Abstract
Mass-balance models using stable isotopes of hydrogen and oxygen provide useful estimates of the water balance of lakes, particularly in the absence of instrumental data. However, isotopic mass balances are rarely compared directly to measured water fluxes. Here we compared instrumental and isotope-based determinations of water fluxes in seven connected lakes over 12 years to quantify how agreement between the two approaches is affected by lake type and its position in the landscape. Overall, lake-specific ratios of evaporation to inflow (E/I) from instrumental measurements (median, x̃ = 0.06, median absolute deviation, MAD = 0.06) agreed well with isotopic estimates using headwater models (x̃ = 0.14, MAD = 0.08), with the exception of one lake with limited channelized inflow of surface waters (x̃instrumental = 0.51 vs. x̃headwater = 0.24). Isotope-instrument agreement improved (x̃ = 0.09 vs. x̃ = 0.03) when basin-specific (‘best-fit’) isotope models also considered local connectivity to upstream water bodies. Comparison among years revealed that mean isotopic E/I values were lowest in 2011 (mean, μ = 0.06, standard deviation, σ = 0.09) during a 1-in-140 year spring flood, and highest during a relatively arid year, 2003 (μ = 0.22, σ = 0.19), while interannual variability in E/I generally increased with distance downstream along the mainstem of the watershed. Similar patterns of agreement between methods were recorded for water-residence time. Isotope models also documented the expected low water yield from lake catchments (μ = 36.2 mm yr−1, σ = 62.3) suggesting that isotope models based on late-summer samples integrate annual inputs from various sources that are difficult to measure with conventional methods. Overall, the strong positive agreement between methods confirms that water isotopes can provide substantial insights into landscape patterns of lake hydrology, even in ungauged systems.
Highlights
Quantification of hydrological processes that regulate the water balance of lakes is essential to both evaluate ecosystem vulnerability to climate change and sustain the health of surface water in the face anthropogenic development (Barnett et al, 2005)
In Last Mountain lake, isotopic values were positioned between the local meteoric water line (LMWL) and the theoretical local theoretical evaporative line (LEL), suggesting a system more influenced by evaporation (Fig. 3)
Lakes in this study spanned a wide range of morphology, hydrological settings and human influence, and can be used as a model drainage system to evaluate the use of isotope mass balances to estimate the hydrology of open lakes
Summary
Quantification of hydrological processes that regulate the water balance of lakes is essential to both evaluate ecosystem vulnerability to climate change and sustain the health of surface water in the face anthropogenic development (Barnett et al, 2005). Analysis of stable isotopes of hydrogen (δ2H) and oxygen (δ18O) has been employed as a reliable means to quantify general hydrological properties of diverse water bodies using limited field data, often with only a single water sample (Ala-aho et al, 2018; Brooks et al, 2014; MacKinnon et al, 2016; Mayr et al, 2007; Pham et al, 2009; Wolfe et al, 2007; Wu et al, 2017; Yu et al, 2002) This isotopic approach has been used to estimate fluxes regulating the water balance of lakes, including evaporation (E) to inflow (I) ratios (E/I) (MacDonald et al, 2017; Narancic et al, 2017; Turner et al, 2014), water residence time (Gibson et al, 2002; Petermann et al, 2018), and water yield (Bennett et al, 2008; Gibson et al, 2010, 2017). Relatively little is known of how the performance of isotope-based approaches may vary on multiannual timescales in comparison to other methods (Gibson et al, 1996; Gibson and Reid, 2014; Longinelli et al, 2008; Tyler et al, 2007)
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