Drought-driven changes in lake areas and their effects on the surface energy balance of Minnesota's lake-dotted landscape

Open water within Minnesota constitutes 11 830 km² and represents a significant proportion of the moisture available on the landscape. Because lakes absorb and store heat in the spring and summer, and release heat in the autumn and winter, they also play a significant role in surface energy budgets. Lake areas fluctuate significantly over time, and thus alter surface albedo and energy partitioning in the expanding and contracting lake margin. Observations of lake areas from the 1930s "Dust Bowl" drought were used to approximate the impact of changes in lake area and volume on surface energy budgets. A statewide map of lake area change constructed from historical aerial photos of 620 lakes shows trends in percent area changes that mimic trends in moisture balance. Based on the aerial photos and an analysis of lake bathymetries, the water volume removed from the land exposed around the lakes during the drought was 9.1 ± 3.0 × 10⁹ m³. The total area of exposed land was 3300 ± 800 km². Based on a balanced energy budget estimate, the transition from standing water to exposed land at lake margins accounted for 1.12 ± 0.31 × 10¹¹ W of energy storage across the state. The decreased heat storage resulted in a statewide average storage flux anomaly of 0.50 ± 0.14 W m^-2 with localized anomalies as high as 33.9 ± 7.1 W m^-2. Large uncertainties exist, however, regarding the partitioning of the energy because of the wide range of potential albedo values for both land and water. As conditions warmed during the twentieth century, lake volumes have increased. Therefore, the assessment herein of the change in heat storage is relevant to comparisons of high regional temperatures in the 1930s and 2000s.