Effects of climate variability on lake evaporation: Results from a long-term energy budget study of Sparkling Lake, northern Wisconsin (USA)

Abstract

Variations in lake evaporation have a significant impact on the energy and water budgets of lakes. Understanding these variations and the role of climate is important for water resource management as well as predicting future changes in lake hydrology as a result of climate change. This study presents a comprehensive, 10-year analysis of seasonal, intraseasonal, and interannual variations in lake evaporation for Sparkling Lake in northern Wisconsin (USA). Meteorological and lake temperature measurements are made at a raft on the lake and are supplemented by radiation measurements from a nearby airport. The data are analyzed over 14-day periods from 1989 to 1998 (during the ice-free season) to provide bi-weekly energy budget estimates of evaporation rate (along with uncertainty estimates). The mean evaporation rate for Sparkling Lake over the study period is 3.1 mm day −1, with a coefficient of variation of 25%. Considerable variability in evaporation rates is found on a wide range of timescales, with seasonal changes having the highest coefficient of variation (18%), followed by the intraseasonal (15%) and interannual timescales (12%; for summer means). Intraseasonal changes in evaporation are primarily associated with synoptic weather variations, with high evaporation events tending to occur during incursions of cold, dry air (due, in part, to the thermal lag between air and lake temperatures). Seasonal variations in evaporation are largely driven by temperature and net radiation, but are out-of-phase with changes in wind speed. This presents challenges when calculating evaporation rates by means of the simpler mass-transfer technique. On interannual timescales, changes in summer evaporation rates are strongly associated with changes in net radiation and show only moderate connections to variations in temperature or humidity. Nonetheless, we are able to identify a simple, empirical relationship for estimating interannual evaporation rates that is more accurate than the mass transfer technique.