Driving Mechanism And Multi-objective Planning Of Water Environment Change In Ulansuhai Basin

Each component of the complex social-economic-ecological system in the basin mutually promotes and constrains one another,so achieving the coordinated and healthy development of social economy and eco-environment was of great significance to the social progress.This has become a hot topic in the study of the sustainable development of the basin.Water resource was one of the most important resources for human production and livelihood.Understanding the current state of the water environment in the basin,analyzing the non-point source pollution load and environmental tolerance of the basin based on the driving mechanism of pollutant concentration change in the water body,and exploring multi-objective planning and sustainable development in arid and semi-arid regions were all crucial in the context of rapid socioeconomic development and ongoing deterioration of the water environment in the basin.In this paper,Ulansuhai Basin,situated in arid and semi-arid region,was selected as the research area.On the basin scale,the response relationship and driving mechanism of lake water quality factor concentration changes with variations in water environment,land use/cover patterns,and climate factors were explored using a hybrid evolution algorithm,econometric analysis model,and generalized additive model.Furthermore,the spatial and temporal distribution patterns of non-point source pollution in the basin were analyzed based on spatial and attribute data of watersheds.The fuzzy multi-objective programming model of the basin was constructed to elucidate the solution results of social-economic-environmental objectives in different scenarios and the optimal decision of variables such as the planting area of main crops and the number of livestock under the state of coordinated and sustainable development of the basin,so as to provide theoretical support for the comprehensive management of the basin.The main conclusions of this study were as follows:1)Based on the Mann-Kendall trend test,the change rates of total nitrogen,total phosphorus and chemical oxygen demand in Ulansuhai from 2000 to 2022 were-2.377,-1.595 and-3.516,respectively,and all of them showed a decreasing trend,while the change rate of chlorophyll-a concentration was 0,showing no changing trend.The comprehensive pollution index of surface water quality in the basin was above the moderate pollution standard line(>0.7),but the lake water quality gradually improved from mild eutrophication to medium nutrition in the past 7 years.In the past 20 years,the concentration of nitrogen elements in Ulansuhai Lake has kept at a high level,and there was an imbalance of N/P.2)An innovative hybrid evolution algorithm was used to construct an"IF-THEN-ELSE"single rule set model for the water quality data of Ulansuhai Lake from 2010 to 2020.Among the 100 models constructed,the model with the highest degree of fitting was selected(R~2of total nitrogen,total phosphorus and chlorophyll a were 0.84,0.81 and 0.59,respectively).Among them,electrical conductivity,p H and transparency were the ecological thresholds selected by total nitrogen,total phosphorus and chlorophyll-a models,respectively.Sensitivity analysis showed that the total nitrogen was negatively correlated with water depth and transparency,but positively correlated with dissolved oxygen and chemical oxygen demand.At the ecological threshold p H>8.92,the concentration of total phosphorus was significantly higher than that of p H<8.92,indicating that alkaline environment was conducive to the growth of TP,and with the increase of water temperature,the concentration of total phosphorus increases rapidly.Chlorophyll-a was negatively correlated with transparency,but positively correlated with permanganate index,ammonium nitrogen and water temperature.3)Based on econometric model and generalized additive model,the effects of land use patterns and meteorological factors on lake water quality in Ulansuhai Basin in recent 20 years were explored.The concentration of total nitrogen and total phosphorus was negatively correlated with the area of forest land and water body,but positively correlated with the area of cultivated land,impervious water surface and grassland.The concentrations of dissolved oxygen,total nitrogen and total phosphorus were positively correlated with annual precipitation(P<0.1),the concentration of dissolved oxygen,chemical oxygen demand and total phosphorus had a significant nonlinear relationship with the maximum monthly precipitation(P<0.05),chemical oxygen demand and total phosphorus concentration were negatively correlated with annual maximum temperature(P<0.01 and P<0.1),chemical oxygen demand was significantly positively correlated with annual mean temperature(P<0.05).Grey correlation analysis showed that the contribution rate of human disturbance such as land use mode to water quality change was higher than that of natural factors.4)The SWAT model was used to simulate non-point source pollution in Ulansuhai Basin,and 103 sub-basins were divided.The runoff data from 2010 to2015 and 2016 to 2018 were taken as the data of model calibration period and verification period,respectively.The R~2of calibration period was 0.74,NES was0.81,and the R~2of evaluation period was 0.78,NES was 0.67.The results of SWAT model showed that the TN and TP load decreased significantly from 2010 to 2019,and the pollutant concentration in Ulansuhai Basin was the highest in spring,followed by summer,and maintained at a low level in autumn and winter.The spatial distribution of pollutants showed that the output of pollutants was lower in the northeast and higher in the southern Hetao irrigation area.5)The Dillon model and Vollenweider model were used to calculate the total amount of pollutants that Ulansuhai Lake could withstand per unit time when total nitrogen,total phosphorus and chemical oxygen demand reached Class III,Ⅳand V water quality standards respectively during 2014 to 2018.The load of TN into the lake exceeded the water environmental capacity of Class V water quality standard in5 years;the load of TP into the lake exceeded the water environmental capacity of Class IV water quality standard in 2016 and 2018;the load of TP into the lake exceeded the water environmental capacity of Class III water quality standard in the remaining three years.In 2014 and 2015,the load of COD into the lake exceeded the environmental capacity of COD water under the Class V water quality standard,and from 2016 to 2018,it exceeded the environmental capacity of COD water under the Class IV water quality standard.6)In order to strengthen ecological protection and high-quality development of Ulansuhai Basin,a fuzzy multi-objective programming model was established for it.The results showed that compared with water demand,water environmental capacity was the main factor restricting the economic development of the basin,and the proportion of cattle and pigs raised was kept at 1.5:1.When the number of sheep raised is about 20 times that of cattle,and the ratio of planting and animal husbandry is 1.14:1(mu:animal),the economic and environmental development of the basin was the most coordinated and the objective function was in line with the expected value.In conclusion,this study proposes to use hybrid evolution algorithm to simulate and predict lake water quality and analyze driving force,and use fuzzy nonlinear algorithm to solve multi-objective planning problems such as basin economy and environment,so as to provide theoretical support for the study on response mechanism of lake water quality change and environmental factors under the background of global climate change.It provides data reference and technical support for the coordinated development of regional economy and ecological environment at river basin scale.