SEASONAL FLOW FREQUENCY ANALYSIS

Abstract

Numerous hydrologic designs require seasonal discharge estimates. Given the seasonal variation in the distribution of rainfall, significant differences can exist between the annual maximum and seasonal maximum T-yr return period discharges. Recommendations for making seasonal flood frequency analyses for gauged and ungauged locations are presented. When performing a seasonal frequency analysis of gauged data, missing discharge data will generally be a problem. Two potential solutions to the problem of incomplete records are introduced. First, a maximum likelihood approximation that replaces missing discharges below a threshold is developed and tested. Results of simulations indicate that it is more accurate than the method demonstrated in Bulletin 17B. Second, the ratio of the measured instantaneous to mean daily discharge is regionalized. This provides a method of replacing missing, below threshold, discharges. The ratio of measured instantaneous discharges above a threshold and the corresponding mean daily discharges can be used for predicting missing instantaneous discharges in seasonal records. Once regionalized, the method can be used for developing seasonal frequency curves at ungauged locations with either the USGS regression equations or a rainfall-runoff model. The Eastern Coastal Plain data are used to demonstrate the development of regional index ratios.