Natural and anthropogenic controls on lake water-level decline and evaporation-to-inflow ratio in the conterminous US
Lake hydrology is integral to lake ecological condition, but human-related water needs and changing climate conditions may alter hydrology beyond natural ranges. Separating natural and human effects can support lake management and assessment and requires a clear conceptual understanding of the multifaceted drivers of hydrologic variation across ecoregions and lake types. We constructed a conceptual model of morphometry, watershed hydrology, climate, and human water management drivers of lake water levels and evaporation-to-inflow ratio (E:I) across the conterminous US. Using a structural equation modeling framework, we tested our model using data from the US EPA National Lakes Assessment surveys. We applied the model to 1) a national subset of lakes with minimal water management infrastructure to establish natural background drivers of lake hydrology and 2) at ecoregional scales to explore how regional variation in water management and climate influence lake hydrology. Our model fit the national and ecoregional datasets (e.g., RMSEA < 0.08) and predicted lake E:I better than vertical water-level decline, showing that E:I is affected by watershed-level attributes and annual climate, whereas vertical decline may be related to predictors at finer spatial and temporal scales. For lakes with minimal water management presence, water-level decline was greatest in hydrologically-restricted lakes with drought conditions, and E:I was greatest in shallow lakes with little inflowing water. Anthropogenic effects varied by ecoregion and likely reflect differences in regional water management strategies. In the West, water management indicators were related to greater vertical decline, whereas in the Midwest, these indicators were related to more stable and full lake levels even during drought conditions. Our analyses indicated that water management infrastructure on lakes is widespread but their effects on lake hydrology are variable and may be adaptable to changing environmental conditions.