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

Abstract

We used water δ 2 H and δ 18 O from ca. 1000 lakes sampled in the 2007 U.S. Environmental Protection Agency's National Lakes Assessment (NLA) to assess two hydrological variables—evaporation as a percentage of inflow ( E : I ) and water residence time ( τ ) for summer 2007. Using a population survey design, sampled lakes were distributed across the conterminous U.S., and results were scaled to the inference population (∼50,000 U.S. lakes). These hydrologic variables were related to lake nutrients and biological condition to illustrate their usefulness in national water quality monitoring efforts. For 50% of lakes, evaporation was < 25% of inflow, with values ranging up to 113% during the 2007 summer. Residence time was < 0.52 yr for half of the lakes and < 1.12 yr for 75% of lakes. Categorizing lakes by flow regime, 66.1% of lakes were flow‐though lakes (60% or more of the water flows through the lake, E : I < 0.4), 33.6% were restricted‐basin lakes (40% or more of the lake inflow evaporates, 0.4 < E : I < 1), and < 0.3% were closed basin (all water entering the lake leaves through evaporation, E : I > 1). While climate patterns drove some of the spatial patterns of E : I and τ , variation in lake depth and watershed size (influencing precipitation volume) were also significant drivers. Lake hydrochemistry was strongly correlated to E : I and more weakly related to τ . Lakes in poor biological condition (based on a predictive model of planktonic taxa) were significantly more evaporated than lakes in good biological condition.