Study On The Transport Balance Of Carbon,Nitrogen And Oxygen In Water Environment System Of Small Watershed On Monthly Time Scale

This paper studies a small watershed where the spatial scale of the confluence area is much smaller than that of the atmospheric rainfall meteorological process.The operation of small watershed water environment system is mainly driven by local rainfall,and there is no external runoff inflow.During rainfall,the runoff of small watershed rivers is large,the transport capacity and reoxygenation capacity are strong.When there is no rainfall,the runoff is small,the transport capacity and reaeration capacity are weak.The transport of carbon,nitrogen and oxygen in small watershed rivers is not uniform in time and space.This is because the carbon and nitrogen transported from land into river water may be deposited in river bed or stagnant in river water in non rainfall period,and can not be exported in time.During the rainfall period,the deposited carbon and nitrogen are washed back to the water body under the action of fast river velocity.The uneven distribution of time and space may lead to the continuous high concentration of carbon and nitrogen in the river water,thus depleting the dissolved oxygen in the water and leading to the collapse of the water ecosystem.The reason is that in the period of high-intensity rainfall,a large amount of deposited carbon and nitrogen will return to the water body,which may cause the concentration of carbon and nitrogen in the river to rise rapidly in a short period of time,and the concentration of dissolved oxygen in the water body to drop;in the non rainfall period,the carbon and nitrogen deposited at the bottom of the river bed will also slowly release due to the change of temperature or wind disturbance,consuming the dissolved oxygen in the water body.In the natural state,the deposition,input and output of carbon and nitrogen,as well as the process of water reaeration and oxygen consumption are in equilibrium.When the load of carbon and nitrogen released by human activities is large,the transportation process of water environment system may be unbalanced.Therefore,in order to control the water quality of small river basin and keep its natural transportation balance,it is necessary to fully study the transportation and deposition process of water environment system in order to formulate effective water environment control strategies.Taking the basin of Fuxi River as an example,this paper analyzes the operation mode of water environment system in Fuxi River Basin,deconstructs the basin environment and water system structure,analyzes the hydrodynamic characteristics of the main rivers in the basin,traces the evolution process of water environment system and water quality state.Setting six control sections,according to the rainfall process from 2014 to 2017,this paper calculates the time scale of the peak transport of the upstream concentration of each control section in the rainfall process of each return period and the time scale of the peak transport of each control section in the typical rainfall process,and analyzes the transport mode of carbon,nitrogen and oxygen in the basin.In the monthly time scale,the input conditions of carbon and nitrogen input sources upstream of each control section and the operation characteristics of the reoxygenation system are analyzed.On this basis,carbon and nitrogen load is measured with(COD+1.33NH3-N),and the transportation balance of carbon and nitrogen and oxygen in each control section from 2014 to 2017 is calculated.The input and output of carbon and nitrogen load and the temporal and spatial distribution of accumulation in the upper reaches of each control section under the monthly time scale are studied,and the output mode of carbon and nitrogen and oxygen in the river is analyzed.The calculation of transportation balance shows that the balance difference of the control sections of Leigongtan,Changtu River and liaojiayan with short transportation time and small confluence area is large,and the maximum balance difference is 31.01%.The balance difference of the control sections of Shuanghekou,Tanyansuo and Dengguan with long transport time and large confluence area in the upper reaches of the river is small,the maximum is 18.53%.The possible factors influencing the calculation results are analyzed.At the same time,the accumulation of carbon and nitrogen load in the upstream of each control section is calculated in the monthly time scale,and the monthly average accumulation distribution of the upstream of each control section from 2014 to 2017,maximum accumulation of carbon and nitrogen load between control sections and maximum accumulation of carbon and nitrogen load upstream of each control section are obtained.The results show that the amount of carbon and nitrogen accumulation in the channel from Shuanghekou to Tanyansuo is the largest and the maximum carbon and nitrogen accumulation is 5.6746 million kg.