MODELING OF UNDERWATER LIGHT IN FRESH-WATER LAKES USING SURVIVAL AND FAILURE TIME ANALYSIS

1. This study presents a new approach to modelling subsurface irradian ce using concepts from survival and failure time analysis. The model a pplies a modified Weibull distribution function to predict downwelling irradiance. Data sets from forty-seven Norwegian sites show extremely high coefficients of determination, up to 99.99%, when analysed by th e Weibull model. 2. The uncritical use of a single k(d) value to model underwater light conditions is likely to result in poor estimates of received irradiance. This error may amount to several hundred per cent . The practice of force-fitting linear least-squares regressions to lo g-transformed irradiance data inevitably leads to highly biased estima tes of the true fraction of incident irradiance entering the water. 3. Wave effects causing fluctuations of subsurface irradiance are modell ed with synthetic data and compared with field observations. Fluctuati ons of surface elevation by waves produce skewed frequency distributio ns of the underwater light field. The result of these effects, which a re to reduce the accuracy of estimated model parameters, can be largel y eliminated by carefully designing field procedures used for the acqu isition of subsurface light data.