The Characteristics And Response Mechanism Of Hydrology And Hydrodynamics And Eutrophication In Poyang Lake

With the change of River-Lake relationships,the hydrology and hydrodynamics of Poyang Lake show obvious adverse variation and the water quality process present new feature,which become the focus of attention and provide a typical case for establishing the congnition of hydrology and hydrodynamics and eutrophication based on the the theory of ecological hydraulics in large river-communicating lake.The paper focuses on the ecological hydraulics problem of the response mechanism of hydrology and hydrodynamics and eutrophication in Poyang Lake.Based on the systemed measured data and the Poyang Lake hydrodynamics and eutrophication model eatablisid in the paper,the response mechanism of hydrology and hydrodynamics,the water quality process and the evolution law of eutrophication under the low water level regulation are mainly discussed.The main conclusions are as follows:(1)The quantitative analysis of the characteristics of hydrology and hydrodynamics and the response mechanism to the variation of River-Lake relationshipsThe analysis method of the hysteresis of water area-stage curve is established,the Poyang Lake hydrodynamics model is constructed,and the characteristics of hydrology and the response mechanism to the variation of River-Lake relationships in Poyang Lake are quantitatively analyzed.The water level of the river lacustrine phase transition in the waterway and the southern main zone based on the change of the water area is proposed.The 8m water level of Xingzi station can be used as the transition level for the waterway;10m can be used in the water-falling period for the southern zone,and 8m can be used in the water-rising period.Compared with the situation of 1956?2002,the low water level problem is serious under 2003?2016,the water level drops obviously in October,the decline of stations is 1.28?2.33m,the increase of the rate of water withdrawal is 0.09?0.062m/d;the time of the water level below 10m in Xingzi station is advanced 30d,the duration time is prolonged 48d;the average flooded days in the lake is reduced 15d.The hysteresis of water area-stage curve is obvious in Poyang Lake,and three different hysteresis directions are found,including counter-clockwise,clockwise,and splayed,and the hysteresis in Tangyin is the weakest.The reason for hysteresis is that the water surface gradient during the water-rising period is larger than the water-falling period.Upstream catchment inflows and Hukou stage reflecting the downstream condition are the main factors that influence hysteresis.Catchment inflows are positively related with the hysteresis and the Hukou stage is the opposite.Compared to the situation of 1956?2002,the hysteresis increase under 2003?2016 and the enlargement range is 0.006?0.046.The concept of water age is introduced to analyze the water exchange capacity of Poyang Lake,and the quantitative analysis of the hydrodynamic characteristics and the response mechanism to the variation of River-Lake relationships are made.The annual average water age in Hukou station is 16.36d,and the water age of summer,autumn winter and spring is 22.55,22.36,10.69 and 9.66d respectively.The water age in summer and autumn is obviously greater than that in winter and spring.The space-time difference of water age in the lake zone is obvious,the water age of the main river is smaller,and the water age of the beach,the inner-lake and the lake bay area is larger.The catchment inflow and Hukou stage are the main factors that affect the water age.The larger the inflow is,the smaller the water age is,and the opposite is the Hukou stage.Compared with 1956?2002,the annual mean age of Hukou under 2003?2016 decreased by 4.85d;the water age of some inner-lakes is increased because of the early departure from the main river.(2)The quantitative study of the characteristics of eutrophication and responce to the variation of hydrodynamic conditions in Poyang LakeBased on the long sequence measured water quality data,the eutrophication characteristics of Poyang Lake are quantitatively studied.The concentrations of TP,TN,NH3-N in the dry season are generally bigger than those in the wet season,and the difference between CODMn is not obvious.The chlorophyll concentration in the summer and autumn is higher than those in the winter and spring.The nutrient concentration,algal concentration and comprehensive trophic state index in the southern zone are generally higher than those in the waterway.In 2011 and 2012,Poyang Lake is mildly eutrophic,and 2013?2016 is the middle nutritional state and the 2014?2016 year shows an upward trend.The eutrophication model of Poyang Lake is built,and the reponse of eutrophication in Poyang Lake to the change of hydrodynamic condition is quantitatively analyzed.Compared with the situation of 1956?2002,the average annual concentration of TP in each control station under 2003?2016 increase 10%?18%,and the average annual concentration of TP increase 7%?15%;the algae concentration in each station decrease and the annual average algal concentration decrease 9%?19%;the nutritional grade increase,and the annual comprehensive nutritional status index of each station increase 05?1.1.The identification method of eutrophication risk region based on the water age is proposed.The results show that the lake bay area in the eastern lake,two lake forks in the southern lake2 and the inner-lake in the western lake are the regions with greater risk of eutrophication.(3)The quantitative analysis of the evolution law of hydrology and hydrodynamics and eutrophication in Poyang Lake under the low water level regulationBased on the established Poyang Lake hydrodynamic and eutrophication model,the evolution law of Poyang Lake hydrology and hydrodynamics and eutrophication in Poyang Lake under the low water level regulation is analyzed by typical year scenario analysis.Water level control measures can lift the water level of dry season and alleviate the problem of dry water problem to a certain extent.The average water level of Xingzi station raise 3.06,1.96 and 1.31m in the three typical years.The average surface area increase by 207,210 and 194km2,respectively;The average flow velocity in the waterway decrease 56%,44%and 15%respectively,the change of velocity in the southern lake zone is not obvious,the flow pattern in the whole lake is not changed;the average flooded days increase by 15,15 and 14d,respectively.Water level control measures change the water area-stage relationship during the regulation period,and exacerbated the hysteresis.The regulation of water level will increase the water age of Poyang Lake and reduce the exchange capacity of water in the lake area.The average water age of Hukou increase by 4.02,4.85 and 3.74d respectively during the regulation period.The regulation of low water can reduce the concentration of TP and TN,the influence of the waterway is obvious,the influence of the main lake in the south is limited;the concentration from Septemper to November decrease obviously,the concentration after December is basically the same as before the construction;the average concentration of TP during regulation period decrease by 0-10%and TN concentration decrease by 0?3%.The concentration of nutrients decrease,but the water age increase,and the chlorophyll concentration increase.The change from Septemper to November is more obvious;the increase of chlorophyll concentration in the regulation period is 0-14%.The impact of regulation on the comprehensive nutritional status index is not obvious,with a variation range of-0.4?0.35.