Using water stable isotopes to assess evaporation and water residence time of lakes in EPA's National Lakes Assessment

Stable isotopes of water (δ¹⁸O and δ²H) can be very useful in large-scale monitoring programs because water samples are easy to collect and water isotopes integrate information about basic hydrological processes such as evaporation as a percentage of inflow (E/I), water residence time and water yield from the watershed. Traditional techniques to acquire such information require significant time and effort beyond what is possible in a one-day assessment. Thus, measurements of these important hydrological processes are often excluded from spatially extensive monitoring programs such as EPA's National Aquatic Resource Surveys. We used the stable isotopes of water (δ²H and δ¹⁸O) analyzed from water collected from 1000+ lakes across the country in EPA's 2007 National Lakes Assessment to derive information these hydrological variables. Water isotopes for precipitation inputs were estimated using the spatially explicit models found at Waterisotopes.org. Lake water isotopes ranged from 5 to -20 ‰ for δ¹⁸O and 20 to -135 ‰ for δ²H with d-excess values (an indicator of evaporation) ranging from 13 to -43 ‰. Lakes were more enriched than the local precipitation with dual-isotope slope median of 5.1, indicating evaporation as the cause for the enrichment over precipitation. Using isotope evaporation theory, we estimated E/I, water residence time, water yield and the runoff ratio for lake watersheds across the USA. For 50 % of the lakes, evaporation was less than 20 % of the inflow, with values ranging from 135 % to no detectable evaporation. Strong spatial patterns in E/I were found across the USA with the West and mid-West being more evaporated than the East Coast. While climate patterns drove some of the spatial patterns of evaporation in lakes, variation in lake water residence time was also a driver. To assess the importance of these hydrological variables to lake biological condition, we rated lakes for high E/I by comparing them to reference lakes within the same ecoregion. Interestingly, we found that highly evaporated natural lakes were four times more likely to be in poor biological condition compared to less evaporated lakes, but evaporation was not correlated with biological condition in man-made lakes (i.e. reservoirs). We speculate that this link to lake condition comes from a strong correlation between E/I and lake total nitrogen concentration. Water samples for lake water isotopes were also collected during the 2012 National Lake Assessment so that temporal trends can begin to be assessed in the future.