Time dependent transport mechanisms in freshwater lakes.

The development and implementation of a totally predictive model for the annual thermal structure of a freshwater lake or reservoir is described in detail. Water quality depends to a great degree on dissolved oxygen profiles and nutrient concentrations and is thus governed by the temperature profile in the lake. The numerical model described here discusses fully this latter problem and also indicates strongly the direction for further research into the introduction of comprehensive biochemical cycles to improve this calculation. All possible variations in external and internal parameters are included. These values can be observed for a specific lake and future behaviour predicted on the basis of them. For an unbuilt reservoir, mean values can be taken for the parameters that cannot otherwise be determined from climatological, meteorological or geophysical sources. The transport mechanisms responsible for the development of the thermal profile during the year are determined completely by those parameters. The numerical representation is based on the assumption of horizontal homogeneity so that the one-dimensional heat transfer equation can be solved using a finite difference grid and a forward time step of one day. Decreasing the time step and modifying daily mean values to allow for diurnal variation in solar elevation permits the model to be used over a shorter time scale. The surface energy budget and wind speed (which it is found must be modified for lakes of small surface area) are the main forcing functions for the vertical mixing. It is shown that the average annual temperature structure of the lake is stable over a period of many years irrespective of the initial conditions imposed on the temperature profile. The problems of validating this model 'climate' against the observations of a single year (termed the lake 'weather' ) are evident.