Evaluation of Peak Discharge Transposition

Discharge estimates obtained from gauged data are generally considered to be more accurate than model-estimated discharges. While most designs do not occur at the location of a gauging station, many designs are required at sites near but not at the gauged location. Transposition methods transfer discharge estimates from a gauged location to a nearby location on the same river. Despite their frequency of use, little is known about their accuracy and sensitivity. Because the state of Maryland is considering using discharge transposition, an assessment of the accuracy and sensitivity of two methods was undertaken: the area-ratio method and Sauer's weighting function method. Gauged data from nine states were used to evaluate the two methods for recurrence intervals of 2, 10, and 100 years. The criterion used to reject a data pair on the basis of time-sampling variation was when the T-year flood for the larger drainage area was less than the T-year flood for the smaller drainage area. Approximately 50% of the station pairs of gauged data had to be discarded because the data were collected during different periods, which is known as the time-sampling-variation problem. Sauer's method provided slightly better accuracy than the area-ratio method. Sensitivity analyses of the two methods are used to assess their rationality. Overall, both methods provide improved accuracy when the ungauged site is near the gauged site. The accuracy results suggest that Sauer's method can be reasonably applied if the drainage area of the ungauged site is within ±25% of the area of the gauged station, but the sensitivity analysis suggests that the method should be applied with caution because of its potential irrationality.