Appendix A: A Practical Guide to Effective Discharge Calculations
At ungaged sites, and gaged sites where records are found to be unrepresentative of the flow
regime, it is necessary to synthesize a flow duration curve. There are two possible methods of doing this,
using records from nearby gaging stations within the same catchment or hydrological region.
Catchment Flow Duration Curve Method (Basin-Area Method)
This method relies on the availability of gage data from a number of stations along the same river
as the ungaged study site. First, flow duration curves for each gaging station are derived for the common
period of record. Provided there is a regular downstream decrease in the discharge per unit watershed
area, then a graph of discharge for a given exceedance duration against upstream basin area will produce
a power function best fit regression line with negligible scatter. For example, Figure 2 shows this
relationship for the River Wye, UK (Hey, 1975). The equations generated by this method enable the flow
duration curve at an ungaged site on that river to be determined as a function of its upstream watershed
area. Flow frequencies for selected discharge classes may then be extracted from the flow duration curve
for the ungaged site.
Downstream Daily Flow Duration Curves for the River Wye, UK Based on
Data from Gaging Stations Collected Between 1937 and 1962 (adapted
from Hey, 1975).