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.