3.3 Regional patterns in spatial variation of FDCs
The estimation parameter α and β of FDCs at 224
hydrological stations were fitted and calculated in the middle and lower
reaches of the Yangtze River Basin,then α and β of PDC of
these stations in corresponding watersheds were calculated too using the
Tyson polygon method. As depicted in Figure 5 , α andβ of different flow duration curves present different regional
distribution patterns.
According to gamma distribution estimation parameters, in stochastic
process, shape parameter α represents the number of events, while
the scale parameter β indicates the occurrence rate of the event,
i.e β =,For a fixed time rate, if we expect more time to occur,
i.e the larger the α , the longer the waiting time T will
be; For a fixed number of events , when the event rate is high, it means
that the smaller β , the shorter the waiting time T will
be.
From Figure 5 , it can be seen that the value α of FFDCs
is basically smaller than that of other duration curves. Among the four
duration curves, the value α of SFDC is relatively larger, due to
the shorter duration time of fast flow and longer duration time of slow
flow. The steeper the slope of the area, the shorter the diffusion time
of the duration curve, that is the smaller the value α will be.
In contrast, the spatial distribution of the values α of PDCs and
FFDCs and those of TFDCs and SFDCs have a similar regional pattern.
However, the value α of PDCs is generally greater than FFDCs, and
the value α of SFDCs is generally greater than TFDCs. What’s
more, the range of value α of PDC and FFDCs is much narrower than
that of TFDCs and SFDCs, which is consistent with Cheng’s research.
Similarly, among the four duration curves, the value β of FFDCs
is relatively smaller, due to the higher occurrence rate of fast flow,
considering the shorter duration of fast flow and the longer duration of
slow flow.
Overall, the regional patterns of α and β in 224
hydrological stations in the Figure 5 indicate that they are
influenced by climate differences, but their differences within the same
spatial patterns related to climate zones are also significant. This may
indicate that other secondary climate and physiological characteristics
may also be the reasons for the differences in FDC shape. Possible
factors could include climate seasonality, groundwater contribution,
vegetation, slope, and watershed shape. The following analysis aims to
determine which of these controls may dominate.