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.