A method for determining runoff coefficients indirectly without direct measurement of volume of runoff was developed for drainage areas of selected storm-water basins on Long Island, N.Y., to expedite evaluation of basin performance. The method requires a maximum-stage gage to record the maximum water level attained in the basin during the storm, and a precipitation gage to record storm intensity at regular intervals. The maximum volume of runoff impounded in the basin during a storm is calculated from precipitation data, an arbitrarily estimated runoff coefficient, and the basin's dimensions and infiltration rate. The calculated result is then compared with the recorded maximum water level, and the process is repeated with increased or decreased coefficients on a trial basis until two of the resulting water-storage maxima closely bracket the recorded maximum. The runoff coefficient in effect during that storm is then interpolated or derived graphically from the calculated water-storage maxima. Results of data analyses suggest that a close approximation of a basin's infiltration rate may be used instead of a measured infiltration rate to calculate the runoff coefficient. Differences between measured and calculated runoff coefficients averaged less than 20 percent when based on average infiltration rates adjusted for water temperature and less than 10 percent when based on measured infiltration rates. Accuracy of the calculated runoff coefficient tends to decline as the interval between precipitation measurements increases. Precipitation data collected at 5- and 15-minute intervals gave accurate runoff coefficients regardless of storm duration, but data collected at 30- or 60-minute intervals gave widely varying results.
