A stratification module for an integrated lake simulation model : application on Lake Victoria, East Africa

Proper management of natural resources, such as lakes, requires a comprehensive understanding of the interactions within ecosystems, and their response to anthropogenic pressures. Integrated ecological models will be helpful tools in investigating and understanding the functioning of ecosystems and their management problems. Main objective of this study was to build a lake stratification module which will be part of an integrated ecological model. A one-dimensional thermal stratification model for a lake was built using the ""M axima"" simulation program. A diffusion approach which considers the calculation of effective diffusion coefficient between layers was selected to model stratification. The internal validation of the model showedgood results. The model was then applied to one year data from Lake Victoria and a comparison was made between simulations of shallow (5.5 m) and deeper (30 m) parts of the lake. The model was able to simulate seasonal variation of water temperature as in measured values for both stations but suppressing stratification levels for some periods. For both stations the values of simulated temperature were lower than the measured temperature. Gamma, a coefficient which determines the density gradient adjustment for the value of effective diffusion between layers, was calibrated to correct for stratification levels. A gamma value of 0.1 (instead of 0.05 as used initially) enabled the model to reveal more correct stratification levels for the whole periods of occurrence. Correction for low simulated temperature was done by calibrating cloudiness condition from 0.5 to 0.8 and a correction of the atmospheric temperature values to the value representing local conditions. The study concluded that modelling thermal stratification may contribute to improved performance of the integrated ecological model in simulating other water quality parameter especially in stratified conditions. The thermal stratification model can simulate water temperature at difference depths, and detect the position of the thermocline. Internal validation of the model showed expected results but the model needs to be tested with more reliable and detailed data for longer periods of time before being coupled to the main integrated model