The Influence of Horizontal Dispersion on Residence Times in Shallow Lakes

This study presents results of circulation and residence time in lakes influenced by wind-induced mixing investigated with numerical simulations. The study area is Lake Kasumigaura, a continuous lake system primally consisting of two lakes, West Lake and North Lake. Although metrological conditions and depths are similar for both lakes, the surface area and shape of the lakes are very different. A numerical model resolves the primary features of the wind-driven circulation in the lake system and is forced by observed river discharges and wind stress. A passive tracer released from the river mouths is used to estimate the residence time and evaluate mixing processes. Wind-driven flow dominates the kinetic energy in the lakes and induces chaotic motions leading to tracer dispersion which is largely influenced by the shape of the lakes. Results indicate that the residence time is much longer in the well-mixed middle basin of West Lake than North Lake. The estimated horizontal tracer diffusivity is approximately three times larger for the large West Lake than for the small-narrow North Lake. This study suggests that the surface area and shape of lakes in which the flow is predominantly wind driven largely influences circulation, water exchange processes and residence times in lakes.