3.1 Performance of the mixed gamma Distribution
Normalized sequences were used to construct PDC, TFDC, FFDC, and SFDC
for 698 weather stations and 224 hydrological stations. Under
interannual variations, annual precipitation records for 56 years and
annual flow records for at least 20 years were constructed,
respectively. According to respective normalized sequences, gamma
distribution was performed on each duration curve and moment method were
used to determine model parameters α and β . The estimated
parameter , α , β were used to predict the PDC and FDCs via
formula (3). In this paper, the Nash-Sutcliffe efficiency coefficient
(Nse ) and the Goodness of fit (R2 ) of
PDC and FDCs fitted by gamma distribution were used to evaluate the
fitting effect and accuracy.
For each duration curve, the aggregated statistical data of Nseand R2 obtained from the mixed gamma
distribution fitting of normalized duration curves for 698 weather
stations and 224 hydrological stations are shown in Figure 3 .
The upper and lower whiskers represent the maximum and minimum values
and the upper and lower lines of the box represent the third and first
quartiles. It can be seen from the Figure 3 that although
visual differences of selected sites based on typical climate conditions
are found in Figure 2 , the mixed gamma distribution still fits
the empirical duration curves: PDC, TFDC, FFDC and SFDC well.