4.2. Contributions of Water Discharge Change and Human Activities to Sediment Load
Human activities may have either decreasing or increasing effects on sediment load in the basin. The construction of reservoirs and soil and water conservation projects play a major role in the decrease in sediment load. By contrast, road and house construction, deforestation, and cultivation play a major role in the increase in sediment load. Completely separating the effects of various activities in the Jinsha River Basin is difficult, considering that collecting relevant data is difficult at present. Accordingly, human activities in this study are studied as a whole.
First, the effects of water discharge and human activities on sediment load are estimated by dividing the study period into (1) the basic period with weak human interference and (2) the measure period with strong human interference. Previous analysis has shown that no evident trend change in water discharge and sediment load was observed in the Jinsha River before 1998, indicating that water discharge changes and human activities had minimal influence on sediment load. After 1998, abrupt change points in sediment load have been observed in all regions of the Jinsha River Basin except in the upstream region of the Zhimenda Station because of the deepening influences of several human activities, such as construction of reservoirs and soil and water conservation projects. Therefore, this study unifies the estimation period from 1998 to 2015. The period before the abrupt change is called the “the basic period” (P0). The periods after abrupt change is divided into “the first measure phase” (P1) and “the second measure phase” (P2) according to the occurrence time of abrupt change point (
Figure 3).
Based on the relationship between water discharge and sediment load building in the basic period, the theoretical annual average sediment load in measure period (MSL) can be estimated. Then, the annual average sediment reduction load caused by the change of water discharge in each region can be estimate by subtracting the annual average sediment load in basic period (BSL) from MSL. Last, the annual average of sediment load variation (
) caused by the variation of water discharge in each region can be estimated by accumulating the variation of sediment load in different measure periods then dividing the total time. The calculation formula is Equation (16). What deserve our attention is that the variation of sediment load in the upstream region of the Zhimenda Station is caused by the variation of water discharge, so Equation (16) can be still used to estimate the variation of sediment load based on the data of annual average sediment load from 1966 to 1998 and from 1999 to 2015.
where
and
are the coefficient and index of the relationship between the sediment and water of the basic period, respectively;
is the sequence number of the measure period (
= 1 or 2);
is the number of measure period;
is the annual average water discharge at each measure period;
is the time of each measure period;
is the annual average sediment load in the basic period; and
is the total time from 1998 to 2015 (
= 18). If
< 0, then the variation in water discharge results in the decrease in sediment load; otherwise, the variation in water discharge leads to the increase in sediment load.
The annual average sediment reduction load caused by human activities in each region can be estimated by subtracting the variation load caused by the variation of water discharge in that period from the difference between the annual average sediment load in that period and that in the previous period. The calculation formula is as follows:
where
is the annual average sediment load of each measure period. The remaining symbols are the same as those in Equation (16).
Table 6 shows the change of sediment load variation in the Jinsha River Basin due to the variation of water discharge and human activities from 1998 to 2015, in which the decreasing load is more than the increasing load. The annual average sediment reduction load is approximately 99.57 × 10
6 t/y for the entire basin. The contributions of the changes of water discharge and human activities to the variation of sediment load are 18.9% and 81.1%, respectively.
In order to study the variation of sediment load from 1966 to 2015 further, the Jinsha River Basin is divided into two parts by Shigu Station (i.e., upstream region of Shigu Station and downstream region of Shigu Station) by us. Making the summation statistics of
and
in above regions based on the results in
Table 6, we find the facts that the influence of human activity is still greater than that of water discharge change, generally, in spite of the variation of sediment load differs in different region in above regions. The increase of water discharge and human activities, such as the building of house, have led the increase of sediment load in the upstream region of Shigu Station. And the increase of annual average sediment load by water discharge change and human activities is 4.58 × 10
6 t/y and 8.52 × 10
6 t/y, and the contributions are 35.0% and 65.0%. By contract, the trend of sediment variation in the downstream region of the Shigu Station is mainly decreasing, the annual average reduction load of sediment by water discharge change and human activities is 23.39 × 10
6 t/y and 89.28 × 10
6 t/y, and the contributions are 20.8% and 79.2%. Large reservoirs are mainly located in the downstream of Shigu Station, so one of the reasons for huge sediment reduction load in this region is the operation of reservoirs, successively.
Considering that many large reservoirs have been operated since 2011 in the middle and lower reaches of the Jinsha River basin, the entire study period is divided into two periods (i.e., from 1998 to 2010 and from 2011 to 2015) to analyze the variation of sediment load after 1998, further. Results show that the annual average sediment load reduction by human activities from 1998 to 2010 and from 2011 to 2015 are 68.21 × 106 t/y and 112.91 × 106 t/y, respectively. Furthermore, the intensity of sediment load reduction in the latter period is approximately 1.7 times that of the former period. Therefore, the operation of large reservoirs in the middle and lower reaches have an evident effect on sediment load reduction.
4.3. Macroeffect of Sediment Reduction in Watershed
The effects of sediment load reduction in the Jinsha River Basin are mainly reflected in the sedimentation of cascade reservoirs, the erosion in the downstream channel, and the considerable decrease in sediment load into the Three Gorges Reservoir.
4.3.1. Effect of Cascade Reservoirs on Sedimentation
The operation of the Xiangjiaba and Xiluodu Reservoir started in 2012 and 2013, respectively. Chengdu Engineering Corporation Limited [
54] predicted that the annual average sediment load retention by the Xiluodu Reservoir was approximately 208 × 10
6 t/y in the first 20 years of its operation based on water discharge and sediment load from 1964 to 1973. Zhongnan Engineering Corporation Limited [
55] predicted that the annual average sediment load retention by the Xiangjiaba Reservoir was approximately 41.5 × 10
6 t/y in the first 20 years of its operation, considering the effect of the Xiluodu Reservoir. Thus, the total annual average sediment load retention by the two reservoirs is approximately 250 × 10
6 t/y. The earlier estimation in this study shows that the annual average sediment load retention by the Xiangjiaba and Xiluodu Reservoir would be 93.47 × 10
6 t from 2013 to 2015, which would be 156 × 10
6 t/y less than the predicted value of the two reservoirs. In the corresponding period, the annual average sediment load of the import control station of Huatan is 70.2 × 10
6 t/y, which is 104 × 10
6 t/y less than the value during the feasibility study period. Therefore, the deposition rate of the Xiangjiaba and Xiluodu Reservoir would be considerably lower than the predicted value due to the notable decrease in sediment in the reservoirs.
4.3.2. Effect on Downstream Channel Erosion
First of all, the actual sediment concentration in the downstream region is far away from the state of theoretical sediment transport capacity after the construction of reservoirs [
56,
57].
Figure 5a shows the result of the particle size distribution of sediment changes at the Pingshan Station in recent years. As shown in the figure, the sediment at Pingshan Station has already been refined due to the retention effect of the reservoirs in the middle reaches of the Jinsha River and the main tributaries before the construction of the Xiangjiaba and Xiluodu Reservoir. The sediment was further refined after the construction of the Xiangjiaba and Xiluodu Reservoir. Accordingly, the percentage of sediment with a particle size of less than 0.062 mm increased from 77.1% between 1988 and 2012 to 96.7% in 2015. The sediment load at Pingshan Station decreased from 222.61 × 10
6 t to 0.6 × 10
6 t in the corresponding period. Secondly, the water flow regulation of reservoir will also cause the erosion or sedimentation of river channel [
58]. The specific regulation is mainly manifested by the method of decrease of flood peak flow, increase of the duration of middle water flow and increase of water flow in dry season. For example, the average water flow of Pingshan Station is 1690 × 10
9 m
3/s in dry season from 2013 to 2015, which is 10.9% higher than that before the operation of Xiangjiaba Reservoir. Due to the increase of water flow, the capacity of sediment transport of river is enhanced, which leads to the erosion of river channel in dry season. Under the combined effect of sediment load reduction, sediment thinning and adjustment of water flow by reservoirs, the downstream channel of the Jinsha River exhibits a considerable channel erosion phenomenon, which is common in the downstream of the Three Gorges Reservoir [
59]. The reach between the Xiangjiaba Reservoir and Yibin City is 29.8 km. Statistics show that the total channel erosion in this reach is 22,244 × 10
3 m
3, and the intensity of erosion is 746 × 10
3 m
3/km. Most erosion locations are concentrated in the main channel of the river bed. The cross section of the Xiangjiaba is taken as an example (
Figure 5b), and the cumulative maximum depth of erosion reaches 7.3 m.
4.3.3. Effect on Sediment into the Three Gorges Reservoir
Presently, the Three Gorges Reservoir is the largest in the world [
60]. The sum of sediment load of Zhutuo Station, Wulong Station and Beibei Station in the upper reaches of the Yangtze River can be roughly equal to that of into the Three Gorges Reservoir. According to statistics, the total sediment load into and out of the Three Gorges Reservoir (
Table 7) were approximately 2160 × 10
6 t and 520 × 10
6 t from 2003 to 2015, respectively. Thus, the total and annual average sediment load retention by the reservoir were 1640 × 10
6 t and 126 × 10
6 t/y in the corresponding time. However, the actual annual average sediment load retention by the reservoir is only approximately 40% of that in the predicted study [
61,
62].
The Jinsha River is the main source of sediment into the Three Gorges Reservoir. During the predicted study stage of the Three Gorges Reservoir, the sediment load of the Jinsha River accounted for 46.1% of the sediment load into the Three Gorges Reservoir. Before the operation of the Xiangjiaba and Xiluodu Reservoir, the annual average sediment into the Three Gorges is approximately 202.71 × 106 t/y from 2003 to 2012, which is 62.7 % less than that in the predicted study stage. The annual average sediment load from the Pingshan Station is approximately 141.63 × 106 t/y, which is 43.6% less than that in the predicted study stage from 2003 to 2015. The sediment load from the Jinsha River decreases considerably due to the operation of the Xiangjiaba and Xiluodu Reservoir. The annual average sediment load into the Three Gorges Reservoir decreases by 417.16 × 106 t/y, 488.56 × 106 t/y, and 511.98 × 106 t/y in 2013, 2014, and 2015, respectively, compared with the annual average sediment load of feasibility study stage. The reduction of annual average sediment load from the Jinsha River is 248.97 × 106 t/y, 248.79 × 106 t/y, 250.4 × 106 t/y, and that account for 59.7 %, 50.9 %, 48.9 % of the reduction sediment load into the Three Gorges Reservoir in corresponding time. Thus, the sediment load reduction in the Jinsha River Basin is an important factor for the regulation of sediment load reduction into the Three Gorges Reservoir.