The complex equation of keeping Tahoe blue

The rallying slogan for preservation efforts at Lake Tahoe has long been “Keep Tahoe Blue”.  However, it has only been in recent years that the scientific possibility of meeting this imperative has been systematically evaluated through the use of numerical models. The cornerstone of this modeling approach is the Tahoe Clarity Model. This is in fact a nested set of models – a one-dimensional hydrodynamic model, an ecological model for describing algal growth in response to varying nutrient and light conditions, a particle settling model for describing the evolution and fate of fine inorganic particles in the water column, and an optical model for predicting lake clarity as a function of the modeled distributions of chlorophyll and fine particles. The model has been calibrated and validated using measured input and response data collected over a 4 year period. Model results indicate that while light absorption by algae accounts for approximately 25% of the clarity loss, the majority of light attenuation is due to scattering by inorganic particles. Modeling potential, future load reduction scenarios, it is evident that the preservation of Tahoe’s clarity is possible on time scales of 20 years.