Using a continuous function for residence time to quantify the impact of climate change on the dynamics of thermally stratified lakes

The residence time of a lake is usually expressed as an annual average. This takes no account of the short-term fluctuations in the rainfall which result in a pronounced seasonal variation in the effective residence time. In this paper, we describe how we combine the regional output of an established Global Climate Model (GCM) with a new definition of residence time to quantify the effect of future changes in the weather on the dynamics of thermally stratified lakes. Illustrative examples are taken from a series of lakes in the English Lake District and demonstrate that some lakes respond to changes in the rainfall that extend over two, three or even four seasons. The critical factors influencing the hydraulic responses of the selected lakes were the size of each basin and the ratio of epilimnetic to hypolimnetic volume. In the smaller lakes, the effective residence time was just a few days whilst the large lakes had residence times that were close to one year. This division of lakes into single season and multi-season lakes is functionally very important since the projected changes in the weather may well be confined to a particular time of year.