Statistical results show that the mean lake inflow increases by 21%, during the dry period, and the mean Yangtze River discharge rises by 14% (Fig. 4c and 4d). It is puzzling why the water level of Poyang Lake has decreased (Fig. 4a and 4b) even though the river-lake discharge has increased. The seemingly contradictory phenomena virtually implied that the dry-season water level reduction of Poyang Lake was consider to be caused by the river-lake system’s channel change.
Yao et al. (2018) applied a physically based hydrodynamic model to calculate the response of water level decline to the bathymetric changes of Poyang Lake during the dry season. This study further develops previous work by involving the effect of the Yangtze River channel change on the dry-season water level reduction and simplifies the analysis tools by using a data-driven LSTM model. The unevenly spatiotemporal distribution of water level changes during the dry periods are the results of the RLWS changes involving lake bottom topography and river channel changes owing to natural evolution and anthropogenic activities. It is known from previous researches that Poyang Lake is being in a relatively stable tectonic development stage (Wu et al., 2015). Therefore, the main factors affecting bottom topography of the area must be attributed to scouring and siltation alterations of the river-lake watercourse system under the complex interaction among hydrodynamic, sediment transport and human activities (Gao et al., 2014; Wu et al., 2015).
The bottom topography change of Poyang Lake plays a leading role in declining water level, especially in the northern outlet channel. On the one hand, the decreasing sediment load into Poyang Lake slowed down deposition of five rivers’ alluvial cones in recent years. The implementation of the policy of converting cultivated farmland to lake has weakened sediment concentration, controlled soil erosion and intercepted sediment load (Feng et al., 2011; Min et al., 2011). Gao et al. (2014) stated that the average annual sediment load of five rivers decreased by 64% during 2003–2010, compared to the reference during 1956–2002. On the other hand, severe scouring from central lake to northern outlet increases the sediment load entering into Yangtze River and decreases the central-to-northern elevation of lake bottom topography. Since the government banned sand mining in the Yangtze River, sand mining was mainly concentrated in the northern river-lake connected channel during 2001–2007, then expanded to the lake's central part recently (Wu et al., 2007). According to statistics, the average dredging depth was 4.95 m and the accumulated sand mining volume was up to 1.29×109 m³ by 2010 (Jiang et al., 2015). The intensification of sand mining strengthened the hydrodynamic conditions in the central lake, deepen and widened the lake outflow channel, and increased suspended sediment content at Hukou (Hu et al., 2011; Lai et al., 2014d; Yao et al., 2019). Yao et al. (2018) resulted that the bed erosion of the northern outlet channel averaged 3 m, decreasing the water level by 1.2–2 m in the northern channels and increasing the average annual outflow by 6.8%. Zhang et al. (2016b) pointed out that the difference between Poyang Lake’s sediment input and sediment output converted from positive before 2000 to negative in recent years.
Meanwhile, the channel morphology of Yangtze River has been subject to considerable down cutting since the TGD operation, further accelerating flow and sediment discharge capacities of the lake into the river. As the results of sediment retention and clear water releasing by the TGD, the riverbed turned from depositional before the dam construction to erosional afterwards, leading to varying degrees of channel sinking along the thalweg throughout the main river course (Dai and Lu, 2014; Lai et al., 2014b; Yang et al., 2014). It is worth mentioning that the degree of erosion was related to the distance to the TGD. The sediment carried by the upstream streamflow has been heavily trapped as it passed through the Three Gorge Dams, only transporting 21%, 47% and 56% of the original sediment load at Yichang, Hankou and Datong, respectively (Zhao et al., 2017). Dai and Liu (2013) defined the rate of erosion as the amount of erosion per unit length of the river, quantified the average erosion rate from Hankou to Hukou (0.1–0.2 ×106 t/km) and from Hukou to Nanjing (smaller than 0.1 ×106 t/km), and classified the former as intermediately erosional and the latter as weakly erosional. This was supported by our findings in this study (Fig. 6).
Up to now, we have known that the dry-season water level variation, caused by the river-lake watercourse system change, showed an unevenly decreasing trend across the Poyang Lake, no matter under which water condition. In 1956–2000, the mean dry-season waler levels at Duchang, Xingzi and Hukou decreased by 1.70 m, 1.12 m and 0.36 m, respectively. In 2006–2016, the mean dry-season waler levels at Duchang, Xingzi and Hukou decreased by 1.62 m, 1.24 m and 0.38 m, respectively. The channel morphology variation of Yangtze River led to a water level reduction of about 0.6 m at the lake outlet. The hydraulic connection between the Yangtze River and the lake decreased with the increasing distance from the lake outlet, thus the influence of the Yangtze River channel morphology variation on the water level of Poyang Lake weakened correspondingly (Li et al., 2016). Yao et al. (2018) deemed that the bathymetric changes of the lake caused mean water level to decrease by 1.2–2.0 m in the northern lake, during the low water level period. In a word, the bottom topography alteration of Poyang Lake makes a dominating contribution to decline the central and northern lake water levels, while the impact of the Yangtze River’s channel change on the water level reduction cannot be ignored.