dynamics during both ice‐free and ice‐covered periods, providing an improved tool for understanding the annual water cycle of lakes.

“…The ratio of evaporation to inflow ( E / I ) is a key parameter to indicate the water balance for open water bodies and has been applied widely to study lake evaporation in different geographical and climatic scenarios (Biggs et al, ; Gibson, Birks, & Yi, ; Kebede, Travi, & Rozanski, ; Mayr et al, ). In low‐altitude regions, E / I ratio can vary due to the influence of groundwater (Isokangas, Rozanski, Rossi, Ronkanen, & Kløve, ), seasonal flood (Brock, Yi, Clogg‐Wright, Edwards, & Wolfe, ), wetland (Gibson, Prepas, & McEachern, ), or chains of lakes (Gibson & Reid, ). Lakes at high altitudes show distinct patterns of evaporation for unique climatic and hydrological regimes.…”

Section: Introductionmentioning

confidence: 99%

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

Cui

Tian

Biggs

et al. 2016

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.