典型桥墩大跨梁桥施工全过程抗震性能综合决策∗

在满足永久状态桥梁抗震设计要求的前提下,针对典型桥墩大跨梁桥施工全过程抗震设计的难题,通过综合考虑施工全过程抗震设计中的各种关键因素,联合应用能力需求比法和层次分析法,建立施工全过程抗震性能综合决策的框架。然后提出合理的决策策略,定量决策出典型桥墩大跨梁桥施工全过程综合性能最优的抗震设计方案。研究结果表明,在施工全过程的抗震能力满足要求的条件下,典型桥墩大跨梁桥:花瓶形实体桥墩连续梁桥、矩形空心薄壁桥墩连续梁桥、双柱实体桥墩连续梁桥施工全过程综合抗震性能的权重之比为0.340∶0.327∶0.333。花瓶形实体桥墩连续梁桥施工全过程综合抗震性能为最优。所提出的方法和建立的框架,可以广泛应用于各类桥梁施工全过程抗震设计及抗震性能综合决策。     

基于决策树的统计预报模型在臭氧浓度时空分布预测中的应用研究

近地面层臭氧是光化学污染的主要污染物之一.臭氧污染不仅严重影响着空气质量并且危害人类健康与动、植物生长.本研究以徐州市为研究区,基于环境监测站连续监测数据分别采用分类回归树(CART)、随机森林(RF)和M5模型树方法建立臭氧浓度统计预报模型,选取1、4、7、10等4个月作为季度代表进行区域臭氧浓度预测的研究.以2015整年逐小时徐州市国控大气监测站实时监测的臭氧浓度(因变量)和气象因子数据(自变量)为训练样本建立臭氧浓度统计预报模型.模型验证结果表明,总体上3种决策树模型能够较好的预测臭氧浓度动态变化特征,月尺度上预测值与观测值相关系数均值为0.68,平均绝对误差和均方根误差均值分别为21.63μg·m~(-3)和27.42μg·m~(-3).在此基础上,基于站点观测所建立臭氧统计预报模型,以WRF模型模拟的气象场作为输入,预报区域网格化臭氧预报值,并发现臭氧浓度空间分布与站点观测特征总体一致性较好.经与观测值进行对比,结果表明两者相关系数均值为0.58,平均绝对误差及均方根误差分别为29.38μg·m~(-3)和37.15μg·m~(-3),预报准确率均高于75%.同时利用周步长观测值与预测值建立的多元线性集合预报回归方程对3种决策树模型的预报值进行修正,在一定程度上提高了预报值的精度.     

城市边缘区农户耕地利用功能对土地转出的影响——武汉、成都、苏州1 022户农民的典型实证

在快速城市化及城乡社会经济转型过程中,城市边缘区农户家庭的耕地利用功能较传统方式已发生根本转变,农业劳动力的农地生存依赖性弱化、生计多元促进土地转出市场活跃。论文以武汉、成都及苏州1 022户农户调查数据为实证,分析城市边缘区农户家庭耕地利用功能对土地转出决策的影响。研究表明：1）城市边缘区农户家庭的耕地利用在食物生产、经济贡献及养老就业等传统功能上明显弱化,在满足家庭成员未来选择及后代馈赠需求等功能有所增强,且存在地区差异。2）城市边缘区农户土地转出意愿强烈,转出率达59.39%,土地转出存在地区差异。苏州乡镇经济发达、农民非农收入比例高,农户土地转出意愿最强烈、转出市场也最活跃;而武汉、成都作为中西部的典型城市,城市边缘区农户的土地转出决策相对滞后。3）整体而言,农户家庭耕地利用功能对土地转出意愿及行为均具有负向影响,表明家庭耕地功能依赖性愈强、土地转出决策越弱。建议健全农村社会及养老保障体系,强化城市边缘区耕地资源产权及生态环境建设,促进耕地资源养老保障及选择馈赠功能的释放,确保农田生计依赖性弱化的农民依托市场转出土地,实现农地资源优化配置、适宜规模经营。     

A CRITIQUE OF WATERSHED MANAGEMENT EFFORTS IN THE LOWER MISSISSIPPI ALLUVIAL PLAIN1

ABSTRACT: Integrated watershed management in the Lower Mississippi Alluvial Plain (Delta) requires blending federal, state, and local authority. The federal government has preeminent authority over interstate navigable waters. Conversely, state and local governments have authority vital for comprehensive watershed management. In the Delta, integrating three broad legal and administrative regimes: (1) flood control, (2) agricultural watershed management, and (3) natural resources and environmental management, is vital for comprehensive intrastate watershed, and interstate river basin management. Federal Mississippi River flood control projects incorporated previous state and local efforts. Similarly, federal agricultural programs in the River's tributary headwaters adopted watershed management and were integrated into flood control efforts. These legal and administrative regimes implement national policy largely in cooperation with and through technical and financial assistance to local agencies such as levee commissions and soil and water conservation districts. This administrative infrastructure could address new national concerns such as nonpoint source pollution which require a watershed scale management approach. However, the natural resources and environmental management regime lacks a local administrative infrastructure. Many governmental and non governmental coordinating organizations have recently formed to address this shortcoming in the Delta. With federal and state leadership and support, these organizations could provide mechanisms to better integrate natural resources and environmental issues into the Delta's existing local administrative infrastructure.