Research On Cross-watershed Effluent Trading Planning Model Under Uncertainty

Multiple rivers flowing into the same bay can be correlated in water quality process and management.They together determine the environmental status of the bay.Therefore,it is necessary to use advanced water quality process simulation model,water quality process uncertainty analysis method and mathematical programming method to analyze the pollutant migration and transformation processes and conduct single-watershed and cross-watershed integrated management.It can coordinate the water quality objectives of bay and watershed,deepen the system management effect improve the scientific decision-making,and achieve optimal water quality management.The multi-watershed integrated management system involves sub-systems such as water quality management,pollutant migration and transformation and social economy.Each sub-system affects each other and contains a large number of uncertain factors,such as uncertainty of meteorology,heterogeneity of underlying surface,and randomness of nutrient loss.How to predict water quality processes and develop multi-watershed water quality planning is the key problem in the integrated watershed management considering complexities and uncertainties.In this study,two agricultural watersheds（i.e.Daguhe and Moshuihe watersheds）are selected as the study areas.TP and NH₃-N are water quality factors,and a Bayesian simulation-based multi-watershed effluent trading designing model（BS-METM）system is applied to water quality management in the two watersheds.Several works have been done:（i）The migration and transformation process of non-point source pollutants in Daguhe watershed is simulated based on SWAT model.Bayesian inference of model parameters is conducted based on MCMC method.And the uncertain non-point source pollutant loadings are simulated and predicted.The results can provide random water quality inputs for effluent trading model.（ii）The mathematical programming methods are introduced to multi-mechanism effluent trading model.Then the parameters’randomness in trading system can be addressed under three trading mechanisms:intra-watershed trading,cross-watershed trading and non-trading mechanisms.In this study,the influence of important factors（i.e.Total control of environmental allowance and the uncertain water inflow）on the effluent trading system is analyzed.The interaction between water inflow and trading mechanism is deeply discussed.The results are helpful for decision makers to obtain the optimal industry planning and the optimal effluent trading planning schemes under various uncertainties and systemic risks,providing the decision-making basis for multi-watershed management.Specific research results show that:（ⅰ）The total excess TP and NH₃-N and total TP and NH₃-N trading amounts are decreased due to the increase of the environmental allowance,thus the net system benefits of the system are increased.（ⅱ）The total trading amounts would be increased under high water availability level because of the existence of demand and more supply for TP and NH₃-N permits,leading to decreased total excess TP and NH₃-N emissions and high level of net system benefits.（ⅲ）The pollution sources in the two watersheds almost have no residual permits under the low water availability level,so it is not necessary to trading.Under the medium and high water availability level,the net system benefits of the trading mechanisms are higher than that of the non-trading mechanism,so the system is suitable for trading mechanisms（i.e.intra-watershed trading mechanism and cross-watershed trading mechanism）.（ⅳ）The net system benefits under cross-watershed mechanism are higher than that under intra-watershed trading mechanism under medium and high water availability levels.Therefore,the cross-watershed trading mechanism is recommended under medium and high water availability levels.However,under cross-watershed trading mechanism,less NH₃-N permits are withdrawn from the trading market.Therefore,the environmental allowance of NH₃-N should be reduced and strictly managed.The results can obtain the optimal watershed management scheme under various uncertainties and risks,and finally answer the scientific questions of whether or not to conduct cross-watershed effluent trading and how to do it.