住房成本与通勤成本的空间互动关系——来自北京市场的微观证据及其宏观含义

住房成本和通勤成本是城市空间结构中两个相互关联的内生变量。居民在进行居住区位 选择时, 会在这两者之间进行权衡, 从而形成住房价格、建筑密度和通勤行为在空间上的变化规 律。本文回顾了通勤成本和住房成本间的理论均衡关系, 探讨了各种现实情况是如何影响这个理 论关系的; 并利用大规模调研所采集的北京市微观个体数据, 提供了这一互动关系的一些定量证 据; 最后从两个角度探讨了这种互动关系在城市管理的公共政策方面的宏观含义: 考虑通勤成本 后的住房支付能力评价, 以及就业与居住过度分离的社会效果分析。     

动机、成本、风险：家长视角下研学旅游产品价值感知研究

以中小学生自然教育研学旅游作为具体情境,基于539份家长问卷数据阐释研学旅游动机与价值感知之间的关系,并通过成本感知和风险感知探讨其内在机制和边界条件。结果表明,家长感知的研学旅游动机越强烈,越容易感知到研学旅游产品所传递的价值,验证了研学旅游情境下动机对价值感知的积极影响,也形成了对动机与价值之间关系研究的理论补充。并且,这一动机除了直接作用于价值感知外,还会通过成本感知的部分中介作用影响价值感知,揭示了中国家长对于教育价值的重视会削弱其对“利失”的感知。此外,进一步验证了风险感知的积极影响,即家长感知研学旅游的不确定性水平越高,动机对价值感知的积极影响越明显,且会抑制成本感知对价值感知的负向作用,但风险感知水平达到一定高度时,其对成本感知与价值感知间的抑制作用将不显著。因此,面向需求设计打造产品,突出产品价值、加大宣传营销力度、把控活动安全、合理制定价格,获取家长群体的认同接受,将是研学旅游进一步拓展市场的重要考量。     

山东地区多台波速比研究

文中从多台计算波速比方法的原理入手,对不同公式计算的波速比差异进行了深入分析,认为在原理一致的条件下,波速比差异主要与拟合过程中自变量的选取及2组数据的相关系数有关。利用2002年1月至2005年7月山东省境内及江苏省北部共52个数字及模拟台站的地方震P波和S波到时资料,选取有4个以上记录台站的地震。整个研究区域的平均波速比为1.716,反映了这个地区的波速比相对于华北其他地区偏高。将研究区域划分为1°×2°的单元,并剔除距离震中过远的台站,以保证区域特征能得到反映。通过对比不同单元的平均波速比发现,34°~37°,115°~119°的平均波速比明显高于其他单元,而它们正好分布在郯庐断裂的两侧,应该与该地区上地壳的岩石性质及震源深度分布范围较宽导致的平均效应有关     

Impact of artificial subsurface drainage on groundwater travel times and baseflow discharge in an agricultural watershed,Iowa (USA)

Pollutant delivery through artificial subsurface drainage networks to streams is an important transport mechanism, yet the impact of drainage tiles on groundwater hydrology at the watershed scale has not been well documented. In this study, we developed a two‐dimensional, steady‐state groundwater flow model for a representative Iowa agricultural watershed to simulate the impact of tile drainage density and incision depth on groundwater travel times and proportion of baseflow contributed by tile drains. Varying tile drainage density from 0 to 0.0038 m^?1, while maintaining a constant tile incision depth at 1.2 m, resulted in the mean groundwater travel time to decrease exponentially from 40 years to 19 years and increased the tile contribution to baseflow from 0% to an upper bound of 37%. In contrast, varying tile depths from 0.3 to 2.7 m, while maintaining a constant tile drainage density of 0.0038 m^?1, caused mean travel times to decrease linearly from 22 to 18 years and increased the tile contribution to baseflow from 30% to 54% in a near‐linear manner. The decrease in the mean travel time was attributed to decrease in the saturated thickness of the aquifer with increasing drainage density and incision depth. Study results indicate that tile drainage affects fundamental watershed characteristics and should be taken into consideration when evaluating water and nitrate export from agricultural regions. Copyright © 2011 John Wiley & Sons, Ltd.     

Travel time controls the magnitude of nitrate discharge in groundwater bypassing the riparian zone to a stream on Virginia's coastal plain

Groundwater that bypasses the riparian zone by travelling along deep flow paths may deliver high concentrations of fertilizer‐derived NO₃^? to streams, or it may be impacted by the NO₃^? removal process of denitrification in streambed sediments. In a study of a small agricultural catchment on the Atlantic coastal plain of Virginia's eastern shore, we used seepage meters deployed in the streambed to measure specific discharge of groundwater and its solute concentrations for various locations and dates. We used values of Cl^? concentration to discriminate between bypass water recharged distal to the stream and that contained high NO₃^? but low Cl^? concentrations and riparian‐influenced water recharged proximal to the stream that contained low NO₃^? and high Cl^? concentrations. The travel time required for bypass water to transit the 30‐cm‐thick, microbially active denitrifying zone in the streambed determined the extent of NO₃^? removal, and hydraulic conductivity determined travel time through the streambed sediments. At all travel times greater than 2 days, NO₃^? removal was virtually complete. Comparison of the timescales for reaction and transport through the streambed sediments in this system confirmed that the predominant control on nitrate flux was travel time rather than denitrification rate coefficients. We conclude that extensive denitrification can occur in groundwater that bypasses the riparian zone, but a residence time in biologically active streambed sediments sufficient to remove a large fraction of the NO₃^? is only achieved in relatively low‐conductivity porous media. Instead of viewing them as separate, the streambed and riparian zone should be considered an integrated NO₃^? removal unit. Copyright © 2011 John Wiley & Sons, Ltd.     

ISLAND SOJOURNS

ABSTRACT. Islands have long held a central place in Western cultures' mythical geographies. They have been associated for centuries with heroic journeys and holy quests, imagined realms of magical transformations. Islands have also been sites of significant rites of passage, and they continue to perform this function in the modern secular world. Today, popular islomania is expressed in the frequency of seasonal sojourning on European and American archipelagos. No longer destinations of permanent residence, islands now provide access to a sense of temporal and spatial rootedness that is no longer available on mainlands. They loom large on the mental maps even of those who rarely, if ever, visit them.     

基于网络流理论的输油管道的最大输送问题研究

在天然气管道网络系统的工艺设计方案中,对布站的站点选择主要取决于其运营成本的相互比较,但鉴于需求和供应之间的关系,还必须考虑管道的输送能力和站点的容量等问题。对于这样复杂化的问题,利用网络流理论的最大流问题和最小费用的最大流问题,建立了输油管道最大输送问题的网络优化模型,并通过实例说明其有效性和可靠性。为输油管道网系统的构建提供了一种新的优化方法,在天然气管道输送决策管理中具有一定的理论参考价值和现实意义。     

Life-cycle cost analysis of design practices for RC framed structures

The objective of this study is to perform life-cycle cost analysis on three design practices namely weak ground storey, short and floating columns and their combinations. Life-cycle cost analysis is recognized as the only suitable tool for assessing the structural performance when the structure is expected to be functional for a long period of time. Life-cycle cost analysis is considered in this study assessing the behaviour of the three design practices against earthquake hazard. Although, a number of checks are performed in order to reduce the influence of these design practices on the seismic behaviour of reinforced concrete (RC) framed structures, it was found that the total life-cycle cost of partially infilled RC designs is significantly increased compared to that of the fully infilled one. Through the test example examined in the framework of this study general conclusions are obtained regarding the behaviour of the three design practices.     

Empirical predictors of annual bed load travel distance,and implications for salmonid habitat restoration and protection

Measurements of annual travel distance (L_b) of bed load sediment at 16 locations in Alaska, the intermountain USA, west coast USA and Scotland are strongly correlated with bankfull channel width (r² = 0·86, p < 0·001). Travel distance of particles is probably limited by trapping in bars, which have a longitudinal spacing proportional to channel width. Increased abundance of woody debris reduces bar spacing and may reduce L_b. Longer cumulative duration of bed load transporting flows in a year appears to increase L_b. Other predictors of annual travel distance such as stream power per unit length, drainage area and bankfull discharge were less well correlated with L_b (r² ranging from 0·27 to 0·51). Stream power per unit bed area, basal shear stress and slope were not significantly related to L_b (r² < 0·05). Most correlations were improved when regressions were limited to data from the west coast USA. Travel distance estimates can be used to help identify reaches that may take longer to recover from large, short‐term increases in sediment supply. Published in 2001 by John Wiley & Sons, Ltd.     

工程结构抗震设防标准的决策分析

提出了工程结构抗震设防标准的决策方法,该决策方法以结构初始造价分析和地震损失分析为基础。建立了结构初始造价与设计烈度的关系,并提出了地震损失的估计方法。使用该决策方法导出了最佳设计烈度和重现周期的解析表达式,从而得出了对抗震设计具有重要意义的结论。