effect of decay coefficient uncertainty on the results of zarjoob river water quality modeling

many constants and coefficient effect on the modeling results. in many cases, it is possible these constants and coefficients differ from their real values in the environment, but the modeling results adapted with real boundary conditions due to other factors adjustment during modeling. uncertainty analysis is a tool for assessing the effect of uncertainty of various factors on the modeling results. as well, it can show the need for more data to improve load estimations, initial conditions or decay rates, etc. the effect of decay coefficient of cbod uncertainty on the dissolved oxygen concentration in zarjoob river (gilan) was studied by qual2e model. sampling for water quality parameters, flow rates, depths, and cross sections was done in august and september for model calibration and october for its verification in six stations during year 2005. meteorological data was obtained from the nearest station in rasht city. the effect of algae, sediment oxygen demand, and nbod was considered as part of other do coefficients. decay and reaeration coefficients obtained by model calibration. single factor method used for uncertainty analysis. the results show sensitivity level of dissolved oxygen to decay coefficient in downstream was about -0.6 in august and in fifth station, the sensitivity level was -6 before the do reaches to zero in july. the reason of high sensitivity is sharp reduction of do from station 4 to 5 and its low level in this station. it shows the need for more time and money for measuring or more precise estimation of decay rates.