Effect Of Different Outflows From The Three Gorges Reservoir On The Spawning Hydrodynamics Of Sedentary Fish In Dongting Lake

The operation of the Three Gorges Reservoir has changed the hydrological and hydrodynamic conditions and ecological environment of the Dongting Lake area,thereby affecting the habitat of the sedentary fish spawning in the lake area.Therefore,this paper studies the influence of the Three Gorges Reservoir on the hydrodynamics of sedentary fish spawning.Through historical data collection,data analysis,numerical simulation and other methods,the range of hydrodynamic demand parameters for the natural reproduction of sedentary fish in Dongting Lake was obtained,and the habitat suitability curve of the sedentary fish during spawning and breeding period was drawn.The model builds a habitat model for ecological fluxes required for sedentary fish spawning in Dongting Lake.On this basis,the quantitative relationship between different outflow flows of the Three Gorges Reservoir and the weighted available area of sedentary fish spawning habitats was established,and then the ecological flow suitable for sedentary fish spawning was obtained.The effects of different discharge flows from the Three Gorges Reservoir on the spawning habitats of sedentary fish were analyzed.The main contents and conclusions are as follows:(1)Analyze the impact of the operation of the Three Gorges Project on the water level of Dongting Lake.The correlation analysis was carried out on the driving factors of the water level of Dongting Lake,and the discharge flow of the Three Gorges,Qingjiang River and the"four rivers"(Xiangjiang,Zishui,Yuanjiang,Lishui)were selected as input variables,and the improved LSTM model was used to predict the water level of representative sites in the lake area(Chenglingji,Lujiao,Yangliutan,Nanzui).Aiming at the problems of the traditional LSTM neural network in the water level prediction,the network training is blocked,the generalization ability is weakened and the hyperparameter selection efficiency is low.The LSTM network is improved.Based on the improved model,the annual average water level simulation error of each station does not exceed 0.05m,and the maximum error is 0.24m.The overall prediction result of the model has a high accuracy.Compared with the traditional LSTM model and BP neural network,the average root mean square error of the improved LSTM model is reduced by 58.8%and 65.95%respectively;the average absolute error is reduced by 60.62%and 61.15%respectively;the prediction time is shortened by 62.1%compared with the traditional LSTM model.Overall,the improved LSTM model has higher water level prediction accuracy,and its comprehensive ability is better than the other two models..(2)Simulation of sedentary fish spawning habitats in Dongting Lake.By collecting the spawning location,time,quantity and corresponding hydrodynamic conditions of sedentary fish,the habitat suitability index is correlated with the hydrodynamic factors(water depth,flow velocity)of the sedentary fish habitat,and the suitability standard is formulated and quantified.The preference degree of sedentary fish to habitat hydrodynamic factors was obtained,and the habitat suitability index of the influencing factor was obtained,and the habitat suitability curve of sedentary fish during spawning and breeding period was drawn.It is clear that the appropriate velocity range for spawning of sedentary fish in Dongting Lake was 0-0.6m/s,the optimum velocity range was 0.1-0.5m/s,the appropriate depth range was 0.5-2.0m,and the optimum depth range was 0.8-1.5m.(3)The effects of different discharge flows from the Three Gorges Reservoir on the spawning habitats of sedentary fish were analyzed.Based on the constructed physical habitat model,the changes of water level and flow velocity in the Dongting Lake area under different discharge flows from the Three Gorges Reservoir were calculated,as well as the corresponding weighted available area of sedentary fish spawning habitats in Dongting Lake,and then the sedentary fish was quantified.The ecological flow for spawning is suitable,and the impact of the operation of the Three Gorges Reservoir on the spawning habitat area of sedentary fish in the Dongting Lake area is quantitatively analyzed.The water levels of Chenglingji and Lujiao in East Dongting Lake increased by 2.82m and 2.41m respectively,and the average flow rate in the lake area increased by 0.15m/s;the water levels of Yangliutan and Nanzui increased by1.36m and 1.79m respectively,and the average velocity of Dongting Lake in the south and west increased by 0.13m/s and 0.14m/s respectively.With the increase of the outflow flow of the Three Gorges,the weighted usable area of the habitat of the sedentary fish shows a trend of first increasing and then decreasing.The curve analysis shows that the suitable ecological flow range of the sedentary fish is 18500m~3/s-24500m~3/s,the most suitable ecological flow is 22500m~3/s.The variation trend of the weighted usable area of different lake areas and the response trend to the outflow of the Three Gorges Reservoir are significantly different.The weighted usable area of East Dongting Lake showed a gradual upward trend,and the overall growth rate of the weighted usable area of South Dongting Lake is not large,showing a double peak curve.The weighted usable area of the West Dongting Lake showed a trend of first increasing and then decreasing,and its overall change range was obvious.The research results are of great significance for improving the natural proliferation level of fish and protecting the fishery resources and biodiversity of Tongjiang lakes.