Lake Water Levels and Associated Hydrologic Characteristics in the Conterminous U.S.
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.