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Stable isotope estimates of evaporation: inflow and water residence time for lakes across the United States as a tool for national lake water quality assessments
Citation:
Brooks, J. Renee, J. Gibson, S. Birks, Marc H. Weber, Kent D. Rodecap, AND John L. Stoddard. Stable isotope estimates of evaporation: inflow and water residence time for lakes across the United States as a tool for national lake water quality assessments. LIMNOLOGY AND OCEANOGRAPHY. American Society of Limnology and Oceanography, Lawrence, KS, 59(6):2150-2165, (2014).
Impact/Purpose:
Under the Clean Water Act, the EPA must report on the condition of the Nation’s waters. EPA has devised the National Aquatic Resource Survey’s as a statistically valid approach for reporting on condition. The assessments collect information on a a wide variety of biological, recreational, chemical and physical indicators in the NARS. During the 2007 National Lakes Assessment (NLA), while the information collected was extensive for water quality and biological diversity, hydrological indicators were initially limited to lake area and depth, basin area, as well as annual precipitation and other climate data. One way to improve the NLA would be to include more detailed measures of lake hydrology in national surveys. Yet direct measurements of many hydrologic processes cannot be conducted in a spatially-extensive survey where sites are randomly distributed and therefore often remote, and where data collection is constrained by the number of measurements that can be conducted in a single day. In this paper, we use a method based on stable isotopic ratios from water to estimate lake water evaporation as a proportion of inflow (E/I) and lake water residence time (τ) for the lakes in the 2007 NLA. We extrapolate those results to the entire inference population of nearly 50,000 lakes within the USA.
Description:
Stable isotope ratios of water (delta18O and delta2H) can be very useful in large-scale monitoring programs because water samples are easy to collect and isotope ratios integrate information about basic hydrologic processes such as evaporation as a percentage of inflow (E/I) and water residence time (τ). Because of time constraints, measurements of these processes are often excluded from spatially-extensive monitoring programs such as Environmental Protection Agency’s National Lakes Assessment (NLA). We used water delta2H and delta18O from ca. 1000 lakes sampled in 2007 and distributed across the conterminous U.S. to assess these hydrological variables and scaled them to the inference population (~50,000 lakes). For 50 % of lakes, evaporation was less than 26 % of inflow, with values ranging up to 113 %. Categorizing lakes by flow regime, 63.6% of lakes were flow-though lakes (E/I < 0.4), 36.3 % were restricted-basin lakes (0.4 < E/I < 1), while less than 0.1 % were closed basin (E/I > 1). While climate patterns drove some of the spatial patterns of E/I and τ, variation in lake depth was also a significant driver. Lakes in poor biological condition (based on a predictive model of planktonic taxa) were significantly more evaporated than lakes in good biological condition. We speculate that this link to lake condition came from a strong positive correlation between E/I and lake total nitrogen concentration. Water samples for isotopic analysis were collected during the 2012 National Lake Assessment, so temporal trends can begin to be assessed in the future.