Citation:

Fergus, E., J. Renee Brooks, Phil Kaufmann, A. Herlihy, A. Pollard, M. Weber, AND Steve Paulsen. Lake Water Levels and Associated Hydrologic Characteristics in the Conterminous U.S. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION. American Water Resources Association, Middleburg, VA, 56(3):450-471, (2020). https://doi.org/10.1111/1752-1688.12817

Impact/Purpose:

Constructed reservoirs and natural lakes serve as sources of water for human activities, habitat for wildlife, and buffers against adverse effects of drought and flooding. Population increases, land use intensification, and climate change have great potential to lower lake and reservoir levels. Lake shoreline zones are hot-spots for biological diversity, ecological processes, and human activity. Complex physical habitat at these land-water interfaces promotes interchange of water, nutrients and energy; and provides diverse habitat for aquatic and terrestrial organisms alike. Reductions in near-shore habitat complexity resulting from lake drawdown can therefore have deleterious impacts on the ecological integrity of lakes and their riparian areas. Across the United States, lake hydrologic condition and its major drivers are poorly understood, and the U.S. EPA National Lakes Assessment (NLA) does not have a formal indicator of hydrologic condition. Using NLA measures of water-level drawdown, and stable isotope ratios determined from water samples, we describe the variation of hydrologic characteristics among lake types (natural vs. reservoir), across regional settings, and over time. Our study is the first U.S. national-scale assessment of lake hydrologic characteristics, demonstrating that NLA’s measures of water level drawdown and lake water balance (using water stable isotopes) quantify ecologically-relevant lake hydrologic characteristics and are well-suited for regional and national lake monitoring. Identifying the regional patterns of lake and reservoir drawdown and evaporation can help inform management strategies to identify water bodies that are particularly vulnerable to changing water use demands and climate. In addition, this study helps provide the lake and landscape context to better understand drivers of lake hydrologic condition across the nation.

Description:

Water withdrawal and changing climate conditions may alter lake water levels beyond normal ranges. Documenting potential changes requires understanding lake hydrology across regional environmental gradients. We assessed hydrologic characteristics of lakes in the conterminous U.S. using the U.S. EPA National Lakes Assessment 2007 and 2012 datasets. We examined 1) regional patterns of water-level drawdown, proportion of evaporated inflow (E:I), and water residence time in natural and man-made lakes and 2) how they changed between the 2007 and 2012 surveys. Approximately 40% of U.S. lakes in 2007 had Medium water-level drawdown, 37% Small, and 21% Large. While in 2012, the majority (>75%) of lakes had no measurable drawdown. Across regions, ~ 30% of man-made lakes in western regions and ~ 30% natural lakes in southern and coastal plains had Large drawdown in both years. For half of U.S. lakes, ~20% of inflowing water was evaporated (E:I) indicating that flow-through processes dominate water regimes (~ 80% of inflow). Water residence time for 75% of U.S. lakes was less than 1.2 years and was longer in natural vs. man-made lakes. Changes in drawdown and E:I between survey years aligned somewhat with changes in regional weather and drought. In 2012, lake drawdown decreased in regions with increased precipitation, cooler temperatures, and less drought-like conditions. However, man-made lakes in the western mountains consistently had large drawdown suggesting that water management may override weather effects. This study provides the context to assess local and regional lake hydrologic condition and inform management decisions in changing environmental conditions.

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