全文获取类型
收费全文 | 873篇 |
免费 | 68篇 |
国内免费 | 42篇 |
专业分类
测绘学 | 6篇 |
大气科学 | 76篇 |
地球物理 | 116篇 |
地质学 | 166篇 |
海洋学 | 5篇 |
天文学 | 2篇 |
综合类 | 8篇 |
自然地理 | 604篇 |
出版年
2024年 | 4篇 |
2023年 | 4篇 |
2022年 | 22篇 |
2021年 | 28篇 |
2020年 | 26篇 |
2019年 | 29篇 |
2018年 | 36篇 |
2017年 | 30篇 |
2016年 | 37篇 |
2015年 | 51篇 |
2014年 | 30篇 |
2013年 | 55篇 |
2012年 | 61篇 |
2011年 | 59篇 |
2010年 | 38篇 |
2009年 | 48篇 |
2008年 | 44篇 |
2007年 | 46篇 |
2006年 | 36篇 |
2005年 | 41篇 |
2004年 | 23篇 |
2003年 | 32篇 |
2002年 | 21篇 |
2001年 | 20篇 |
2000年 | 16篇 |
1999年 | 24篇 |
1998年 | 12篇 |
1997年 | 20篇 |
1996年 | 26篇 |
1995年 | 17篇 |
1994年 | 17篇 |
1993年 | 5篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
排序方式: 共有983条查询结果,搜索用时 62 毫秒
281.
Longitudinal dunes are the most widespread dune types in the world sand seas but comprehensive study on the sand surface stability
is scarce. The southern part of Gürbantünggüt Desert is mainly covered by longitudinal dune in which fixed and semi-fixed
dunes occupy over 80% of the total area. Systematic analysis on the climatic conditions, the soil moisture and vegetation
distributions, and the sand surface activities showed that the fixed and semi-fixed dunes are in a comprehensive low-energy
wind environment. Snow cover and frozen soil provide a good protection to the ground surface in winter. The temporal distribution
of precipitation and corresponding variation of temperature create a favorable condition for the desert plants growth, especially
for the ephemeral plants. The occurrence of effective winds for sand moving in April to June coincides with the stage of relatively
wet sand surface and good vegetation cover, which effectively keep the sand surface stable at the interdune and the plinth
of the dunes. Activity sand surface appears only at the crest and the upper part of the sand dunes. 相似文献
282.
E. P. Hamerlynck T. E. Huxman R. S. Nowak S. Redar M. E. Loik D. N. Jordan S. F. Zitzer J. S. Coleman J. R. Seemann S. D. Smith 《Journal of Arid Environments》2000,44(4):425
Of all terrestrial ecosystems, the productivity of deserts has been suggested to be the most responsive to increasing atmospheric CO2. The extent to which this prediction holds will depend in part on plant responses to elevated CO2under the highly variable conditions characteristic of arid regions. The photosynthetic responses ofLarrea tridentata , an evergreen shrub, to a step-increase in atmospheric CO2(to 550 μmolmol−1) were examined in the field using Free-Air CO2Enrichment (FACE) under seasonally varying moisture conditions. Elevated CO2substantially increased net assimilation rate (Anet) in Larrea during both moist and dry periods of the potential growing season, while stomatal conductance (gs) did not differ between elevated and ambient CO2treatments. Seasonal and diurnal gas exchange dynamics in elevated CO2mirrored patterns in ambient CO2, indicating that elevated CO2did not extend photosynthetic activity longer into the dry season or during more stressful times of the day. Net assimilation vs. internal CO2(A/Ci) responses showed no evidence of photosynthetic down-regulation during the dry season. In contrast, after significant autumn rains, Amax(the CO2saturated rate of photosynthesis) and CE (carboxylation efficiency) were lower in Larrea under elevated CO2. In situ chlorophyll fluorescence estimation ofLarrea Photosystem II efficiency (Fv/Fm) responded more to water limitation than to elevated CO2. These findings suggest that predictions regarding desert plant responses to elevated CO2should account for seasonal patterns of photosynthetic regulatory responses, which may vary across species and plant functional types. 相似文献
283.
风沙活动区工程线路走向与风沙危害程度的关系--以塔里木沙漠公路为例 总被引:9,自引:2,他引:7
在风沙活动区,公路或铁路等工程线路的走向无疑会对其本身及防护体系所受的风沙危害程度产生影响。本文以塔里木沙漠公路为例,提出在将风况与公路走向结合起来考虑的同时,以风沙活动强度垂直分量的转化率作为评价公路走向的一种数学方法,并用该方法对沙漠公路不同路段走向进行评价与说明。 相似文献
284.
60ka来腾格里沙漠东南缘风成沉积与沙漠演化 总被引:11,自引:3,他引:8
对腾格里沙漠东南缘风成沉积相典型剖面(中卫南山剖面)年代的测定,建立了60 ka以来腾格里沙漠演化的时间序列,并结合各地层粒度组分分析结果认为60 ka以来腾格里沙漠演化共经历了5个阶段。60~20 ka B.P.,腾格里沙漠的范围远没有到达现在的位置;20~10 ka B.P.,沙漠向东南方向大规模扩展,奠定了东南缘现代沙漠环境的格局;8.1~5.2 ka B.P.,沙漠东南部得到固定,沙漠范围缩小;11~8.1ka B.P.与5.2 ka B.P.至现代两阶段和现代沙漠环境接近。 相似文献
285.
286.
287.
古尔班通古特沙漠生物结皮小尺度分异的环境特征 总被引:12,自引:5,他引:7
古尔班通古特沙漠广泛存在的生物结皮,对地貌部位有极强的选择性分布,其实质是生物对环境条件综合适应的一种生态表现。2002年3~11月在个体沙丘表面初步开展了生物结皮类型小尺度环境分异规律研究。结果表明:苔藓结皮、地衣结皮和藻类结皮依次分布于垄间、沙垄两坡中部和坡上部,从垄间往垄顶,生物结皮盖度逐渐减小,厚度变薄,抗压性减弱。苔藓结皮分布区的物质组成以细沙和极细沙为主,春季表层土壤水分在5%以上,短命植物广泛发育,基质稳定;坡中部的地衣结皮分布区以细沙为主,春季表层土壤水分在4%左右,短命植物亦有广泛分布,地表受风沙活动影响较小;沙垄上部和顶部的藻类结皮分布区,是沙垄表面活动性最强和土壤水分最差的区域,物质组成以中沙为主,高等植物中白梭梭为优势种,可见藻结皮是三类结皮中最能适应恶劣环境的生物结皮类型。 相似文献
288.
乌兰布和沙漠北部全新世地貌演化 总被引:5,自引:2,他引:3
根据对乌兰布和沙漠北部地区全新世风沙和湖泊沉积记录及其沉积时代进行分析,认为全新世以来,乌兰布和沙漠北部地区地貌演化经历了4个主要的发育时期:Q33~Q41湖泊地貌发育时期,Q41风沙地貌发育时期,Q42湖(河)(屠申泽)地貌发育时期和Q43风沙地貌发育时期。全新世中期屠申泽最为繁盛的时期,湖泊相互沟通,范围几乎占据了整个乌兰布和沙漠北部地区,之后经历了自南向北的萎缩、分化过程,而屠申泽的萎缩、分化过程正是乌兰布和沙漠晚全新世风沙地貌发育并不断扩张的过程,这是一个以自然环境变化为主因、以人为影响为辅因的变化过程。 相似文献
289.
塔里木盆地南缘水资源开发与绿洲的生态环境效应 总被引:20,自引:10,他引:10
主要讨论塔里木盆地南缘的自然环境特点、水资源及其开发利用, 揭示在百年来气候变化和人类活动影响下, 特别是近50a来, 对沙漠南缘的生态脆弱带的绿洲产生明显的生态环境负效应, 引起的生态环境退化极不利于区域可持续发展。为此需要从区域生态环境战略考虑, 调整水资源配置和优化利用, 实施天然绿洲与人工绿洲相应的保护和并重发展, 逐步恢复天然绿洲的水分生态平衡。这样, 才能遏制住强劲的区域土地退化态势, 抵御沙漠化灾害, 并走向良性循环。 相似文献
290.
古尔班通古特沙漠短命植物分布及其沙面稳定意义 总被引:7,自引:3,他引:4
Based on systematically monitoring plants on dune ridges in the southern part of the Gurbantunggut Desert in 2002, this paper, from the angle of dune stabilization by vegetation,describes the temporal and spatial distribution patterns of ephemeral plants on isolated sand dunes,analyses the natural invasion processes of ephemeral plants on human-disturbed sand surface and expounds the importance of ephemeral plants in stabilizing sand dune surface. A total of 45 plant species were identified in the study area, 29 of which are ephemeral plants. Ephemeral plants sprouted in early April and completed their life-circle within about two months. Just as aeolian sand activities came to the strongest stage from April to June in desert regions of northern Xinjiang, the total coverage of trees, shrubs and herbs of long vegetational period on most dune ridges was less than 10%, while the mean coverage of ephemeral plants reached 13.9% in April, 40.2% in May and 14.1% in June. Therefore ephemeral plants acted as the major contributor to dune surface stabilization in the Gurbantunggut Desert.Investigations of vegetation restoration on engineering-disturbed dune surface show that ephemeral plants first recolonized the disturbed dune surface. 相似文献