The report presents a methodology to assess the probability distribution of Dissolved Oxygen (DO) based on the Streeter-Phelp’s equation. The methodology involves the use of first order analysis and Monte Carlo simulation to analyze the uncertainty associated with DO. The mean and standard deviation of the parameters have been assumed as same for the study’s probability distributions.

It has been recognized that processes in natural stream environments are inherently random. The ability to quantify the probabilistic status of stream environments is of vital importance in water quality management decision-making.  

The critical DO is found to be approximately normally distributed. The mean value of DO at the critical location is found to be more or less same, irrespective of the method used. But the level of uncertainty used associated with DO is found to be considerably different. The Monte Carlo simulations with log-normally distributed input provide lower uncertainty in the DO levels.  

Monte Carlo simulation with log-normally distributed input variables is established to be a preferable method for the estimation of travel time. The study infers that for the overall DO profile in the stream, first order analysis which predicts the same DO levels as that obtained by Monte Carlo simulation with a lower level of uncertainty, is found to be reasonable.

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