Analysis of geomagnetic variations in south-western Nigeria

Geomagnetic variations, observed at 11 sites in south-western Nigeria, have been analysed to derive interstation transfer functions with the site at Ile-Ife as reference station. The study involves frequencies from 1 to 6 c.p.h. The reference station Ile-Ife is 160 km northward of the continental slope off the Nigerian coastline and 400 km southward of the dip equator. The analysis has been carried out separately with selected data sections of a few hours length during daytime and during the night. Thus expressed linear relations between field components are in the case of day events of dual implication: (1) for the source field structure of the equatorial electrojet, (2) for internal conductivity conditions, including the coast effect from the Bight of Benin. Conductivity anomalies are the sole cause for an observed spatial variability of night events. A 2-D thin-sheet conductivity model has been derived taking both the source and the coast effect into consideration. This model provides a reasonably good fit between observed and computed transfer functions during day and night.     

外资效应、经济复杂度与内资企业出口市场拓展

外资企业对内资企业出口能力提升和出口市场拓展具有重要影响。本文基于2000—2016年中国地级市出口数据,分析中国内资企业出口市场拓展格局,研究外资效应对内资企业进入新出口市场和出口额增长的影响,并依据经济和技术复杂度检验外资效应的区域和产业异质性。描述性分析发现：① 内资企业出口在国内呈多级连片格局,逐渐向非洲和拉丁美洲等新兴市场拓展。② 处于低复杂度城市/产业的内资企业更倾向于拓展新出口市场,处于高复杂度城市/产业的内资企业则倾向于提升出口额。计量模型结果表明：① 外资企业推动了内资企业出口额增长,但抑制了其进入新出口市场,分别表现出溢出效应和竞争效应。② 从区域和产业异质性来看,外资企业仅在高复杂度城市/产业阻碍了内资企业出口市场拓展,表现出竞争效应;而在低复杂度城市/产业则表现为溢出效应。③ 相较于低经济复杂度城市,高经济复杂度城市的内资企业受到的竞争和溢出效应均更强。     

The role of shear and tensile failure in dynamically triggered landslides

Dynamic stresses generated by earthquakes can trigger landslides. Current methods of landslide analysis such as pseudo-static analysis and Newmark's method focus on the effects of earthquake accelerations on the landslide mass to characterize dynamic landslide behaviour. One limitation of these methods is their use Mohr–Coulomb failure criteria, which only accounts for shear failure, but the role of tensile failure is not accounted for. We develop a limit-equilibrium model to investigate the dynamic stresses generated by a given ground motion due to a plane wave and use this model to assess the role of shear and tensile failure in the initiation of slope instability. We do so by incorporating a modified Griffith failure envelope, which combines shear and tensile failure into a single criterion. Tests of dynamic stresses in both homogeneous and layered slopes demonstrate that two modes of failure exist, tensile failure in the uppermost meters of a slope and shear failure at greater depth. Further, we derive equations that express the dynamic stress in the near-surface in the acceleration measured at the surface. These equations are used to approximately define the depth range for each mechanism of failure. The depths at which these failure mechanisms occur suggest that shear and tensile failure might collaborate in generating slope failure.     

Crayfish and fish as bioturbators of streambed sediments: Assessing joint effects of species with different mechanistic abilities

Many studies illustrate that bioturbating animal species individually affect aquatic sediments through diverse mechanistic abilities, whereas assessments of joint effects of such species on sediments are relatively rare. Such joint effects have implications for real systems, in which different bioturbators coexist, but are difficult to predict for two reasons. First, they can be additive (being the sum of the individual effects of each species) or they can be positive or negative interactive (being greater or smaller than the sum of the individual effects). Second, if interactive, they can depend on biotic interactions that affect the bioturbating activities of the species and/or they can depend on physical interactions among bioturbator-induced sediment modifications. Using experimental streams, we assessed such joint effects on gravel–sand sediments for flow and sediment conditions preferred by barbel (Barbus barbus) but also used by gudgeon (Gobio gobio) and, in a second experiment, for flow and sediment conditions preferred by both male crayfish (Orconectes limosus) and gudgeon. These species have different mechanistic abilities to affect gravel and/or sand in stream beds. In each experiment, we measured (i) the transport of gravel and sand at baseflow (during 12 experimental days); (ii) four sediment surface characteristics (after 12 d); and (iii) the critical shear stress (τ_c) causing incipient gravel and sand motion during experimental floods (after 12 d). Gudgeon contributed differently to the joint effects in the two experiments, which related to its individual weight, prevailing baseflow shear stress, sediment particle weight, and sediment mixture (availability of surface sand). Overall, the species pairs had predominantly negative interactive joint effects on the sediment variables assessed by us. Both a literature survey and observations during the experiments provided no evidence for direct biotic interactions between barbel and gudgeon or crayfish and gudgeon, so one would reasonably associate their negative interactive effects on the sediments with physical interactions among bioturbator-induced sediment surface modifications. Individually, each species reduced the percentage of sand in the surface layer and the surface algal cover to relatively low values so that the species pairs could not accomplish much greater joint effects on these variables, explaining their negative interactive effects on them. As algal cover particularly affected the τ_c for gravel and sand, the negative interactive effects of the animals on this surface variable chained toward the τ_c for the sediments, on which the species pairs also had negative interactive effects. Such chained negative interactive effects on sediment variables are seemingly a general pattern of joint bioturbator effects on aquatic sediments, i.e., the many so far described single-species effects should be smaller than their sum if the species coexists in nature.     

土地变化的生态效应研究方法

土地变化研究是全球土地计划(GLP)的重要组成部分,并逐渐演化成新的研究领域——土地变化科学(LCS)。土地变化对陆地生态系统造成了深远影响,其生态效应研究在全球和区域尺度上广泛开展。该文对土地变化的大气、水文、土壤、生物单要素效应及区域综合生态效应的主要研究方法和技术手段进行总结。结果表明,在观测和试验基础上,抽象出数理模型对土地变化的生态效应进行模拟以及定量评价是主要研究方法;借助于多时空尺度模型,土地变化的大气、水文效应研究已经相当深入,但区域综合生态效应的研究还比较薄弱,主要以影响评价为主;建立多元素耦合、多尺度集成的机理模型和综合定量评价是今后发展的重要方向。     

论生态城市绿地系统的综合效益

绿地是配合环境创造自然条件,适合种植乔木、灌木和草本植物而形成一定范围的绿化地面或区域。城市绿地系统是生态城市一个重要的生态环境指标,其效益是综合的、多元化的、多方位的,不仅为生态城市创造出较高的生态效益,产生社会效益,而且也具有经济效益。     

A review on eco-hydrological effects of condensation water

Almost half the world is suffering from drought at different levels. Since arid and semi-arid regions are characterized by extreme shortage and precipitation uncertainty, water availability is the most important limiting factor in arid lands, and any additional source of moisture, such as condensation water, may have a positive impact upon the ecosystem. Therefore, extensive research has been carried out on condensation. Based on articles and relevant documents on the subject, this paper reviews systematically and comprehensively the latest research achievements in several main aspects, including eco-hydrological effects of condensation water on plants, small animals, microbiotic crusts, soil moisture balance, ground water recharge and anthropogenic utilization. History and trends of research about eco-hydrological effects of condensation water are intensively analyzed in order to summarize existing problems and offer opinions and suggestions for future study.     

印度对布拉马普特拉河的开发及其跨境影响

通过研究印度对布拉马普特拉河的开发现状及计划,分析其开发对上下游国家的影响,发现印度在布拉马普特拉河流域已建水电工程8座,在建1座,拟建19座,并且布拉马普特拉河在印度的连通计划中占有举足轻重的地位,JTF(布拉马普特拉河-恒河)和MSTG(布拉马普特拉河-恒河)工程分别占印度北部水系连通计划总体投资的12.04%和14.53%。这些开发计划使中印的边界问题更加复杂,并且当中国开发雅鲁藏布江时,将会对印度的水利工程的运作带来影响;印度为了缓解能源问题以及调节印度境内的水电工程,在不丹境内建造了大量的水电工程,这使不丹更加依赖印度,并给不丹带来生态灾难;孟加拉国位于印度的下游,上游过多的水电工程使其旱季供水不足影响农田灌溉,改变河流水文条件影响其渔业,并使海水倒灌至使土地盐碱化。     

凝结水的生态水文效应研究进展

世界上近一半的国家和地区不同程度地受到干旱问题的影响。干旱区的最显著特征是降水的极端稀少和不确定。任何降雨以外的水资源输入,即使数量很微小,也对荒漠生态系统的维持和存续有重要意义,因此凝结水作为可能性水源成为研究热点。基于文献资料,本文从干旱、半干旱地区凝结水对生物的影响,对生物结皮及土壤水分平衡的作用,对地下水的补给以及凝结水的人为利用等几个方面阐述国内外的凝结水研究现状,着重分析凝结水生态水文效应方面的研究历程和趋势,总结凝结水研究中存在的问题,展望未来研究工作的重点。     

航空和高铁对中国城市创新能力的影响

交通运输是创新网络中人才流、资本流等知识与技术流动的物理空间承载,其对城市的创新能力影响已成为经济地理学的交叉前沿热点。论文基于2007—2018年中国城市尺度数据,以航空和高铁运输为例,构建交通运输对城市创新能力影响效应的理论框架,采用双向固定效应面板回归模型,实证检验航空和高铁对城市创新能力的多重异质性机制,并探讨了知识传播、资本积累、产业升级在交通运输与创新能力之间的中介效应。研究发现：① 航空和高铁建设均对城市创新能力有显著正向影响,高铁对城市的创新溢出效应约为航空的3倍。② 航空和高铁对不同类型城市的创新溢出效应存在显著异质性。城市等级异质性方面,航空和高铁对中心城市创新能力的正向影响强度高于非中心城市。人口规模异质性方面,航空对大、中城市创新能力提升有显著正向影响,对小城市有抑制作用;高铁运输则对不同人口规模城市的创新能力均有正向影响,呈现大城市>中等城市>小城市的态势。区域异质性方面,航空和高铁对东、中、西、东北地区的创新能力均有不同程度的提升作用,表现出显著的“马太效应”,东部地区优势地位凸显。③ 航空和高铁均可通过促进技术转移、风险资本配置、外商资本配置间接促使城市创新能力提升。此外,航空还能够通过促进产业升级间接促使城市创新能力提升。     

DEM resolution dependencies of terrain attributes across a landscape

This paper documents resolution dependencies in terrain analysis and describes how they vary across landform location. Six terrain attributes were evaluated as a function of DEM resolution—slope, plan curvature, profile curvature, north–south slope orientation, east–west slope orientation, and topographic wetness index. The research highlights the effect of varying spatial resolution through a spatial sampling/resampling scheme while maintaining sets of indexed sample points at various resolutions. Tested sample points therefore coincide exactly between two directly compared resolutions in terms of their location and elevation value. An unsupervised landform classification procedure based on statistical clustering algorithms was employed to define landform classes in a reproducible manner. Correlation and regression analyses identified sensitive and consistent responses for each attribute as resolution was changed, although the tested terrain attributes responded in characteristically different ways. These responses displayed distinguishable patterns among various landform classes, a conclusion that was further verified by a series of two‐sample, two‐tailed t‐tests.     

全球升温对中国区域温度纬向梯度的影响

利用中国气象中心160站点的实际观察资料,对中国半个世纪的气温变化分时间段进行了计算分析。用各个站点52年的气温序列斜率与纬度和海拔分别做了相关分析,发现气温变化幅度和纬度有着很好的相关性,特别是在冬季;但气温变化和海拔没有相关性。另外,通过对不同季节的温度变化情况进行计算和纬向分析,发现随着全球气温的持续上升,气温的纬向梯度在变小,变化的幅度是冬季大、夏季小,高纬度地区大、低纬度地区小。最后指出造成气温纬度梯度减小的主要原因是大陆度随纬度的不同分布。     

灾害天气对山地高速公路建设管理的影响分析——以山西省为例

利用1971-2000年30a山西省107个地面观测站气象观测资料以及高速公路管理部门有关资料，分析了山西省高速公路沿线大雾、降雪（积雪）、结冰等恶劣天气的分布特征及其成因，讨论了其影响，提出了一些合理化建议。     

农村社区农户互动效应的定量评估——来自河南省杞县孟寨村的实证

信息流动会使农户产生互动学习效应,进而影响农户的农业生产率。基于河南省孟寨村第8小组农户的普查,获得有关农户亲戚、邻居和与之地块邻近农户的社会经济特征和大蒜产量的信息,对这些群组内部的互动学习效应进行检验,并识别不同类型的互动效应。结果表明:内生互动效应仅存在于亲缘群组;情境互动效应存在于地邻群组和邻里群组,但在两个群组中存在一定的差异;关联效应在三个群组均存在,与亲缘群组相比,邻里群组和地邻群组中关联效应更大,但在亲缘群组中,关联效应远小于内生互动效应,在邻里群组和地邻群组中也远小于情境互动效应。研究结论对国外学者的发现进行了修正。这种理论研究具有重要的政策含义。     

Research progress on behaviors and environmental effects of mercury in the cryosphere of the Tibetan Plateau:a critical review

The behavior and fates of environmental pollutants within the cryosphere and the associated environmental impacts are of increasing concerns in the context of global warming.The Tibetan Plateau(TP),also known as the"Third Pole",represents one of the most important cryospheric regions in the world.Mercury(Hg)is recognized as a global pollutant.Here,we summarize the current knowledge of Hg concentration levels,pools and spatio-temporal distribution in cryospheric environments(e.g.,glacier,permafrost),and its transfer and potential cycle in the TP cryospheric region.Transboundary transport of anthropogenic Hg from the surrounding heavily-polluted regions,such as South and Southeast Asia,provides significant sources of atmospheric Hg depositions onto the TP cryosphere.We concluded that the melting of the cryosphere on the TP represents an increasing source of Hg and brings a risk to the TP environment.In addition,global warming acts as an important catalyst accelerating the release of legacy Hg from the melting cryosphere,adversely impacting ecosystems and biological health.Furthermore,we emphasize on the remaining gaps and proposed issues needed to be addressed in future work,including enhancing our knowledge on some key release pathways and the related environmental effects of Hg in the cryospheric region,integrated observation and consideration of Hg distribution,migration and cycle processes at a key region,and uses of Hg isotopic technical and Hg models to improve the understanding of Hg cycling in the TP cryospheric region.     

20世纪下半叶干旱对新疆农业的影响及灾害链效应

利用历史灾害数据对20世纪下半叶新疆农业的干旱影响及灾害链效应进行了评价。同时对干旱灾害的时间特征也做了分析。结果表明,1950-2001年的52年里,新疆年平均农田旱灾成灾面积为109.25×103hm2,其中.2000年最高达312.0×103hm2,1950年最低仅为2.01×103hm2。1950-1997年48年年平均受灾人口与减产粮食分别为285.63×103人和174.85×103t。从1950-2001年,新疆农田成灾面积大致上呈上升趋势,虽然其间也有暂时的下降时段,即20世纪70年代早期和90年代早期。1950-1997年受旱人口和减产粮食也存在大致的上升趋势。有两个时期(20世纪60年代和80年代到90年代早期)干旱灾害相对较严重。另外,还发现干旱灾害与沙尘天气和草原蝗灾有明显的相关性,表现出了明显的干旱灾害链性。本评价的结果暗示干旱及其相关灾害的增多可能意味着天气和气候极端事件的增多;20世纪80年代中期以来,尽管区域年降水和径流量的增多对减轻干旱及其灾害具有很大的正效应,但由于降水和径流的年内分布不均匀性,加上耕地向易旱区的扩张,区域干旱灾害还是比较严重,并表现出了与降水和径流增加不一致的趋势。