Study On The Rules Of River-lake Water Exchange In Mid-lower Reaches Of Yangtze River

At present, the research on the river-lake relation is one of the popular topics which attracted various attentions from researchers and the whole society. Scientific understanding and proper management on the river-lake relation is the key to keep river healthy, as well as to build a harmonious human-water and human-land relationship. Water and sediment exchanges of river-lake system are the core issues of the relation of river-lake system, which are the bond of the evolution of the relation of river-lake system. Variations in the water and sediment and exchanges of them in the river-lake system are one of the driving forces in the evolution of the relation of river-lake system. Moreover, water exchange in the river-lake system is the carrier of the sediment exchange between rivers and lakes, which means more water more sediment and no water no sediment. Consequently, the study of water exchange in the river-lake system is the premise and basis for clarifying the problem of water and sediment exchange between river and lake, or even the relation of river-lake system.The relation of river-lake system in the Dongting Lake and the Poyang Lake at the middle and lower reaches of the Yangtze River has become the focus to properly handling the man-water relationship or even the man-land relationship of the Yangtze River. Since the First Yangtze River Forum Annual Meeting was held successfully in Wuhan City in April 2005, Protection and development declaration for Yangtze River received enthusiastic response from the whole society. At present, the ideas of 'Maintain a healthy Yangtze River, Promote harmonious man-water relationship'are being actively practised. Beyond all doubt, after the completion of the Three Gorges Project (TGD), it received remarkable achievements in controlling floods, avoiding peak discharge et al. Especially, in the flood period of 2010, when the peak flow was close to that of 1998, it effectively controlled the upper Yangtze River flooding, reduced the amount of flood in the Jingjiang reach and the Dongting lake, and mitigated the flood disaster of the Jingjiang reach. It is necessary to change the contemporary ideas on water management, and to explore a reasonable way to reach a harmonious man-water relationship in lake area, so that the potentialities of lake regulation of river-lake system can be brought into full function. Simultaneously, it is in favor of maintenance of a healthy Yangtze River arid Estuary. Basic theories of general hydrology, engineering hydrology, stochastic hydrology and water resources sicence, as well as various methods of time-series statistical analysis, Mann-Kendall tendency and mutation inspection, Wavelet Analysis etc., were applied in this paper. By using qualitative and quantitative analysis, and with the aid of computer technology, statistical analysis software SPSS 15.0, scientific calculation software Matlab R2007b, geographical information system software ArcGIS and MapINFO, drawing software CorelDRAW X3 etc., this paper, based on the analysis of water exchange between mainstream and lakes, received the rules of water exchange between the Dongting Lake and the Yangtze River as well as between the Poyang Lake and the Yangtze River, deduced a quantitative formula of river-lake water exchange intensity which has been successfully used in the research on the water exchange of the Dongting Lake and the Poyang Lake. In this paper, the author emphatically analyzed the variations in the water exchange between two lakes and the mainstream of the Yangtze River before and after the operation of the Three Gorges Reservoir (TGR) and exchange processes in special hydrologic years. The purposes are to probe the theory of the water exchange of river-lake system, to provide scientific basis and some suggestions for rational utilization of water resources, and for maintenance of a healthy relationship of river-lake system as well as a healthy estuary.ⅠThe impact of the TGR on the variations in the runoff of the Yangtze RiverSince the operation of the TGR, the proportion of the runoff of the mainstream of the Yangtze River in the dry season in that of a whole year has risen. In the meantime, the proportion of runoff in October, which is the last month of the flood season, has gradually decreased. This phenomenon was particularly prominent at Yichang station. In conclusion, the runoff of the Yangtze River has changed after the completion of the TGD. The results from these analyses on the impact of the TGR on the mainstream runoff indicate that the operation of the TGR intensified the trend of the decrease in the flood season runoff in the middle and lower reaches of the Yangtze River and enhanced the tendency of the increase in the dry season runoff. Furthermore, the jumping point of the mainstream runoff sequence in 2002 was related to the impounding of the TGR in 2003. Therefore, the operation of the TGR has changed the distribution of the runoff within a year downstream of the dam, which made the proportion of each month's runoff within a year equably.ⅡThe rules of river-lake water exchange before and after the operation of TGR i) The rules of water exchange between the Dongting Lake and the mainstream of the Yangtze River. After the operation of the TGR, the total runoff of Three Inlets during 2003-2009 decreased by 220×10~8m~3, compared with 1998-2002. The total mean flowin September and October of Three Inlets decreased by 870m~3/s during 2003-2009. The rate of decrease was 38.9%. During 2003-2009, when the flow at Yichang was 50000 m~3/s and 30000 m~3/s, the split ratio of Three Inlets increased, but when the flow at Yichang reduced to 10000 m~3/s, the split ratio of Three Inlets decreased, compared with that during 1998-2002. After the operation of the TGR, 2003-2009, compared to the previous data, the outflow of Chenglingji (Qilishan) was more affected by the waterback effect of trunk stream of the Yangtze River when the flow at Yichang was biger, while the outflow of Chenglingji (Qilishan) changed little when the flow at Yichang was medium or low, compared with that of 2000-2002.ii) The rules of water exchange between the Poyang Lake and the mainstream of the Yangtze River. After the completion of the Three Gorges Project (TGP), the annually mean discharge changed little, but proportion of the runoff in the dry season in the annual runoff of the mainstream of the Yangtze River in 2000 increased, compared with previous data. In September and October, this was also the reservoir impounding water time, the proportion of the runoff of this period within a year increased, compared with 1990s. The increase in the runoff at Hukou during September and October in 2010 was was caused by the lower water level, lower flow and weak backwater effect of the Yangtze River. The coefficient of water exchange I_p equals 0.21, without backward water from the mainstream of the Yangtze River to the Poyang Lake in this year, which also shows this point.The phenomenon of backward flow from the Yangtze River to the Poyang Lake at Hukou changed from decade to decade. The number of years and days when there was backward water declined in 2000s when compared with that during 1960s and 1980s, and the water volume of backward water also decreased simultaneously.ⅢResults of river-lake water exchange with the runoff of different frequenciesⅰ) Under the conditions with runoff of different frequencies, the water exchange between the mainstream of the Yangtze River and the Dongting Lake was computed with the water balance principle. The Four Tributaries experience 5% high flow year when the mainstream of the Yangtze River is in 5% rich water year.The water volume of flood diversion to the Dongting Lake from the mainstream is 658×10 m and the lake supplies3191×10~8m~3 of water for the Yangtze River at the same year. When Three Inlets and Four Tributaries experience 50% normal year, the water volume of flood diversion to the Dongting Lake is 545×10~8m~3 with supplying 2403×10~8m~3 of water for the mainstream, Whereas when the Three Inlets and Four Tributaries are in 95% low flow year, the Dongting Lake only supplies 1873×10~8m~3 of water for the Yangtze River.ii) Under the conditions with runoff of different frequencies, the water exchange between the mainstream of the Yangtze River and the Poyang Lake was computed by using the water balance principle. The annual runoff of Hukou is 2300×10~8m~3,1440×10~8m~3,1175×10~8m~3 and 930×10~8m~3 when the frequencies are 5%,50%,75% and 95%.IV River-lake water exchange process in special hydrological yearsi) Main conclusions on water exchange between river and lakes in the typical low flow years(1) The water exchange coefficient was 0.57 between the Yangtze River and the Dongting Lake and 0.56 between the Yangtze and the Poyang in 1978. Water exchange was stable between the mainstream of the Yangtze River and the Dongting and the Poyang Lake. The backward flow from the river to the lake appeared in this year. In 2006, the water exchange coefficients of these two systems were 0.89 and 0.51 respectively, the former showed that there was strong water supply from the lake to the river, while the latter revealed a situation of approximately stable water exchange between the river and the lake.(2)The annual runoff distribution of the middle and lower reaches of the Yangtze River became more uniform after the operation of TGR in 2006. The runoff increased, especially in dry season, which led to the water level rise in the channel, affecting the water exchange process between the river and lakes, which resulted in rich water in the Yangtze River in the dry season, compared to other low flow years.(3) 1978 and 2006 were both typical lower flow years of the Yangtze River basin, the water supply, therefore, from river-connected lakes in the middle reach to the mainstream was more obvious, especially in 2006. For instance the Poyang Lake supplied 1564×10~8m~3 of water for the river, which was much more than the normal year.ii) Main conclusions on water exchange between river and lakes in typical flood years (1) Water exchange coefficient of Dongting-Yangtze was 0.35 in 1954, and 0.34 for Poyang-Yangtze. Water exchange was strong between the both two lakes and the river, and they played the main role in flood diversion, runoff regulation and storage. The two coefficients in 1998 were 0.46 and 0.39 respectively. The strength of the Dongting-Yangtze water exchange was close to the normal year, whereas the main function, flood diversion, was much weeker than that in 1954. The situation was similar in the Poyang lake, despite that the flood diversion was relatively stronger.(2) The volume of the water supply from the Dongting and the Poyang Lake to the Yangtze River was 5248×10~8m~3 and 2481×10~8m~3 in 1954, which accounted for nearly 38.6% and 19.0% of the runoff at Datong Station during the same period. For the year of 1998, the volume of the water supply was 3994×10~8m~3 and 2650×10~8m~3, which accounted for 32.1% and 21.3% of the runoff at Datong.ⅲ) The Dongting and the Poyang Lake played an important role in water regulation and storage for the Yangtze River either in high or low flow years. In flood years, the two lakes store water and smooth the peak flow of the flood, reducing the downstream flooding threatening.In low flow years, the two lakes play a prominent role in water supply and maintaining a certain flow downstream.Ⅴ. Achievements on quantified analysis of water exchange between the river and the lakeWater exchange coefficients of the Yangtze-Dongting and the Yangtze-Poyang were able to indicate the real water exchange between the two lakes and Yangtze over the years. Therefore, it is a practical way for common use to quantitatively analyze the river-lake water exchange.