Optimal Allocation Of Water Resources In The Central City Of Pingliang Based On Carbon Emission Reduction And Sponge City

On 22 September 2020,General Secretary Xi Jinping proposed at the 75th session of the United Nations General Assembly that"China will increase its independent national contribution,adopt more vigorous policies and measures,strive to peak CO2 emissions by2030 and work towards achieving carbon neutrality by 2060".Urban water resources emit a large amount of greenhouse gases in the process of water extraction,supply and use,so introducing carbon emission optimisation targets in the process of optimising water resources allocation will not only effectively reduce carbon emissions,but also play an extremely strong role in demonstrating carbon emission reduction.Since 2016,China’s sponge cities have gradually evolved from pilot to region-wide promotion of sponge city construction demonstrations.In May 2022,Pingliang was selected for the second batch of national region-wide promotion of sponge city construction demonstrations,which put forward certain requirements on the recycled water reuse rate and rainwater utilisation rate in Pingliang.Therefore,the integration of reclaimed water and rainwater and other non-conventional water resources into the integrated and optimal allocation of urban water resources is an inherent requirement for systematically promoting the construction of sponge cities.Based on this,this paper takes the central urban area of Pingliang City as the research object,considers the water distribution relationship between various water-using sectors and water supply sources,and establishes a multi-objective optimization model for the integrated allocation of conventional and non-conventional water resources in the central urban area of Pingliang City,taking carbon emissions,economic benefits,social benefits,wastewater emissions and groundwater abstraction as the objective funcstions.The advanced NSGA-Ⅲalgorithm is used to solve the model and determine the water resources allocation plan for each planning year under different guarantee rates based on the generated Pareto optimal solution set and multi-objective planning characteristics.The aim is to achieve effective and rational use of conventional and non-conventional water resources while reducing carbon emissions,and to promote the sustainable development of water resources in the central city of Pingliang.The main research elements are as follows:(1)Analysis of the current situation and degree of exploitation of conventional and non-conventional water resources.The degree of exploitation of surface water and groundwater is6.08%and 72.71%respectively,with surface water still having a certain potential for exploitation;the reuse rate of recycled water in 2021 is only 16%;the utilisation rate of rainwater is only 0.13%,with a low degree of exploitation of non-conventional water resources.Based on the historical data of the central city of Pingliang,the water demand and available water supply under each guarantee rate in the planning year of the central city of Pingliang is predicted,and the balance of supply and demand analysis is carried out.When surface water and groundwater are supplied to various water-using sectors in the central urban area of Pingliang City,water shortage problems occur to varying degrees in each planning year,with water shortage rates of 16.07%,22.21%,25.23%and 27.81%respectively;when surface water,groundwater,recycled water and rainwater are supplied to various water-using sectors in the central urban area of Pingliang City at the same time,no shortage problems occur.(2)Establish a multi-objective water resources optimization allocation model with the objective functions of carbon emissions,economic benefits,social benefits,wastewater emissions and groundwater abstraction as the objectives and relevant constraints.The parameters of each model were determined and the NSGA-Ⅲalgorithm was used to solve and calculate the water resources optimization allocation model for the central city of Pingliang.Based on the optimization results,six optimal water resource allocation options were selected,with Option 1 focusing on carbon emissions,Option 2 on economic benefits,Option 3 on social benefits,Option 4 on wastewater emissions,Option 5 on groundwater abstraction and Option 6 on balancing carbon emissions,economic benefits,social benefits,wastewater emissions and groundwater abstraction.Based on the water resources regulations in the study area,the priority given to domestic and industrial water consumption,and the overall water shortage rate in the study area,Option 6 is recommended as the optimal water resources allocation plan.(3)Carbon emission benefit analysis of water resource optimization results for the planning year.Analyze the total carbon emissions in the past years,the total carbon emissions in the planning year and the carbon emissions in each month.The results showed that the carbon emissions per unit of water supplied in the planning year were around 1.60 kg/m~3,which was less than the average carbon emissions per unit of water supplied in the past year of 1.73 kg/m~3.The average carbon emissions per capita and the average carbon emissions per unit of area in the past year were used to calculate the carbon emissions in the planning year as the traditional allocation plan,and then compared with the optimal allocation plan.The results show that,after the optimized allocation,the carbon emissions are reduced,and the carbon emission reduction in the planning year is 28800～39800 t based on the per capita carbon emission in the calendar year,and the cumulative economy can be obtained at the trading price of 0.9807～9.9478 million yuan;based on the carbon emission per unit area in the calendar year,the carbon emission reduction in the planning year is 58400～75800 t,and the cumulative economy can be obtained at the trading price is 1.5947～16.1868 million yuan.(4)Analysis of non-conventional water resources utilization as a result of optimized water resources allocation in the planning year.After the optimal allocation of water resources,the reuse rate of recycled water in the planning years reached 50%,60%,59%and63%respectively;the utilization rate of rainwater reached 11%,14%,20%and 25%respectively.The average values of the proportion of non-conventional water use to total water supply in the central city of Pingliang in each planning year under different guarantee rates were 33.77%,36.40%,34.57%and 34.39%respectively,much greater than other regions in China.This shows that the optimal allocation of water resources has improved the comprehensive use of non-conventional water resources such as recycled water and rainwater,which is conducive to the sustainable development of water resources in the region.(5)Analysis of economic benefits and wastewater emissions as a result of the optimized allocation of water resources in the planning year.The economic benefits and wastewater discharges were compared between the previous years and the planning year.The results show that the economic benefits in the planning year increased by more than 84.20%compared to the average economic benefits in previous years,and the reduction in wastewater emissions was more than 17.41%.The economic benefits and wastewater discharges for the planning year were analyzed in terms of unit water volume,per capita water volume and unit area.The economic benefits and wastewater discharges for the planning year were calculated using the average values of unit water volume,per capita water volume and unit area for the calendar year as the traditional allocation scenario,and then compared with the optimized allocation scenario.The results show that after water resources optimization,the economic benefits are improved,with a cumulative increase in economic benefits of RMB79.3～117.8billion in the planning year,and the wastewater emissions are reduced,with a cumulative reduction of 3.7905～1.1561 million t.The results of the optimized allocation are then analyzed in terms of social benefits,water consumption and water supply.The results show that,after optimizing the allocation of water resources,the central city of Pingliang can meet the water demand under all guarantee rates in the planning year,and increase the use of recycled water and rainwater in the planning year,thus indirectly reducing the amount of groundwater abstraction and helping to protect groundwater resources.