Calculating stream reaeration coefficients from oxygen profiles

The stream reaeration coefficient inferred from a single-station diurnal oxygen curve analysis is a function only of the photoperiod duration and of the time lag between solar noon and oxygen maximuni-it is independent of rates of primary production and respiration. Solutions of this function, already presented in this journal as graphs by Chapra and Di Toro in 1991, arise from numerical solution of an implicit transcendental equation and so do not have a simple analytical expression. A simple alternative approximate logistic curve-fit equation is presented here, enabling quite accurate direct calculation of the coefficient from the time lag and photoperiod duration alone. A temperature correction factor is also included. The equation may be useful for incorporation in decision support systems.