5 Discussion
5.1 Impact mechanism of
reservoir
One of the main functions of the reservoir is to regulate runoff to
reduce the influence of floods. Usually, a part of the reservoir
capacity is discharged before the wet season arrives, and some of the
peak flow is intercepted during the wet season. This regulation will
obviously reduce flood peaks and increase baseflow during dry season,
but will not directly affect the total runoff. In this study, the
average streamflow of the downstream Zhenxi station decreased by about
14% after the construction of the reservoir, which may be mainly due to
the increased direct consumption of water surface evaporation and
indirect consumption of drinking water, farmland irrigation, etc. (T.
Maavara, Lauerwald, Regnier, & Van Cappellen, 2017; Vorosmarty et al.,
1997). In other words, the regulation of runoff by the reservoir
directly leads to the reduction of peak flow and the increase of
baseflow, and the increased direct and indirect consumption of reservoir
storage leads to the reduction of average or total streamflow. Some
studies have well illustrated the above point of view: Piman, Lennaerts,
and Southalack (2013) showed that hydropower dams in the tributaries of
the Mekong River will lead to a 25% decrease in the streamflow during
the wet‑season and an increase of 95% during the dry‑season; In
addition, Piman, Cochrane, and Arias (2016) also pointed out that the
impact of hydropower dams on the downstream daily average streamflow is
greater, with the maximum daily streamflow decreasing by 36% and the
minimum increasing by 168%. Vorosmarty et al. (1997) pointed out that
the loss of Lake Nasser on the Nile River caused by evaporation and
leakage accounts for about 13% of its inflow every year, Lake Kariba on
the Zambezi River loses about 20% of its inflow, while the smaller Tiga
Reservoir loses about 26% of its upstream inflow. Shiklomanov (2000)
pointed out that the reservoir‑based evaporation would increase the
consumption of the global river runoff by about 5%, which would be
larger if the domestic, industrial and agricultural water consumption
were taken into account.
As mentioned in Section 4.2, after the construction of the reservoir,
the recession coefficient a of downstream Zhenxi station is
significantly reduced, while b is significantly increased, which
leads to the change of the master recession curve and S-Q relationship.
Why is there such a change? We try to find the answer from the scatter
plot of log (|dQ /dt |) vs. log
(Q ). By comparing the log
(|dQ /dt |) vs. log (Q ) points
distribution characteristics before and after the construction of the
reservoir at Zhenxi station, we find that the scatter points at the low
flow stage after the construction of the reservoir are significantly
shifted to the right (Fig. 6a). This also results in a counterclockwise
rotation of the linear regression line, which eventually increases the
slope (b ) and decreases the intercept (a ). This shift
indicates that after the construction of the reservoir, the downstream
baseflow (Q ) increased during the dry‑season, but the recession
rate (dQ/dt) did not change significantly, which can be understood as
the addition of a constant flow (Qcon ) in the
original recession process. To verify this assumption, we add a constant
value (1m3/s) to the master recession curve (MRC) of
Zhenxi station before the construction of the reservoir, and then draw
the log (|dQ /dt |) vs. log (Q )
scatter plots (Fig. 6b). It is found that the scatter points in the low
flow stage are also shifted to the right after increasing the constant
value of 1m3/s to the MRC. The recession coefficienta calculated by the linear regression method is reduced to 0.01,
and b is increased to 1.22, which are basically the same as the
calculated values after the construction of the reservoir. In addition,
it can be seen from Table 2 that after the construction of the
reservoir, the Qmin of Zhenxi station increased
from 0.29m3/s to 1.38m3/s, increased
by 1.09m3/s, and the 7Qmin increased
by 7.03m3/s, which should not be a coincidence. That
is to say, after the construction of the reservoir, the variable water
storage of the basin is increased, and a relatively constant flow (about
1m3/s) is added to the baseflow recession process of
downstream Zhenxi station, which causes the log
(|dQ /dt |) vs. log (Q ) scatter
points to shift to the right in the late recession stage, and finally
causes the recession coefficient a to decrease and b to
increase. This relatively constant flow may come from the continuous and
stable drainage of the reservoir, the continuous leakage, and the
continuous return flow of domestic, agricultural and industrial used
reservoir water. Specific sources still require detailed field
investigations.
Figure 6. -dQ/dt vs. Q scatter plots in double logarithmic
coordinate system, with the Vog extraction method. The lines in the
figure are linear regression lines. (a) Observed values before and after
the construction of the reservoir, (b) calculated values before and
after adding a constant flow.
5.2 Master recession analysis considering constant addition
flow
It can be seen from Fig. 6b that after adding a constant flow, the log
(|dQ /dt |) vs. log (Q ) scatter
points no longer satisfies the linear relationship, and the slope
gradually becomes larger in the late stage of the recession. The
recession coefficients calculated by the traditional linear
parameterization method reflect the average linear approximation of this
nonlinear relationship. Therefore, a new master recession analysis
method should be adopted to analyze the baseflow recession process with
a constant addition flow. For example, we can deduct the constant flow
(Qcon ) before drawing the log
(|dQ /dt |) vs. log (Q ) scatter
plot for parameterization (i.e. all measured recession streamflow minusQcon ), and then add Qconto the master recession curve (MRC). At this time, the equation of the
MRC becomes\(Q_{(t+t)}=Q_{\text{con}}+\left[a\left(b-1\right)t+{(Q_{t}-Q_{\text{con}})}^{1-b}\right]^{\frac{1}{1-b}}\),
where a and b in the formula are obtained by linear
regression of the scattered points after subtractingQcon . After subtractingQcon = 1m3/s from the recession
streamflow of Zhenxi station from 2009 to 2013, the recession
coefficients a and b are 0.022 and 1.10 respectively,
which are close to the analysis results of 1982 to 1986. Finally, the
MRC considering constant addition flow is drawn (Fig. 7). After
comparison, it can be found that the MRC obtained by the average linear
approximation will overestimate the streamflow in the middle recession
and underestimate the streamflow in the later recession, but the
deviation is not obvious as a whole. That is to say, in Sect. 4.2, the
recession analysis results obtained by linear approximation can reflect
the overall change of the recession characteristics before and after the
construction of the reservoir, but there will be some deviations in some
specific stages. The master recession analysis method proposed in this
paper can also be used in basins with inter‑basin water transfer
behavior. When there is stable inflow, Qcon is
positive, and when there is stable outflow, Qconis negative.
Figure 7. Master recession curves with and without constant
addition flow, semi logarithmic coordinate.