Reasons for the homogenization of the seasonal discharges in the Yangtze River


 Allocations of water discharges between dry and flood seasons along the Yangtze River have significantly homogenized during the past decades, mainly due to precipitation change, regulation of key hydraulic works on the mainstream like the Three Gorges Reservoir (TGR), and the construction of numerous dams scattered in sub-basins. To reveal the specific roles of these three major factors in changing the seasonal discharges of the whole Yangtze River, this paper analyzes daily discharges during 1961–2014 at 16 hydrological stations from the far upper reach (the Jinshajiang Reach) to the estuary. We found that precipitation has only homogenized in areas 427 km downstream of the TGR, contributing 9.5–23.6% to the homogenized discharges. Even though the TGR is the largest hydraulic works in the world, it only contributes 17.5–27.2% to the downstream homogenization of seasonal discharge. By comparison, dams in sub-regions are a major contributor (61.1–100%) in the homogenized reach either upper or lower to the TGR. Of all the sub-basins, dams in Hanjiang River basin have the most significant effect (16.9%) on changing the allocations of seasonal discharges to the sea, followed by Wujiang (11.5%), Jialingjiang (10.1%), Yalongjiang (9.4%), Qingjiang (8.4%), and Daduhe-Minjiang (4.7%) river basins.


INTRODUCTION
Seasonal hydrological processes in rivers all around the world have significantly changed under the impacts of rapid climate change and intensified human activities during recent decades (Miao et al. ). As the Earth's surface temperature increases, glaciers melt earlier than before (Sorg et al. ), and peak discharges shift from summer to spring or even winter, as being observed in the Rhine, Rhone, Danube (Huss ), Yellow (Li et al. ) and Naryn (Gan et al. ) rivers. By comparison, variations in precipitation affect seasonal discharges in a faster and more violent way, often leading to flood and drought disasters which can cause huge economic loss and heavy casualties.
On the other hand, the impacts of human activities on seasonal water discharges are also widespread. By storing water discharges in flood seasons and releasing in dry seasons, dams effectively regulate the intra-year distribution of river flows. Moreover, water-soil conservation projects conducted worldwide progressively alter the surface of watersheds, making the characteristics of seasonal water discharge more complicated.
With a drainage area of ∼1,800,000 km 2 and a length of 6,300 km, the Yangtze River is the largest river in China and the world's third longest river. Recent studies have shown that the allocations of discharges between dry and flood seasons are significantly homogenized (i.e. gap in seasonal discharge narrows) across a vast area of the Yangtze River Basin extending from the estuary to the gauging station of Zhutuo in the upper reach (Guo et al. ). Precipitation change and dam operation, in particular, the operation of the Three Gorges Reservoir (TGR), are believed to be key factors behind such rapid changes. Dai et al. () found that climate variability was the dominant factor for the homogenized discharges downstream of the TGR in 2013 when severe droughts occurred. During the period of 2003-2014, the operation of the TGR was generally believed to be responsible for the changes in the intra-year distribution of discharges.
As the most intensively regulated catchment in the world, the Yangtze River Basin contains more than 50,000 dams in addition to the TGR and these dams are scattered in all the major sub-regions, such as the Daduhe-Minjiang River Basin, the Jialingjiang River Basin, the Wujiang River Basin, the Qingjiang River Basin, the Hanjiang River Basin, and the two lake areas of Dongting and Poyang. Given that the total capacity of those numerous dams is very large (154 km 3 , Yang et al. ), they may also lead to significant homogenization effect on the seasonal discharges. However, most previous studies mainly focused on the effects of TGR and precipitation change (Dai et al. ; Han et al. ). The role of hydraulic works in each sub-basin in changing the allocations of seasonal water discharges, especially by comparison with precipitation change and the TGR, has not been quantitatively indicated. This paper extends the study area to the far upper reach of the Yangtze River, the Jinshajiang Reach, and tests the changing trend of discharge allocations between dry and flood seasons during 1961-2014 of the whole Yangtze River. Then, specific roles of all the major factors, such as changes in precipitation, operation of the TGR operation, and constructions of numerous other dams in eight major sub-regions of the Yangtze River Basin, in modifying the patterns of seasonal discharges are estimated, providing a step toward adaptive management of water resources in a changing environment.

Division of sub-regions
The Yangtze River is divided into the upper reach, the middle reach, and the lower reach by the hydrological gauging stations of Yichang and Hukou, with lengths of 4,504 km, 955 km and 938 km, respectively (Figure 1(a)). Located 44 km upstream of the Yichang station, the TGR is regarded as the largest reservoir in the world, which has effectively flattened the intra-year distribution of discharges downstream of the dam since it became operational in 2003. The present study involves ten hydrological stations on the mainstream, including Panzhihua, Pingshan, Zhutuo, and Cuntan stations on the upper reach, Yichang, Zhicheng, Shashi, Luoshan, and Hankou stations on the middle reach, and Datong station on the lower reach ( Figure 1(a)). Control stations of major tributaries, such as Hukou, Huangzhuang, and Chenglingji, and outlet stations, such as Songzikou, Taipingkou, and Ouchikou, are also included (Figure 1(b)). Thus, the whole Yangtze River Basin is divided into eight sub-regions by these stations, i.e. the Yalongjiang River Basin, the Daduhe-Minjiang River Basin, the Jialingjiang River Basin and the Wujiang River Basin in the upper region, and Qingjiang River Basin, Dongting Lake Area, Hanjing River Basin, and Poyang Lake Area in the middle and lower regions (Figure 1(c)). Water discharges of these sub-regions can be either directly derived from the control stations of tributaries and outlets, or approximated by the difference between the two adjacent stations on the mainstream (Figure 1(c)). Collecting data from the meteorological stations in each sub-region (Figure 1(d)), the corresponding precipitation can also be calculated by Thiessen polygon method.

Normalization of discharges in dry and flood seasons
It is commonly recognized that the flood season of the Yangtze River Basin lasts from May to October, during which period the water discharge accounts for 67.9% of the annual total at the Datong station, and the period from November to next April is selected as the dry season (Gemmer et al. ). In order to reflect the relative importance of discharges in dry and flood seasons to the annual total of each year, a normalization method is proposed.
The discharges in dry and flood seasons are changed into dimensionless values between 0 and 1 by dividing through by the total (Equations (1) and (2)).
In Equations (1) and (2) The simulated water level and discharge at the four hydrological stations are in good agreement with the measured data.
The average absolute value of the relative error between the simulated and measured discharges was only 0.8%, and the value of the water level was 3.1%, which demonstrates that the reconstructed discharges are reliable. The reconstructed discharge without TGR operation was also normalized.
Thus, the differences between the reconstructed series without TGR operation and the observed series during the post-TGR period can be estimated (ΔQ h-TGR , dimensionless, see Figure 2).

Corrected Proof
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Impact of TGR and dams scattered in each sub-region
In the pre-TGR period, the total impact of human activities, which can be separated out by comparing the reconstructed discharge series (

Uncertainty analysis
Although the linear correlations between precipitation and discharge have high confidence levels (Figure 4), the uncertainty still needs to be evaluated. By comparing between the discharges predicted by the linear relationship with the measured ones, the mean relative error of each station is found to vary from 0.6 to 1.2% (with standard deviation varying from ± 8.0 to 11.1%), which is acceptable. Furthermore, uncertainty introduced by the inherent relation of the discharge series is also analyzed by a cross test using regression equations of uneven and even years, i.e., the regression equations of uneven years to predict the discharge of even years, and vice versa. The maximum relative error is only À3.3% for the predictions of even years and 5.6% for those of uneven years, with standard deviations of ±10.5% and ±11.3%, respectively.
Clearly, the impact of an inherent relation is relatively low, which implies that the reconstructed discharge series using linear regression equations are reasonable.  Our study also indicates that the TGR has played a positive role in alleviating droughts, rather than aggravating it.