Large lakes as sources and sinks of anthropogenic heat: Capacities and limits

The goal of reducing carbon fuel and thereby saving energy will increase the use of lake water for heating and cooling of riparian infrastructures. This raises the question of which heat use designs meet the ecological and technical requirements for lakes, particularly in regard to climate warming. Thus, this study explores heat use effects on the temperature and stratification of a large, deep, temperate lake by applying the one-dimensional k-epsilon model SimStrat to various forcing scenarios. Several design parameters, such as extraction and discharge depth, and their effects were assessed. Additionally, 21st century climate projections were used to evaluate the effects of climate change relative to those of heat use. Generally, the study showed only minor effects for a realistic heat demand of 2 W m(-2) quite independent of the heat extraction/discharge modes. Mean water temperature changed less than 0.2 degrees C as long as there was no discharge into the deepest layers. Water extraction and discharge at the surface had the least thermal influence. To relate to climate change, heat use was scaled up to +85 W m(-2). Resultant simulations showed that such (unrealistic) anthropogenic, lake-based thermal pollution would have a comparable influence to that of climate change. Conversely, heat extraction could damp or even compensate climate-induced warming. The present study concludes that (i) there are minor effects on water temperatures, stratification, and seasonal mixing due to heat use of up to 2 W m(-2) and (ii) those influences are insignificant relative to the expected climate change.