4 Results
4.1 Impact of reservoir on
runoff
Before the baseflow recession analysis, we first analyze the impact of
the construction of the Chaersen Reservoir on the runoff of the
downstream Zhenxi station. The streamflow duration curves of the two
comparison periods before and after the construction of the reservoir
are plotted, respectively, as shown in Fig. 2. After the construction of
the reservoir, the peak flow of Zhenxi station decreases significantly,
while the low‑flow increases significantly (Fig. 2c). The streamflow of
the two upstream hydrological stations (Suolun and Dashizhai) are not
significantly different in the comparison periods (Fig. 2a, Fig. 2b),
and the deviations in some stages may be mainly caused by the
differences in climate characteristics. In order to quantitatively
analyze the impact of reservoir construction on runoff, we have
calculated some characteristic values of streamflow (maximum:
Qmax, minimum: Qmin, average:
Qmean, seven‑days minimum: 7Qmin) in the
comparison periods, and the percentage bias between them, as shown in
Table 2. The streamflow characteristic values of Suolun station in the
post‑comparison period is generally smaller, with a bias of -16% to
-24%. However, these values of Dashizhai station in the post‑comparison
period are generally larger, with a bias of 11% to 60%. This opposite
deviation may be caused by the spatial difference of climate
characteristics, which will cancel each other when converging to
downstream Zhenxi station. Considering the large difference between the
streamflow magnitude of the two upstream hydrological stations, this
study estimates the impact of climate change on the downstream Zhenxi
station by weighted average of the two opposite deviations, with the
proportion of average streamflow as the weight (Table 2). After
excluding the impact of climate change, the construction of Chaersen
Reservoir resulted in a 14% decrease in Qmean of the
downstream Zhenxi station, a 35% decrease in Qmax, a
380% increase in Qmin, and a 235% increase in
7Qmin.
Figure 2. Streamflow duration curves before and after the
construction of reservoir. (a) Suolun station, (b) Dashizhai station,
(c) Zhenxi station.
Table 2. Characteristic values of streamflow before and after
the construction of reservoir, m3/s.
4.2 Impact of reservoir on baseflow recession
characteristics
According to the method in section 3, the recession coefficients of the
two comparison periods are calculated, and the results are shown in Fig.
3. In general, after the construction of the reservoir, the recession
coefficient a of the downstream Zhenxi station is obviously
reduced, while the coefficient b is obviously increased. No
significant differences are found in the recession coefficients of the
comparison periods at the upstream Suolun and Dashizhai stations (Fig.
3). There are inherent differences in the coefficients calculated by
different methods, Stoelzle et al. (2013) has discussed it in detail.
The recession coefficients obtained by linear regression and binning
methods are not significantly different, but these obtained by the lower
envelope method are significantly lower. Different baseflow recession
segments extraction methods always cause certain fluctuations in the
coefficients. Therefore, we use the same recession analysis method
combination to avoid misjudgment due to the systematic error between
different recession analysis methods (e.g. using the average result of
different recession extraction methods under the same parameter
optimization method, Table 3).
In order to quantitatively analyze the impact of the reservoir, we
calculate the percentage bias between the average results of the two
comparison periods, as shown in Table 3. Under different parameter
optimization methods, the average recession coefficients a in the
post‑comparison period of Suolun station are smaller, with a bias of
-3% to -10%, and the average values of b are larger, with a
bias of 0% to 3%. The average a of Dashizhai station are also
smaller, with a bias of -12% to -15%, and the average b are
slightly larger, with a bias of -2% to 6%. The deviation directions of
the two upstream hydrological stations are the same, so this study uses
the average of the two hydrological stations to estimate the bias caused
by the climate change between the comparison periods of the downstream
Zhenxi station (Table 3). After excluding the influence of climate
change, the recession coefficient a of different parameter
optimization methods decreased by 57% ~ 63% (the
average is 60%), and the b increased by 21% ~
27% (the average is 24%). Although different parameter optimization
methods will get different results, the impact degree of reservoir
construction on the analysis results of different methods is basically
the same.
In order to more intuitively observe the influence of reservoir
construction on the baseflow recession characteristics, we have drawn
the master recession curves of the comparison periods before and after
the construction of the reservoir, the equation
is\(\ Q_{(t+t)}=\left[a\left(b-1\right)t+Q_{t}^{1-b}\right]^{\frac{1}{1-b}}\),
as shown in Fig. 4, Fig. S4 and Fig. S5. When the recession starts with
average streamflow (Fig. 4a), the master recession curves of Zhenxi
station before and after the construction of the reservoir intersects.
And the master recession curve after the construction of the reservoir
is significantly higher in the later stage of the recession, and the
recession rate becomes significantly smaller. However, the master
recession curves of Suolun and Dashizhai stations do not change
significantly before and after the construction of the reservoir. When
the recession starts with same streamflow (Fig. 4b), the master
recession curves of Zhenxi, Suolun and Dashizhai stations before the
construction of the reservoir are basically the same. However, after the
construction of the reservoir, the master recession curve of Zhenxi
station is significantly higher. In other words, the construction of the
reservoir slows the baseflow recession rate and increases the drainage
time, causing the master recession curve to shift upward.
Changes in the recession coefficients and the master recession curve
also mean changes in the basin-scale storage-discharge (S-Q)
relationship. Based on the nonlinear reservoir theoretical model (Sect.
3.2), the S-Q relationship curves before and after the construction of
the reservoir are drawn, as shown in Fig. 5, Fig. S6, and Fig. S7.
Before the construction of the reservoir, the S-Q relationships of the
three hydrological stations are approximately linear, but the
nonlinearity of the S-Q relationship of Zhenxi station increases
significantly after the construction of the reservoir. In addition,
after the construction of the reservoir, the storage capacity of Zhenxi
station increases significantly. When the baseflow is
5m3/s (60th percentile), the storage
capacity is 3.1 times before the reservoir construction; When the
baseflow is 20m3/s (32thpercentile), the storage capacity becomes 2.1 times as before; When the
baseflow is 40m3/s (22thpercentile), the storage capacity becomes 1.7 times as before. That is,
with the increase of baseflow, the increase rate of basin‑scale storage
becomes slower and slower.
Figure 3. Recession coefficients analysis results of different
methods. LR: linear regression, BIN: binning, LE: lower envelope.
Table 3. The average recession coefficients of different
parameter optimization methods. The bias is calculated with
1982~1986 as reference.
Figure 4. Master recession curves, with the recession
coefficients calculated by linear regression (LR) method. (a) Recession
starts with average streamflow, (b) recession starts with same
streamflow.
Figure 5. Storage-discharge (S-Q) relationship curves, with the
recession coefficients calculated by linear regression (LR) method.