Water levels in several NTL-LTER study lakes reached historical lows during the summer of 2009, owing to general decreases in precipitation and increases in total evaporation during the ice-free seasons of 1998-2009. Balancing a water budget for temperate lakes is difficult, requiring detailed measurements of precipitation, groundwater fluxes and surface flows to estimate the amount of water lost through evaporation. The NTL-LTER Trout Bog site has long-term records of meteorological conditions, including terrestrial and solar radiation from 1989 to the present. These measurements have the potential to provide more accurate estimates of evaporative fluxes via an energy budget analysis. We have carried out a detailed 20 year energy budget on Trout Bog (an NTL-LTER study lake) during the ice-free seasons of 1989-2009. Trout Bog is a closed basin system with minimal ground water outflow and no surface water inflows or outflows, where changes in ice-free lake levels are driven by the balance between evaporation and precipitation. Trout Bog is also representative of the median size class of lakes in the area. Compared to pre-1998 conditions, we found an average increase of 3.4 cm/yr of evaporation and an average decrease of 7.5 cm/yr during ice-free periods. Compared to 1990-1997, the mean ice-free period also increased by more than 18 days during 1998-2009.
Figure 1: Cumulative precipitation (black line) and cumulative evaporation in cm (dashed line) during the ice-free period, balanced to yield change in lake storage (grey fill: model, circles: measured). Ice-free duration relative to the 20 year median is shown in intensity of blue (shorter) or red (longer).