Figure 3 Values ofR2 and Nse of the mixed gamma distribution fitting to normalized duration curves. 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.
Among 698 weather stations and 224 hydrological stations, the 75% of the estimated values ofNse and R2 exceed 0.81 and the minimum values are greater than 0.6. This reflects that the mixed gamma distribution can perfectly capture the shape of PDC and FDCs.
From different hydrological stations’ distribution of Nse andR2 in FDC, it can be seen that there is a difference. The values of R2 and Nse of some stations are relative low, indicating that the fitting effect of FDC is not very good at certain hydrological stations or in certain years. On the other hand, the fitting performance of PDCs and FFDCs is superior to that of TFDCs and SFDCs. It may be due to the fact that the gamma fitted data should meet the requirements of independently identically distribution, which presents the characteristic of the random-like properties. Precipitation and surface runoff processes include more complex stochastic processes, which are more able to reflect this randomness.