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1.
利用清代黄河上中游3个站点的志桩尺寸记录,通过建立回归模型反演了1766-1911年的逐年径流量,重建了兰州、青铜峡和三门峡1766-2000年的逐年汛期径流量序列,再结合河源段唐乃亥站1766-2000年的逐年径流量,构建了河源—上游—中游4个站点的径流量序列,这是目前利用历史文献记录能够获得的最为清晰的黄河径流量曲线。研究显示,19世纪中期出现在黄河下游的沉重“河患”是由青铜峡—三门峡河段内的径流量突变所致,而20世纪20年代的枯水时段从河源段到中游段都存在,但其不是突变造成。同时,研究也揭示出太平洋年代际振荡（PDO）与黄河上中游径流量在年代际尺度上存在着阶段性的反相位关系,20世纪前期和中期,8~16年尺度上在4个站点的径流量都有反相位关系出现;在19世纪30-50年代,PDO与流量在4~6年尺度上的反相位关系在兰州和三门峡断面都较为明显。交互小波分析显示,PDO与黄河上中游汛期水量在8~16年尺度上存在着较为明显的反相关关系,但只存在于三门峡和兰州断面。这一关系也许表明,黄河上中游产流区的夏季降雨量与PDO的关系具有较为明显的时间和空间差异性。 相似文献
2.
《自然地理学》2013,34(5):403-437
Local landforms and microtopography control site conditions, spatial patterns, and dynamics in treeline landscapes. Several topography-related treeline types are presented and their responses to a warming climate discussed. On rugged mountain terrain, pronounced changes in vertical range and variety of treeline landscapes will not take place as long as debris slides and avalanches occur regularly. On intensively eroded steep terrain, trees will mainly colonize convexities. On trough shoulders and similar gentle topography with irregular mosaics of convex and concave landforms, tree establishment is most likely to take place on convex topography. On gentle slopes and rolling uplands, where the proportion of wind-swept terrain is comparatively large, microsite facilitation appears to be a precondition for tree establishment. At higher elevations, the relative importance of shelter-providing landforms will increase due to windier conditions. At the beginning of treeline rise, seedling establishment, growth, and survival are closely related to local landforms and microtopography and their effects on site conditions. Later, the feedback from trees on their environment may overrule the effects of landforms. 相似文献
3.
Climate changes at the multi-decadal scale are often associated with multi-decadal phase shifts of the dominant sea surface temperature （SST） pattern, such as the Pacific Decadal Oscillation （PDO）. The PDO may be associated with the North Pacific branch of the Thermohaline Circulation （THC）. Great earthquakes （M 〉8）, particularly along the route of the THC, might modulate the vertical mixing and bring deep, cold water to surface, contributing to multi-decadal changes in surface currents and the PDO. This may eventually lead to multi-decadal climate changes. We tested this hypothesis for the Pacific Ocean where great earthquakes have been frequently recorded. We found associations between the PDO and recurrent earthquakes along the route of the deep currents of the THC in the modern period since 1900, and relationships between hydroclimate change in Monsoonal Asia and historical earthquakes since 1300. However, it should be noted that this hypothesis is very preliminary and has many gaps that needs further evidences from more observational records and modeling studies. 相似文献
4.
On Niwot Ridge, alpine climatic factors dominate today within the upper part of the forest-alpine tundra ecotone on the east slope of the Colorado Front Range. This study provides evidence that the climatic conditions controlling the distribution of the upper limit of tree species and growth forms within the ecotone have changed through time. It appears that much of the present forest-alpine tundra ecotone is a relict of past warm climate episodes. There is much evidence that, in the past, tree forms were more symmetrical and less deformed, tree trunks with larger diameters existed, and the trees were more extensive in areal coverage. However, no evidence exists to show that the upper limit of tree species growth was more than marginally higher in the past than today. The treeline appears to be stable to historical and Holocene climatic fluctuations, but the altitudinal limits of seedling establishment and survival are not stable. The liming of climatic changes in the Front Range and their possible influence on the distribution of the trees within the ecotone are reviewed. Because of its extreme easterly location, slight but significant climatic variations may be observed within the forest-alpine tundra ecotone of Niwot Ridge, but are not observed elsewhere. 相似文献
5.
《自然地理学》2013,34(3):177-191
Incidences of dense fog have been gradually disappearing from large cities around the world. In Los Angeles, the climate records show that dense fog may become another extinct specie. In the last 50 years, dense fog has been reduced by about half at two, busy, coastal Los Angeles airports. Not one, but two factors, can be seen as possible causes. Both the downtown Los Angeles urban heat island and the city's air pollution clean-up show a significant relationship to decreasing fog. This study correlates monthly and annual dense fog data recorded at Los Angeles International and Long Beach International airports since the 1950s with downtown Los Angeles temperature and particulate air pollution data. The El Niño—Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) values also explain interannual dense fog variability, with PDO showing a stronger relationship. Coastal sea surface temperatures also closely follow dense fog frequencies. Recent coastal cooling, since 1998, associated with a shift to the cool phase of PDO, has been associated with a rise in dense fog incidences. 相似文献
6.
Wind as an Ecological Agent at Treelines in North America,the Alps,and the European Subarctic 总被引：2，自引：1，他引：1
《自然地理学》2013,34(3):203-233
The altitudinal treeline ecotone is a windy environment where wind velocities and directions are controlled by local mountain topography and also by the distribution pattern and structures of tree stands. Wind may override the role of heat deficiency in determining treeline position, spatial pattern, ecological conditions, and tree growth. Regular strong permanent winds restrict tree height and usually cause asymmetric and suppressed growth forms that are common in the treeline ecotone. Apart from direct physiological and mechanical effects on trees and ground vegetation, wind also disperses seeds, relocates snow, and locally erodes soils in the treeline ecotone. Wind effects must be considered an important factor that may delay or even preclude establishment of seedling trees on wind-swept terrain. Discussions of a potential climatically driven upward shift of the treeline at the landscape and smaller scales should give greater attention to the varying wind effects because warming cannot compensate for these other factors. The relative importance of microsite facilitation providing shelter from the wind will increase in parallel with the upslope migration of the tree limit into a much windier environment. 相似文献
7.
黄土高原甘肃区降水变化与气候指数关系 总被引：2，自引：1，他引：1
基于黄土高原甘肃区34个气象站1961~2010年的逐日降水资料,结合线性趋势法、Mann-Kendall法和反距离加权插值等方法,对黄土高原甘肃区降水量时空变化进行分析,利用Morlet小波、交叉小波变换和小波相干谱分析其周期特征及其与太平洋年代际涛动（PDO）、厄尔尼诺/南方涛动（ENSO）指数相关关系。结果表明：研究区年降水量呈下降趋势,变化倾向率为-15.4 mm/10a,特别是20世纪90年代以来降水量下降趋势明显;就季节变化而言,降水量除冬季呈轻微增加外,其他各季均呈减少趋势,其中以秋季减少最为明显,其次为春、夏季。从空间分布来看,年降水量总体呈减少趋势,其东南地区降水量减少幅度高于西北部。研究区降水量存在2~3 a、4.3~5 a的年际振荡周期特征,呈现了同大尺度气候因子相似的变化特征;降水量与两大气候因子存在着多时间尺度的显著相关性,与PDO和ENSO存在5~5.6 a左右共振周期,位相谱月降水量变化位相比PDO和ENSO提前;低能量区降水量与PDO有3~3.5 a年的显著共振周期且接近同位相变化,与ENSO存在0~3 a和3~6 a呈负位相的共振周期。 相似文献
8.
Fluvial process and the establishment of bottomland trees 总被引：1，自引：0，他引：1
The effects of river regulation on bottomland tree communities in western North America have generated substantial concern because of the important habitat and aesthetic values of these communities. Consideration of such effects in water management decisions has been hampered by the apparent variability of responses of bottomland tree communities to flow alteration. When the relation between streamflow and tree establishment is placed in a geomorphic context, however, much of that variability is explained, and prediction of changes in the tree community is improved.The relation between streamflow and establishment of bottomland trees is conditioned by the dominant fluvial process or processes acting along a stream. For successful establishment, cottonwoods, poplars, and willows require bare, moist surfaces protected from disturbance. Channel narrowing, channel meandering, and flood deposition promote different spatial and temporal patterns of establishment. During channel narrowing, the site requirements are met on portions of the bed abandoned by the stream, and establishment is associated with a period of low flow lasting one to several years. During channel meandering, the requirements are met on point bars following moderate or higher peak flows. Following flood deposition, the requirements are met on flood deposits ;high above the channel bed. Flood deposition can occur along most streams, but where a channel is constrained by a narrow valley, this process may be the only mechanism that can produce a bare, moist surface high enough to be safe from future disturbance. Because of differences in local bedrock, tributary influence, or geologic history, two nearby reaches of the same stream may be dominated by different fluvial processes and have different spatial and temporal patterns of trees. We illustrate this phenomenon with examples from forests of plains cottonwood (Populus deltoides ssp. monilifera) along meandering and constrained reaches of the Missouri River in Montana. 相似文献
9.
Lynn M. Resler 《The Professional geographer》2006,58(2):124-138
This study examines the role of surface geomorphic features in tree establishment at the alpine treeline in Glacier National Park, Montana, through the presentation of a multiscale, conceptual model of biogeomorphic relationships at alpine treeline. Empirical observations gathered through a multiscale field methodology over three summers serve as a base for the model. The model highlights the importance of surface geomorphic features, specifically boulders and terrace risers, in creating favorable local site conditions, largely by protecting seedlings from wind. The sheltering effect of surface features enables initial seedling establishment, and in some cases survival, above current treeline locations, thereby initiating a positive feedback effect that encourages subsequent tree establishment. Geomorphic features are therefore important in linking scales of pattern and process at the alpine treeline ecotone. 相似文献
10.
中国城市空间结构研究评述 总被引：8，自引：0，他引：8
中国正处于城市化快速发展时期, 城市空间结构面临关键转型, 相关研究日益受到重视, 研究数量不断增多, 深度不断加强。因此, 有必要对中国城市空间结构相关研究进行系统总结, 从中找出存在的问题, 以及与国际相关研究的差距。基于对20 世纪80 年代以来中国城市空间结构相关研究成果的数量统计, 将其划分为3 个主要的研究时期:① 20 世纪80-90 年代中期, 为西方城市空间结构理论引入期及国内实证研究的起步期;② 20 世纪90年代中期-21 世纪初, 为中国城市空间结构的研究积累期;③ 21 世纪初至今, 为中国城市空间结构模式的总结及新城市空间现象研究的多元化时期。在对各个时期主要相关研究成果进行梳理和总结的基础上, 对比当前国际研究的最新成果, 提出未来中国城市空间结构的重点研究领域应当集中于:转型期城市空间结构理论模式的探讨, 新城市空间现象的深入探讨, 节约型城市空间增长的研究, 城市空间结构深层机制的分析, 中、微观尺度研究领域的开拓, 新研究方法的应用等6个方面。 相似文献
11.
《The Professional geographer》2013,65(2):124-129
Abstract This study examines the role of surface geomorphic features in tree establishment at the alpine treeline in Glacier National Park, Montana, through the presentation of a multiscale, conceptual model of biogeomorphic relationships at alpine treeline. Empirical observations gathered through a multiscale field methodology over three summers serve as a base for the model. The model highlights the importance of surface geomorphic features, specifically boulders and terrace risers, in creating favorable local site conditions, largely by protecting seedlings from wind. The sheltering effect of surface features enables initial seedling establishment, and in some cases survival, above current treeline locations, thereby initiating a positive feedback effect that encourages subsequent tree establishment. Geomorphic features are therefore important in linking scales of pattern and process at the alpine treeline ecotone. 相似文献
12.
天山乌一号冰川物质平衡特征的统计分析 总被引：4，自引：1，他引：4
利用天山乌鲁木齐河源一号冰川物质平衡的多年观测资料,通过数学统计方法提取主成分,获得影响该冰川物质平衡的主要因素,通过成分矩阵的旋转获得表示冰川物质平衡特征的两个主因子,其中第一主因子为影响冰川物质平衡的常年因子,代表该冰川的多年持续变化方向与趋势,第二主因子为影响冰川物质平衡的年际因子,代表该冰川在平衡年内的物质平衡变化特征。常年因子的持续下降反映了冰川持续后退的现实,年际因子在上世纪80年代中期以后突然强劲上升,实际上是冰川物质平衡水平提高,平衡年内积累量和消融量都增加的表现,清晰地指示了西北地区气候由暖干向暖湿的转型。常年因子是冰川物质平衡变化的主导因子,控制了冰川变化的方向。常年因子具有较好的周期性,以6年周期最为明显,但其形成原因尚不明。年际因子周期性不明显,但可以看出其周期与太阳黑子变化具有相关性,同时受到常年因子的影响。 相似文献
13.
The rate of bluff erosion has been monitored and related processes examined on a Mississippi River meander bend at Port Hudson, Louisiana, from 1971 to mid-1980. The erosion data were analyzed using a stepwise multiple regression analysis to determine the conditions controlling the amount and distribution of bluff erosion. The results indicate a pattern of decreasing bluff erosion that is attributed to downstream migration of the zone of maximum basal scour. Bluff retreat in the period of active basal undercutting (1971 to mid-1975) was 18.9m/year and in the following period of decreased basal scour activity (mid-1975 to mid-1980) was 6.8m/year. Approximately, 61% of the erosion was associated with rising river levels. The data also indicated that 66% of the erosion occurred at bankfull discharge stage or above. Conditions that produce bluff erosion were found to vary between the two eriosion periods and also as river levels rose or fell in each of the periods. During the actively undercutting period, precipitation rate and amount of river rise explained 94% of the bank erosion that occurred as river levels rose. During the falling river phase, days above bankfull stage and amount of river fall accounted for 99% of the bluff erosion. In the period of decreased basal scour, precipitation rate and maximum river stage accounted for 94% of the bluff erosion related to rising river levels, while the erosion during falling river levels could be only tentatively correlated to precipitation rate and factors associated with drawdown. Changes in slope processes as the result of decreased basal scour were also examined. 相似文献
14.
《自然地理学》2013,34(4):302-320
Using a 545-year ponderosa pine (Pinus ponderosa) tree-ring chronology, we examine the drought history of central Oregon to: (1) determine the relationship among drought, ENSO (El Niño/Southern Oscillation), and the PDO (Pacific Decadal Oscillation), and (2) compare the climatic sensitivity of ponderosa pine and western juniper (Juniperus occidentalis) to determine their suitability as interchangeable climate proxies. Our climatic reconstruction explained 35% of the variance in historical Palmer's Drought Severity Index (PDSI) values and revealed severe drought periods during the 1480s, 1630s, 1700s, and 1930s. The most sustained drought period in our reconstruction occurred during the 1930s, with the most severe single drought year occurring in 1489. We found a significant (p ≤ .01) but weak relationship between our ponderosa pine chronology and ENSO and the PDO, explaining 9% and 12% of the variation respectively. Both ponderosa pine and western juniper record periods of severe regional drought, but western juniper is more sensitive to regional and seasonal climatic variations, whereas ponderosa pine is more responsive to temperature change. These differences suggest that their substitutability as climate proxies in dendroecological studies is limited. 相似文献
15.
气候变化和人类活动对我国荒漠化的可能影响 总被引：42，自引：24，他引：18
荒漠化和气候变化作为全球性的重大环境问题,科学界历来对此十分重视。我国是受荒漠化最为严重的国家之一。截止2004年,我国的荒漠化土地面积为263.62万km2,沙化土地面积173.97万km2,分别占国土总面积的27.46%和18.12%。与20世纪90年代中期至20世纪末的情况相比,荒漠化和沙化土地面积、扩展速度均出现减少和减缓的趋势,沙化土地已由90年代末每年扩展3 436 km2转变为每年减少1 283 km2。这是气候变化和人类活动综合影响的体现,也是国家加大生态工程建设力度和加大荒漠化防治投入的结果。我国未来气候变化和人类活动对荒漠化的扩展与逆转过程仍会产生影响。气候变化中的降水变化在大范围内控制着荒漠化土地的扩展与逆转过程,气候暖干化有助于荒漠化的发生与扩展,但在局部地区随着气温升高有降水增多的可能,有利于荒漠化土地的逆转。人类活动对荒漠化的影响有正反两方面的效应,特别是荒漠化地区人口增长以及对农牧产品等物质需求的增加而引起的不合理经济活动是不容忽视的。要遏止荒漠化的扩展,应采取适应与减缓气候干旱化对策,消除人为荒漠化因素,并积极整治荒漠化土地,从根本上促进荒漠化土地的逆转。 相似文献
16.
Although dust storms rarely occur in southern California's deserts, blowing dust often reduces visibility, and large spatial and temporal variability in the frequency of blowing dust occurs throughout the region. On average only 1.3 dust storms occur in the study area each year. The annual average number of dust events (visibility <11 km) is 18.0, with the Coachella Valley being dustiest region, averaging 37.8 dust events each year. Mean annual frequencies of dust events for 1973–1994 are mapped, showing a core of activity centered over the Imperial/Coachella Valley region, with fewer dust events around the periphery of the study area. Most stations show a coherent temporal pattern of dust frequency during the period 1973–1994, with the mid-1970s experiencing the most dust. Blowing dust generally was absent from all stations during 1979–1983, 1987–1989, and 1992–1994. The mid-1980s were moderately dusty and 1990–1991 saw a return to very dusty conditions, possibly resulting from below-normal precipitation and increased anthropogenic disturbances. Dust events in the Mojave Desert characteristically occur during the winter to spring months (February-May), associated with dry frontal activity, and are largely absent during the dry summer months. The Colorado Desert experiences a similar seasonal distribution of dust events, but has more summer events, usually associated with convective thunderstorms. Frequencies of blowing dust have weak, but statistically significant, correlations with mean annual and antecedent precipitation, suggesting that complex processes control dust emission. [Key words: blowing dust, dust storms, Mojave Desert, Colorado Desert, wind erosion.] 相似文献
17.
Njila Souza da Rocha Bijeesh K Veettil Atilio Grondona Silvia Rolim 《Singapore journal of tropical geography》2019,40(3):346-360
Recent decades, particularly since the late 1970s, have witnessed a rapid retreat of glaciers in the tropical Andes. We compiled the changes in glacier surfaces along the eastern cordilleras of the tropical Andes of Peru and Bolivia since the early 1980s from the literature. Water levels from two Brazilian river basins in the Amazon basin (one (Madeira River) glacially fed by meltwater from the Andes and the other (Envira River) non‐glacially fed), were analysed for a 30‐year period between 1985?2014. Furthermore, precipitation data near these two basins were also analysed in order to understand the differential contributions of glacier melting and rainfall. Variations in the water levels from the glacially fed Madeira River showed that some years were associated with higher water levels even when the precipitation remained low during the corresponding season (May‐October). This observation was common when El Niño events occurred during the positive phase of Pacific Decadal Oscillation (PDO). Water levels in glacier‐fed Madeira River were slightly higher during the periods where El Niño and warm PDO co‐occurred. On the other hand, water levels in the Envira River were precipitation dependent; water levels were higher when the rainfall was high. 相似文献
18.
柴达木盆地植被覆盖度的分析与模拟——空间自相关方法的运用(英文) 总被引：3，自引：0，他引：3
Relationship between vegetation and environmental factors has always been a major topic in ecology, but it has also been an important way to reveal vegetation’s dynamic response to and feedback effects on climate change. For the special geographical location and climatic characteristics of the Qaidam Basin, with the support of traditional and remote sensing data, in this paper a vegetation coverage model was established. The quantitative prediction of vegetation coverage by five environmental factors was initially realized through multiple stepwise regression (MSR) models. However, there is significant multicollinearity among these five environmental factors, which reduces the performance of the MSR model. Then through the introduction of the Moran Index, an indicator that reflects the spatial auto-correlation of vegetation distribution, only two variables of average annual rainfall and local Moran Index were used in the final establishment of the vegetation coverage model. The results show that there is significant spatial autocorrelation in the distribution of vegetation. The role of spatial autocorrelation in the establishment of vegetation coverage model has not only improved the model fitting R2 from 0.608 to 0.656, but also removed the multicollinearity among independents. 相似文献
19.
Spanish dehesas and Portuguese montados are similar Iberian silvopastoral systems that provide important environmental services and hold a high biodiversity. However, there is a raising concern for the sustainability of these cultural landscapes, as they suffer from tree recruitment failure. Consequently, trees in those systems are lessening due to tree ageing, diseases and other causes. A key to developing successful management strategies in dehesas is to understand the spatiotemporal behaviour of this process. We analysed the spatial pattern of lost and remaining trees in five dehesas in Extremadura (SW Spain) between 1956 and 2009 based on aerial images. We studied the possible spatial dependency (clustering or segregation) of the lost and remaining trees in areas with low or high land use intensity at different spatial distances. Analyses were performed under the random labelling hypothesis and the critical values were obtained by using 199 permutations of a Monte Carlo test. A linear mixed model was used to investigate the possible factors affecting tree loss. The results indicate a clear segregation between remaining and lost trees in areas of high intensity land use, while in areas with low intensity this pattern was less evident. Also, lost trees were more clustered than the remaining ones. Higher tree loss density occurred at lower slopes, the areas initially occupied by high or medium-density shrubs being less prone to undergo tree loss than those that appeared in 1956 like ploughed lands, grasslands or low-density shrubs. These results show a concentration of the risk of tree loss in certain areas where conservation efforts should be focused. 相似文献
20.
新疆博州地区近46年来的气候变化特征 总被引：23，自引：4，他引：23
根据新疆博尔塔拉蒙古自治州(简称博州,下同)四个气象代表站点的气温及降水资料,利用线性趋势函数及t检验法分析了该地区近46年的气候变化。结果表明,博州年平均气温、平均最高气温、平均最低气温均呈上升趋势,而且平均最低气温上升的幅度远大于平均最高气温的上升幅度。年平均气温及年平均最低气温约在20世纪80年代中期出现了显著的均值突变,而年平均最高气温在80年代末出现突变;除春、夏季平均最高气温无明显变化趋势外,其它各季平均气温、平均最高气温、平均最低气温均呈上升趋势,其中均以冬季增幅最大,秋季次之;80年代冬夜升温最强劲,90年代则明显减弱;夏季的平均气温与平均最低气温均在70年代中期出现突变,比其它任何突变时间都早;暖温年多发生在80年代后,冷温年多发生在20世纪60年代、70年代。年降水量略呈上升趋势,少雨年多在60年代、70年代,多雨年多在近20年,除春季外,其余各季降水略有上升趋势。 相似文献