Xuchun Ye

The influence of river-to-lake backflow on the hydrodynamics of a large floodplain lake system (Poyang Lake, China)

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

Factors influencing water level changes in China's largest freshwater lake, Poyang Lake, in the past 50 years

This study analyses the changing characteristics in the level of Poyang Lake during the period 1960–2010. Results show that the changing stage of annual lake level variations is evident, and average onset time of the lake dry season has advanced since the 1990s. Investigation indicates that the Yangtze River discharge has a greater impact on annual lake level variations than the lake’s catchment inflow. Climate change in the Yangtze River basin since the 1990s is possibly the precondition for the advance of the lake dry season, which is further aggravated by the effects of the Three Gorges Dam in the 2000s.

Investigation of the Variability and Implications of Meteorological Dry/Wet Conditions in the Poyang Lake Catchment, China, during the Period 1960–2010

This paper presents a quantitative investigation of the variability of meteorological dry/wet conditions of the Poyang Lake catchment during 1960–2010 by using the standardized precipitation-evapotranspiration index (SPEI) based on daily observations of 14 meteorological stations across the catchment. Extending from previous studies, the background of the encounter and overlap of multitimescales of meteorological dry/wet episodes for several severe drought and flood events were discussed. In addition, the possible impacts of temperature changes on dry/wet variability were also examined by the comparison of SPEI and standardized precipitation index (SPI) variations at multitimescales during the study period. Major results show that the occurrence of dry/wet condition has become increasingly frequent in the recent two decades, especially the extreme wet episodes in 1990s and the extreme dry episodes in 2000s. Historically, the encounter and overlap of multitimescales of meteorological dry/wet episodes plays an important role in the occurrence of several severe drought and flood events. The study concluded that the impact of temperature anomaly to the dry/wet variability cannot be neglected.

Capabilities of Satellite-Based Precipitation to Estimate the Spatiotemporal Variation of Flood/Drought Class in Poyang Lake Basin

Poyang Lake basin is one of the most frequently affected areas by a variety of flood or drought events in China. Satellite-based precipitation data have greatly improved their temporal and spatial resolution in recent years, but the short length of records limited their applications in some fields. This paper compared and evaluated the creditability of using a short period data series to estimate the statistics characteristics of long period data series and investigated the usefulness of TRMM rainfall data for monitoring the temporal and spatial distribution of flood/drought classes by the Z index method in Poyang Lake basin. The results show that (1) the 1998–2010 data series are sufficiently robust to depict the statistics characteristics of long period data; (2) the intra-annual distribution and interannual variability of flood/drought classes based on TRMM rainfall data matched well with the results from rain gauges data; (3) the spatial agreement between TRMM and interpolated gauges rainfall varied with the precipitation characteristics; and (4) TRMM rainfall data described the similar spatial pattern of flood/drought classes with the interpolated gauges rainfall. In conclusion, it is suitable and credible for flood/drought classes evaluation based on the TRMM rainfall data in Poyang Lake basin.

Effects of spatial information of soil physical properties on hydrological modeling based on a distributed hydrological model

The spatial distribution of soil physical properties is essential for modeling and understanding hydrological processes. In this study, the different spatial information (the conventional soil types map-based spatial information (STMB) versus refined spatial information map (RSIM)) of soil physical properties, including field capacity, soil porosity and saturated hydraulic conductivity are used respectively as input data for Water Flow Model for Lake Catchment (WATLAC) to determine their effectiveness in simulating hydrological processes and to expound the effects on model performance in terms of estimating groundwater recharge, soil evaporation, runoff generation as well as partitioning of surface and subsurface water flow. The results show that: 1) the simulated stream flow hydrographs based on the STMB and RSIM soil data reproduce the observed hydrographs well. There is no significant increase in model accuracy as more precise soil physical properties information being used, but WATLAC model using the RSIM soil data could predict more runoff volume and reduce the relative runoff depth errors; 2) the groundwater recharges have a consistent trend for both cases, while the STMB soil data tend to produce higher groundwater recharges than the RSIM soil data. In addition, the spatial distribution of annual groundwater recharge is significantly affected by the spatial distribution of soil physical properties; 3) the soil evaporation simulated using the STMB and RSIM soil data are similar to each other, and the spatial distribution patterns are also insensitive to the spatial information of soil physical properties; and 4) although the different spatial information of soil physical properties does not cause apparent difference in overall stream flow, the partitioning of surface and subsurface water flow is distinct. The implications of this study are that the refined spatial information of soil physical properties does not necessarily contribute to a more accurate prediction of stream flow, and the selection of appropriate soil physical property data needs to consider the scale of watersheds and the level of accuracy required.

A modeling study of hydrological response to landuse changes based on hypothetical scenarios for the Poyang Lake catchment

Water resources management in the Poyang Lake basin have raised the concern of how human activities affect the local hydrological cycle. To address this concern and related issues, a distributed hydrological model-WATLAC (A Water Flow Model for Lake Catchments), was applied to the Poyang lake watershed to study the hydrological response to different landuse change scenarios. Simulations of canopy interception, evapotranspiration and stream flow were modelled and compared from four different extreme landuse change scenarios which were constructed from the landuse 1996 of the Poyang Lake basin. Major results from the modelling study indicate that through interactions of vegetation with soil and surface waters, land-use change can modify the local hydrological cycle and exert a strong biological control of the annual distribution of streamflow in the basin. Due to the discrepancy and great variation of Leaf Area Index, simulated annual interception and evapotranspiration by the scenario of forest land is much larger than the scenarios of agriculture, shrub and grass lands, while stream flow is relative smaller. The increase of forest cover after returning agricultural lands to forest in Poyang Lake basin can effectively reduces the local flood risk in the lake area.

Long-term trend analysis of effect of the Yangtze River on water level variation of Poyang Lake (1960 to 2007)

The Poyang Lake water level is heavily influenced by the watershed inflows and also by the Yangtze River from the north of the lake basin. Observed river discharge and water level from the Yangtze River and the Poyang Lake catchment are used in this paper to further investigate the variation of the Yangtze River (blocking) effect on the change of Poyang Lake water level. The preliminary results indicate that the strong Yangtze River effect shows a long-term decrease trend according to the increase of warm season rainfall in the Poyang Lake basin. Poyang Lake water level increased obviously during 1960-2000, however, the pattern of annual lake water level variation was somewhat altered due to the modifications to the river-lake interactions caused by various reasons, especially after the completion of the Three Gorges Dam (TGD) project in 2003. In accordance to the regulation of the Three Gorges Reservoir, the Poyang Lake water level varied correspondingly with decrease in October-December and slight increase in January-March. In particular, the operation of the Three Gorges Dam project may promote an earlier coming of low water season for Poyang Lake, and aggravate water supply in the lake area, especially in dry years.

Testing of winter vegetation construction and water purification effect of a cascaded wetland model of Jialu river

In order to test the winter vegetation construction and water purification of wetland in vast northern China, a cascaded surface-flow wetland model with a total area of 7400m2 was constructed along the Jialu river floodplain in Zhengzhou city of Henan province. After winter vegetation such as Potamogeton crispus, Elodea Canadensis and Iris. Sibirica were planted, vegetation development and purification effect was observed continuously. Results show that Potamogeton crispus and Elodea Canadensis survived under very low temperature and the vegetation coverage stood at 50percent, Siberian Iris all survived and issued new leaves. Water purification effect indicates that the average removal rates of nitrogen, phosphorus ammonia and CODCr were 32%, 67%, 55% and 29% respectively with hydraulic loading of 140mm·d-1, and the purification efficiency can be strengthened with gradual development of the vegetation. This study demonstrated that wetland with reasonable winter aquatic plants and species composition can ultimately improve the purification effects of wetlands in north China. In addition, waste heat in urban drainage can provide favorable conditions for winter vegetation in cascaded wetland, so it is feasible to increase appropriate water depth to built submerged vegetation with Potamogeton crispus and Elodea Canadensis.

Lake flooding sensitivity to the relative timing of peak flows between upstream and downstream waterways: a case study of Poyang Lake, China

The relative timing of peak flows (RTPF) from tributaries has significant influence on flood occurrence at their confluence. This study is aimed at 1) analyzing the characteristics of the RTPF of the five 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 behavior 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.

Change of annual extreme water levels and correlation with river discharges in the middle-lower Yangtze River: Characteristics and possible affecting factors

As one of the fastest developing regions in China, the middle-lower Yangtze River (MLYR) is vulnerable to floods and droughts. With obtained time series of annual highest water level (HWL), annual lowest water level (LWL) and the corresponding river discharges from three gauging stations in MLYR that covering the period 1987–2011, the current study evaluated the change characteristics of annual extreme water levels and the correlation with river discharges by using the methods of trend test, Mann-Whitney-Pettitt (MWP) test and double mass analysis. Major result indicated a decreasing/increasing trend for annual HWL/LWL of all stations in MLYR during the study period. A change point in 1999 was identified for annual HWL at the Hankou and Datong stations. The year 2006 was found to be the critical year that the relationship between annual extreme water levels and river discharges changed in the MLYR. With contrast to annual LWL in MLYR, further investigation revealed that the change characteristics of annual HWL were highly consistent with regional precipitation in the Yangtze River Basin, while the linkage with Three Gorges Dam (TGD) operation is not strong. Our observation also pointed out that the effect of serious down cutting of the riverbed and the enlargement of the cross-section area during the initial period of TGD operation caused the downward trend of the relationship between annual LWL and river discharge. Whereas, the relatively raised river water level before the flood season due to TGD regulation since 2006 explained for the changing upward trend of the relationship between annual HWL and river discharge.