2.3 FDC and its corresponding streamflow percentiles
In order to present the characteristics of interdecadal changes, the changes of every 10 years: 1970-1979, 1980-1989, 2007-2016 were counted for total 30 years, providing more observational data for regionalization. Runoff data includes 15 percentiles corresponding to exceedance probabilities:Q 0.3, Q 0.5,Q 1, Q 5,Q 10, Q 20,Q 30, Q 50,Q 70, Q 80,Q 90, Q 95,Q 99, Q 99.5,Q 99.7, representing 0.3%, 0.5%, 1%, 5%, 10%, 20%, 50%, 70%, 80%, 90%, 95%, 99%, 99.5%, 99.7% respectively. These 15 quantile flow values are calculated and are directly related to the return period. Each quantile represents a different part of FDC, ranging from particularly high flow (Q 0.3,Q 0.5, Q 1 andQ 5) to particularly low flow (Q 95, Q 99,Q 99.5 and Q 99.7). For example, taking Wuxi station as an example (see Figure 4 ) and draw its fitting curve using gamma distribution, the quantile corresponding to Q5 is 35.1160m3/s, and the return period of 35.1160m3/s is a 20-year return period (1/p=1/0.05=20). It is noteworthy that each quantile was log transformed (log10(streamflow)) prior to modelling.
The variation characteristic of the Figure 4 shows that although the quantiles are randomly selected, they are used to determine the overall shape of each FDC. Because low and high flow rates are usually more important for drought and flood research, which are related to many processes occurring in ecosystems, the selected exceedance probabilities are closer to both ends to explain very large or very small changes of corresponding quantiles, while the distribution of the center part of the curve is determined by exceedance probabilities which are distributed more evenly and fewer. The overall variation of the curve is significant, with both the high and low flow parts of the curve showing a downward trend, and the FDC showing an S-shape. The change at the low tail of the curve is greater than the change at the front of the curve, indicating that the low flow part has a greater change.
[Insert Figure 4]
Figure 4 Streamflow values corresponding to different probability of exceedance (Year 1980-1989_Wuxi Station). Red marks represent the point of 15 streamflow percentiles analyzed in this paper.
For a more intuitive presentation, the final dataset includes a 10-year combination of 224 sites, with each quantile normalized by the logarithmic coordinate transformation (log (quantile)). The 80% of randomly selected stations were used as training sets, while the remaining 20% were reserved for testing.