STRUCTURAL EQUATION MODELING OF DYNAMICS OF NITRATE CONTAMINATION IN GROUND WATER1

ABSTRACT: Most research on the temporal aspect of nitrate pollution in water resources has focused on surface water. Comprehensive studies on the dynamics of nitrate in ground water are lacking, especially on a drainage basin scale and for relatively long periods of time. In this study, structural equation modeling is applied in investigating the influences of climate, hydrology, and nitrogen management in agricultural production on nitrate concentration in the Big Spring Basin, Iowa, over a 10-year period. The study shows that for given hydrogeological settings, nitrogen management practices and climate are the two most important factors that affect nitrate dynamics. The long-term trend of nitrate is closely related to the nitrogen input primarily determined by management practices. The potential effects of nitrogen management, however, are contingent on the variations of climate. The improvements in water quality (reduced nitrate concentration and loads) in relation to improved nitrogen management are often overshadowed by the impact of climate, especially in extremely dry or wet years. The variations of climate and hydrology have much greater impacts on the nitrate dynamics than the changes in nitrogen input. This study reveals significant seasonal variation in the relations between nitrate concentration and influencing factors, which is also closely related to the seasonal variation in climate. Assessment of management practices and resultant water quality should consider the impact of short- and long-term climate dynamics.     

重庆地区土地人口承载力 SD 模型研究

本文建立了重庆地区土地人口承载力的系统动力学模型,模型包括人口、土地利用、种植业、渔业、林业、畜牧业和粮食储备7个子系统。作者分析了模型的主要结构及其功能,并对模型进行了仿真模拟,同时提出政策建议。本文模型可作为决策者进行决策分析的工具。     

THE EFFECTS OF DIURNAL MIXING ON THERMAL STRATIFICATION OF STATIC IMPOUNDMENTS1

ABSTRACT: The time variation in the temperature distribution of a static water impoundment is predicted. The body of water is modeled as a discrete number of horizontally isothermal layers and the energy equation is solved using an implicit numerical scheme. Vertical energy transport mechanisms included are solar absorption, molecular diffusion, and convective mixing due to nocturnal turnover. The latter mechanism, called diurnal mixing, is found to have a profound effect on the stratification, particularly in the eplimnion patterns for a typical deep static impoundment.     

Modeling hydrology and eutrophication in a Louisiana swamp forest ecosystem

The EPA Storm Water Management Model (1971) was used to model hydrodynamics, nutrient dynamics, and eutrophication in a Louisiana swamp forest ecosystem. The present system of canals and spoil banks in the swamp causes impoundment of swamp areas and does not optimize discharge from the swamp forest. Simulations showed that hydraulics could be managed to increase discharge rates to the lower estuary (22 percent), to increase productivity of the swamp forest (100 percent), and to decrease lake eutrophication (43 percent). This could be done by removing spoil banks in the swamp and allowing upland runoff to pass through the backswamp.     

Salt marsh vegetation change in response to tidal restriction

Vegetation change in response to restriction of the normal tidal prism of six Connecticut salt marshes is documented. Tidal flow at the study sites was restricted with tide gates and associated causeways and dikes for purposes of flood protection, mosquito control, and/or salt hay farming. One study site has been under a regime of reduced tidal flow since colonial times, while the duration of restriction at the other sites ranges from less than ten years to several decades. The data indicate that with tidal restriction there is a substantial reduction in soil water salinity, lowering of the water table level, as well as a relative drop in the marsh surface elevation. These factors are considered to favor the establishment and spread ofPhragmites australis (common reed grass) and other less salt-tolerant species, with an attendant loss ofSpartina-dominated marsh. Based on detailed vegetation mapping of the study sites, a generalized scheme is presented to describe the sequence of vegetation change from typicalSpartina- toPhragmites-dominated marshes. The restoration of thesePhragmites systems is feasible following the reintroduction of tidal flow. At several sites dominated byPhragmites, tidal flow was reintroduced after two decades of continuous restriction, resulting in a marked reduction inPhragmites height and the reestablishment of typical salt marsh vegetation along creekbanks. It is suggested that large-scale restoration efforts be initiated in order that these degraded systems once again assume their roles within the salt marsh-estuarine ecosystem.     

基于系统动力学的地铁火灾调度安全模型研究

全自动驾驶已经成为城市轨道交通建设发展的必然趋势,车辆火灾场景是影响列车全自动运行安全最大的场景之一,因此制定合理的火灾调度安全控制方案具有极其重要的意义。以地铁全自动驾驶中车辆火灾场景为研究对象,将系统动力学理论方法用于构建调度安全影响因子模型。与传统的地铁安全模型不同,该模型以全自动驾驶场景为标准形成影响因子库。通过德菲尔法相关思想的专家评分法确定各子系统权重比,利用Vensim-PLE软件对模型进行仿真试验,对各影响因子进行动态预测,对比不同的安全投入方案及调度人员配置比方案,优选出最有效的火灾调度安全控制方案。     

复杂适应系统视角下矿工自制力影响因素研究

为了提高矿工自制力水平、进一步减少煤矿事故中的人因事故,基于复杂适应系统理论及系统动力学知识,通过Vensim PLE软件研究矿工主体与安检人员、环境、工作任务特征3个系统之间的相互作用,并从矿工个体角度构建自制力影响因素仿真模型。研究结果表明:矿工情绪对自制力提升最为显著,疲劳程度对矿工自制力的抑制作用较突出,过高或过低的自信心水平均会对矿工自制力产生削弱影响;因此,改善工作环境、提高整体组织氛围、保证矿工的情绪稳定性以及通过改善作业制度降低疲劳程度对提高自制力最为有效。煤矿企业应根据各因素在工作时间段的不同作用,结合矿工自身生理心理因素,采取相应措施以提高矿工自制力水平。     

近20年来康定市景观格局演变及其驱动力分析

为了研究近20年来康定市景观格局的演变并分析其主要驱动力,基于1990、2000、2010年的遥感解译数据,在类型和景观水平上计算分析了近20年来康定市的景观格局指数变化、景观类型转移和质心偏移,探讨了康定市景观格局演变的主要驱动力。结果表明:1)康定市景观格局中林地景观和草地景观所占面积最大,为主要优势基质景观类型,对维持区域生态系统稳定起决定性作用,其次依次为其他景观、水域景观、农田景观和人文景观;2)20年来康定市景观破碎化程度有所增加,景观形状整体上趋于简单化,景观连接性和多样性增加,但景观均匀度下降;3)1990—2010年人文景观和林地景观面积增加,而农田景观面积降低,其他类型面积变化较小,农田景观和人文景观受人类干扰较大,破碎化程度增加,斑块形状趋于简单;4)1990—2000年景观类型面积基本未发生转移,2000—2010年农田景观和草地景观向林地景观转移,草地景观和林地景观向人文景观转移;5)近20年来人文景观类型向西偏移了0.058°,向北偏移了0.026°,农田景观类型向西偏移了0.033°,向南偏移了0.014°,而林地、草地、水域和其他景观类型的质心几乎未发生偏移。近20年来康定市景观格局变化是自然因素和人为因素综合作用的结果,特别是社会经济、政策和人口增长等人为因素是康定市景观格局演变的主要因素。     

空中危险接近的系统动力学机理及仿真研究

为了探索空中危险接近的形成过程与反馈机制,运用系统动力学的方法深入分析了空中危险接近影响因素间的复杂相互作用和回馈效应,建立了空中危险接近发生机理的动态模型,包括管制服务、人力资源和安全三大子系统,利用Vensim软件对其进行模拟仿真,可预测不同场景下空中危险接近的变化趋势。结果表明:乐观场景时,2005—2015年空中危险接近数量呈缓慢增长趋势,2015年之后保持不变,这表明在国内航班量稳步增长时,空中危险接近事件的数量不变,事故率越来越低;悲观场景时,空中危险接近数量一直处于上升趋势,2021年之前缓慢增长,这表明虽然设备故障率和管制员的防撞培训频率不高,但缩短TCAS(Traffic Alert and Collision Avoidance System,空中交通预警和防撞系统)反应时间能在短期内明显提高管制员的专注程度,从而延缓空中不安全事件的数量增长,但随着2021年后指挥架次的增加,反应时间越短,空中危险接近事件会越多。     

胞外聚合物对活性污泥吸附生活污水碳源的影响

为探索活性污泥中不同层胞外聚合物(SB-EPS、LB-EPS和TB-EPS)对污泥吸附性能的影响,以培养污泥、污水厂污泥为研究对象,采用加热法分层提取EPS,研究不同层EPS对生活污水中COD的吸附特征,并采用Lagergern单层吸附动力学模型、Ritchie双层吸附模型和颗粒内扩散模型方程进行动力学数据分析.结果表明:两种污泥的原污泥、-SB污泥和培养-LB污泥与Lagergern单层吸附模型和Ritchie双层吸附模型的拟合效果均比较好,说明污泥吸附过程既存在单层的物理吸附过程,又存在多层的物理化学吸附过程.污泥从外层到内层,EPS含量逐步增加,蛋白质与多糖比值均为外层小于内层,但培养污泥LB-EPS中蛋白质与多糖比值大于TB-EPS.两种污泥单位SB-EPS的吸附量分别为0.650 mg·mg~(-1)、3.37 mg·mg~(-1),吸附速率比原污泥分别增加了0.0997 min-1、0.0390 min-1,因此,SB-EPS可吸附有机污染物,但吸附速率较小.两种污泥单位TB-EPS的吸附量分别为1.06 mg·mg~(-1)、0.443 mg·mg~(-1),吸附速率比原污泥分别增加了10.7 min-1、0.183 min-1,因此,TB-EPS结构紧密,导致吸附速率减慢,但其能很好地将吸附上的污染物保存在菌体细胞壁外而不被释放.而LB-EPS对污染物没有储存能力,只有快速传递污染物的能力.