Research On Water Environment Capacity And The Lowest Ecological Water Level,Changhu Lake

Lake is an important part of surface water resources.Due to the rapid economic development and the overexploitation of lakes,lakes have generally experienced problems such as degradation,drastic reduction of water volume,and water pollution.Changhu Lake is the third largest freshwater lake in Hubei Province,and it is the main reservoir lake of the Four Lakes watershed.Since the 1950s,the water quality of Changhu Lake has been deteriorating and the water ecology has been severely damaged.The lowest ecological water level of the lake is the basic requirement of the health of lake ecosystems.Water Environment Capacity accounting is the scientific basis for ensuring the total amount of pollutants.The control of lake pollutants and the improvement of water environment are conducive to the restoration of water ecology.The lowest ecological water level of lakes should not only consider the restoration of water ecosystems,but also ensure that the lakes have enough capacity to absorb pollution.This paper establishes the relationship between the lowest ecological water level of the lake and the water environment capacity,calculates and analyzes the water environment capacity and the lowest ecological water level of the lake,providing technical support for water pollution control and water ecological restoration.The main research work and conclusions are as follows:(1)Kriging interpolation was used to analyze the spatial distribution characteristics of water pollution in Changhu Lake.It was found that the content of the permanganate index and ammonia nitrogen in the waters of Miaohu Lake were the highest,the total phosphorus and total nitrogen were high,and the overall water quality was poor.The permanganate index and ammonia nitrogen in the waters from Phoenix Mountain to Xinyang Village are the lowest,and the total phosphorus and total nitrogen are the highest.The ammonia-nitrogen in Shanwei to Beihe waters is higher and the water quality is better.The water quality in the Dahu is the best,the ammonia-nitrogen in the south is higher,and the lake water quality is generally better.(2)The eutrophication degree of Changhu Lake was evaluated using the comprehensive nutritional status index method.It can be seen that the comprehensive evaluation results of different areas in Changhu Lake have little difference.Except that Guanjukou water area has reached the light eutrophic level,other areas are moderately eutrophic,indicating that all the waters in Changhu Lake have trophic problem.(3)Applying MIKE21 model,uniform mixing model,non-uniform mixing model,Dillon model and Hirota model to calculate the water environment capacity of Changhu Lake,and comparing the applicability of various methods.It can be found that the calculation results using the MIKE21 model is smaller than that of the uniform mixing model and greater than that of the non-uniform mixture model.The result of the uniform mixing model is much larger than that of the other two models,because it takes into account the water quality of Dahu,which has a good water quality,and the water volume is larger.However,the uniform mixing model does not consider the spatial characteristics of the contaminants.Therefore,even if the water environment capacity is high,the water quality at the outlet of the sewage outlet is poor.The non-uniform mixing model only considers the known major point source pollution and tributaries into lakes,and does not calculate other pollution,so the results are smaller than the actual situation.When the water quality of the Dahu reaches the water level ?,the large-area waters of Changhu Lake calculated by the MIKE21 model have exceeded the standard of Class III water quality.In the MIKE21 simulation,the problem of non-point source pollution was also considered,but its calculation not only considered the spatial distribution of pollutants,but also considered the water quality of the entire lake.In comparison,the results were more reasonable.(4)The lowest ecological water level of Changhu Lake was calculated using lake morphology analysis method and natural water level data method,which were 25.65m and 28.64m respectively.According to the analysis of water level characteristics,28.64m is the extreme minimum,and 25.65m is much smaller than the multi-monthly average water level of 29.87m.And their water environment capacity cannot meet the sewage demand.Therefore,these two water levels are not suitable as the lowest ecological water level of Changhu Lake.Using the M-K mutation test,it was found that there was a sudden change in the water level of the Chang Lake in 1993.Therefore,the minimum monthly average water level after 1994 was selected as the minimum ecological water level of Changhu Lake.There is a clear seasonal pattern in the water level of Changhu Lake,so this paper proposes the minimum ecological water level for Changhu Lake each month.