Search

The severe and frequent drought in Poyang Lake, the largest freshwater lake in China, has seriously affected the local water resources security and wetland ecological security. Especially, the impoundment of the Three Gorges Reservoir (TGR) in the Yangtze River further aggravated the drought in lake. To explore the approaches to alleviate drought in Poyang Lake, this study simulated and quantified the influences of increasing water release of the TGR during the dry periods on water regimes of Poyang Lake, including the lake water level, duration of low water, flow velocity, flow field of lake, as well as potential suitable habitat area of migratory birds, based on a hydrodynamic model. Major results showed that the increase of water release of the TGR raised the lake water level, especially in the northern lake area, with the average lake level increase of 0.2–1.08 m and the lowest lake level increase of 0.16–0.68 m. Meanwhile, the duration of low water was significantly shortened, from 127 days to 87–116 days (threshold lake level of 12.0 m) at Xingzi station. In addition, the hydrodynamic simulations revealed that the increasing water release of the TGR reduced the lake flow velocity and discharge of Poyang Lake, which decreased the outflow of lake by 1.66–17.87 × 108 m3, accounting for about 0.47%–5.11% of the total outflow in the dry season. Finally, the potential suitable habitat area with 0.2–0.6 m water depth increased by 1.7–27.9 km2 in December–February, and showed a significant expansion trend in the central area of Poyang Lake. Outcomes of the study are useful for understanding the influences of water release of the TGR on water regimes of downstream lakes, and can provide valuable references for mitigating lake droughts in complex river–lake system. [ABSTRACT FROM AUTHOR]

[Display omitted] • Water release from local reservoirs significantly raised the lake level in dry season. • Release of local reservoirs decreased the duration and severity/intensity of drought. • Exposure duration of wetland was shortened and the area with long duration decreased. • Joint operation of local reservoirs and the TGR has further alleviated the droughts. Lake is an important component of the Earth's surface water bodies. Frequent and serious drought events in lakes have affected the security of regional water resources and caused the degradation of the lake ecosystem. Poyang Lake, the largest freshwater lake in China, has been suffering from increasingly severe hydrological droughts. This study simulated and investigated the effects of water release from local reservoirs in dry season on hydrological droughts in Poyang Lake based on a hydrodynamic model MIKE 21, and quantified the effectiveness of the joint operation of local reservoirs and the Three Gorges Reservoir (TGR). Major results showed that the water level of Poyang Lake during the dry period was significantly raised by the water release from local reservoirs, especially in the northern lake area. The average and the lowest lake level increased from 10.99 m to 11.18–11.59 m and from 9.97 m to 10.14–10.50 m at Xingzi station, respectively. Meanwhile, both the duration of low water and the drought severity/intensity (DS/DI) decreased significantly, with the duration decreasing from 128 days to 105–115 days for typical threshold lake level of 12.0 m, and the DS and DI decreasing from 176.6 m and 1.38 m/day to 117.6–155.3 m and 1.12–1.35 m/day, respectively. Moreover, the release of local reservoirs shortened the exposure duration of beach wetland, from nearly 3 months to about 1.5–2.5 months at the northern channel of Poyang Lake. The area with long exposure duration (>120 days) was also significantly reduced. In addition, the hydrodynamic simulations revealed that the joint operation of local reservoirs and the TGR further raised the average and the lowest lake level and decreased the duration of low water and drought severity and intensity. Outcomes of the study provided valuable references for mitigating lake droughts in complex river-lake-catchment system. [ABSTRACT FROM AUTHOR]

In recent years, dramatic decline in China’s largest freshwater lake, Poyang Lake, has raised wide concerns about water supply and ecological crises in the middle–lower Yangtze River reaches. To assist in resolving the debates regarding the low water regime of the lake, the current study quantitatively assessed the enhanced water level decline from the changing underlying surface in the Poyang Lake region. It is the first time that the magnitude, temporal–spatial difference, trend development and background mechanism of lake water level variation and its causes are studied comprehensively. The results revealed that the changing underlying surface in the lake region has caused an average decline of annual water level of 0.26 m ~ 0.75 m across the lake during 2000–2014, which shows great seasonal and spatial differences. The enlarged outflow cross–section due to extensive sand mining was the major reason for the effect on water level decline in the northern lake. While, increased water surface gradient should be attributed to water level decline in the southern lake. The long–term increasing trend of annual lake water level decline reflects the cumulative effects of lake bottom topography change caused by the continuous south movement of sand mining activities. [ABSTRACT FROM AUTHOR]

The relative timing of peak flows (RTPF) from tributaries has significant influence on flood occurrence at their confluence. This study is aimed at (1) analysing the characteristics of the RTPF of the 5 recharging rivers in the Poyang Lake catchment and the Yangtze River during the period of 1960-2012, and (2) employing a physically-based hydrodynamic model (MIKE 21) to quantify the effects of RTPF on flood behaviour in the Poyang Lake (the largest freshwater lake in China). The results show that short RTPF, or close occurrence of peak flows, triggers flood in the Poyang Lake more easily. More than 75% of total flood events in the study period occurred with RTPF less than 60 days, and more than 55% of the events occurred with RTPF less than 30 days. The hydrodynamic simulation revealed that the date of flood peak in the lake was postponed by 4-7 days and the flood stage raised by 0.69 m because of the delay of peak flows from the upstream rivers/tributaries. On the other hand, earlier start of the Yangtze River peak flow led to flood peak in the lake 6-13 days earlier. Additionally, the duration of high lake water levels was extended by 9-12 days when the RTPF shortened, and the flood hydrograph of the Poyang Lake changed from a flat to a flashy type. These results indicate that an enlarged RTPF between the upstream rivers and the Yangtze River could be an effective way to prevent flood disasters in the Poyang Lake, a method apparently being adopted in the operation of the Three Gorges Dam. The RTPF should be considered and integrated when developing flood prevention and management plans in the Poyang Lake, as well as in other similar regions in the world. [ABSTRACT FROM AUTHOR]

Backflow, the temporary reversal of discharge at the outlet of a lake, is an important mechanism controlling flow and transport in many connected river-lake systems. This study used statistical methods to examine long-term variations and primary causal factors of backflow from the Yangtze River to a laterally connected, large floodplain lake (Poyang Lake, China). Additionally, the effects of backflow on the lake hydrology were explored using a physically based hydrodynamic model and a particle-tracking model. Although backflow into Poyang Lake occurs frequently, with an average of 16 backflow events per year, and varies greatly in magnitude between years, statistical analysis indicates that both the frequency and magnitude of backflow reduced significantly during 2001-2010 relative to the previous period of 1960-2000. The ratio of Poyang Lake catchment inflows to Yangtze River discharge can be used as an indication of the daily occurrence of backflow, which is most likely to occur during periods when this ratio is lower than 5%. Statistical analysis also indicates that the Yangtze River discharge is the main controlling factor of backflow during July to October, rather than catchment inflows to the lake. Hydrodynamic modelling reveals that, in general, backflow disturbs the normal northward water flow direction in Poyang Lake and transports mass ~20 km southward into the lake. The effects of backflow on flow direction, water velocities and water levels propagate to virtually its upstream extremity. The current study represents a first attempt to explore backflow and causal factors for a highly dynamic floodplain lake system. An improved understanding of Poyang Lake backflow is critical for guiding future strategies to manage the lake, its water quality and ecosystem value, given proposals to modify the lake-river connectivity. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In recent years, a dramatic decline in Poyang Lake water levels and a shrinking water surface have raised concerns about water security and the wetland ecosystem. Changes in bottom topography due to sand mining activities in the lake was supposed to be one of the influencing factors of these changes. In response to this issue, the current study analyzed the change of lake bottom topography from observed digital elevation model (DEM) data, and quantitatively assessed the spatial and temporal responses of lake hydrology based on the framework of the neural network and the sediment effect was examined afterward. Results showed a total volume of 11.54 × 108 m3/year (about 0.96 × 108 m3/year or 1.58 × 108 t/year sediment) in net change of lake bottom topography in recent years, among which 97% was directly exported by commercial sand mining. During the study period, 2000–2011, intensive sand mining extended the central part of Poyang Lake and widened and deepened the outflow channel of the northern lake. This great change of lake bottom topography caused an average annual increase of 182.74 m3/s of lake outflow and a decline of 0.23 m–0.61 m in water levels across the lake. However, lake water levels are not consistent and show remarkable spatial and seasonal differences. The effects of changes in lake bottom topography on lake hydrological processes continue to grow as sand mining activities in the lake continue. More research on the environmental impacts is required for sustainable management of the lake ecosystem. [ABSTRACT FROM AUTHOR]

The hydrological regime of a lake is largely dependent on its bathymetry. A dramatic water level reduction has occurred in Poyang Lake in recent years, coinciding with significant bed erosion. Few studies have focused on the influence of bathymetric changes on the hydrological regime in such a complex river-lake floodplain system. This study combined hydrological data and a physically based hydrodynamic model to quantify the influence of the bathymetric changes (1998–2010) on the water level spatiotemporal distribution in Poyang Lake, based on a dry year (2006), a wet year (2010) and an average year (2000–2010). The following conclusions can be drawn from the results of this study: (1) The bed erosion of the northern outlet channel averaged 3 m, resulting in a decrease in the water level by 1.2–2 m in the northern channels (the most significantly influenced areas) and approximately 0.3 m in the central lake areas during low-level periods. The water levels below 16 m and 14 m were significantly affected during the rising period and recession period, respectively. The water level reduction was enhanced due to lower water levels. (2) The water surface profiles adjusted, and the rising and recession rates of the water level increased by 0.5–3.1 cm/d at the lake outlet. The bathymetric influence extended across the entire lake due to the emptying effect, resulting in a change in the water level distribution. The average annual outflow increased by 6.8%. (3) The bathymetric changes contributed approximately 14.4% to the extreme low water level in autumn 2006 and enhanced the drought in the dry season. This study quantified the impact of the bathymetric changes on the lake water levels, thereby providing a better understanding of the potential effects of continued sand mining operations and providing scientific explanations for the considerable variations in the hydrological regimes of Poyang Lake. Moreover, this study attempts to provide a reference for the assessment of similarly dramatic bathymetric changes in complex floodplain lakes. [ABSTRACT FROM AUTHOR]

• Continuous high spatial–temporal resolution inundation dataset was reconstructed. • Inundation dynamics of the lake show obvious spatiotemporal heterogeneity. • The formation mechanism of two hysteretic functions was conceptually generalized. • Seasonal floodplain lakes have crucial impact on the stage-area hysteresis. • Magnitude and direction of stage–area hysteretic relationship changes with time. The hysteretic relationship between the water level and the inundated area is one of the basic non-linear characteristics of lake hydrology. However, it is difficult to obtain this relationship accurately, especially for large floodplain lakes that exhibit time-varying boundaries with rapid water-level fluctuations. Taking the largest lake-floodplain system of the Yangtze River basin – Poyang Lake and its extremely productive wetland – as an example, we investigated the spatial–temporal variation of the stage-area hysteretic relationship in large heterogeneous lake-floodplain systems by adopting the Enhanced Spatial and Temporal Adaptive Reflection Fusion Model (ESTARFM) based on the observed water levels and reconstructed high spatial–temporal resolution inundation datasets using multi-source remote sensing data. The major results indicate that the inundation dynamics in the regions of the main lake and seasonal floodplain lakes are remarkably inconsistent. Concerning the inundation behavior of the river and lake-floodplain, a conceptual model was established to explain the formation mechanism of the counter-clockwise and clockwise stage-area hysteretic relationships in the Poyang lake–floodplain system. Further investigation revealed that seasonal lakes exist widely in floodplain settings and have a crucial impact on increasing the hysteresis of upstream stations and decreasing that of downstream stations. The magnitude and direction of the stage-area hysteretic relationships varied with time in a changing environment. This study extends the understanding of the complexity of hydrological behavior in large heterogeneous lake-floodplain systems, which is of vital importance for lake water resources and ecological management. [ABSTRACT FROM AUTHOR]