Estimation of Flow-Duration Curves at Ungaged Sites in Southern New England

Two sets of regional-regression equations are developed to estimate the daily, unregulated, period-of-record flow duration curve (FDC) at ungaged sites in southern New England. The first method assumes an underlying probability density function (pdf) for daily streamflow whose parameter values are related to the physical characteristics of the ungaged basin. The second method relates flow at selected exceedence probabilities on the FDC to physical characteristics of the ungaged basin. We consider 66 relatively unregulated gages having between 10 and 86 years of continuous, daily-streamflow measurements. A jack-knife procedure is used to compare FDCs estimated from each method to the gage data from which the regression equations were developed. FDC estimates from regression equations developed for individual exceedences led to lower mean square errors than estimates of FDCs that assumed an underlying pdf. L-moment diagrams, probability plots and simulation experiments reveal that daily streamflow are well approximated by a kappa distribution. The first four L-moments are highly correlated with each other, which were used to improve estimates of FDCs based on a regional kappa distribution.