全文获取类型
收费全文 | 2966篇 |
免费 | 407篇 |
国内免费 | 297篇 |
专业分类
测绘学 | 89篇 |
大气科学 | 36篇 |
地球物理 | 1344篇 |
地质学 | 860篇 |
海洋学 | 379篇 |
天文学 | 17篇 |
综合类 | 92篇 |
自然地理 | 853篇 |
出版年
2024年 | 18篇 |
2023年 | 33篇 |
2022年 | 56篇 |
2021年 | 118篇 |
2020年 | 141篇 |
2019年 | 132篇 |
2018年 | 121篇 |
2017年 | 121篇 |
2016年 | 115篇 |
2015年 | 123篇 |
2014年 | 128篇 |
2013年 | 204篇 |
2012年 | 116篇 |
2011年 | 147篇 |
2010年 | 116篇 |
2009年 | 168篇 |
2008年 | 173篇 |
2007年 | 170篇 |
2006年 | 195篇 |
2005年 | 161篇 |
2004年 | 134篇 |
2003年 | 127篇 |
2002年 | 112篇 |
2001年 | 117篇 |
2000年 | 86篇 |
1999年 | 86篇 |
1998年 | 70篇 |
1997年 | 69篇 |
1996年 | 48篇 |
1995年 | 38篇 |
1994年 | 50篇 |
1993年 | 25篇 |
1992年 | 23篇 |
1991年 | 22篇 |
1990年 | 11篇 |
1989年 | 14篇 |
1988年 | 9篇 |
1987年 | 18篇 |
1986年 | 9篇 |
1985年 | 7篇 |
1984年 | 10篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 3篇 |
排序方式: 共有3670条查询结果,搜索用时 15 毫秒
51.
雅鲁藏布江河谷(尤其是"一江两河"区域)风沙化较为严重,制约了当地社会经济的可持续发展,追踪河谷风沙来源对当地风沙治理意义重大。本研究聚焦于拉萨贡嘎机场附近的山南宽谷风沙地貌,采集2个河沙、3个风成沙丘和1个坡积物样品,以及引用前人研究的2个河沙样品。基于碎屑锆石U-Pb年龄及微量元素地球化学特征,进行沙源示踪研究,旨在为当地风沙治理提供科学依据。对360颗锆石进行LA-ICP-MS U-Pb定年,获得322个谐和年龄。分析结果表明,河沙、风成沙丘及坡积物样品碎屑锆石U-Pb年龄分布特征高度相似,主要峰值与冈底斯岩浆弧新生代的两期岩浆活动一一对应;非矩阵多维标度统计技术(MDS)分析也证实了样品与冈底斯岩浆弧有良好的亲缘性;地球化学判别图解显示锆石具有亏损轻稀土元素、富集重稀土元素、明显正Ce异常和明显负Eu异常的特征,亦指示其岩浆成因,表明冈底斯岩浆弧为沙丘、河沙与坡积物中碎屑锆石的主要来源。依据不同类型样品碎屑锆石年龄对比以及锆石年龄相似程度对比,结果发现:1)雅鲁藏布江地区风沙地貌主要是由于枯水季河床水位下降,导致河岸和河漫滩出露地表,就地起沙吹扬到山坡上所形成;2)研究区河沙并非主要从上游搬运而来,更多是拉萨河搬运汇入的泥沙。依据沙源分析结果,欲解决河谷风沙化问题,首要应在河岸和河漫滩种植适宜当地气候的耐旱植物固定河床上的泥沙,这是最直接有效的手段。 相似文献
52.
J.A. Dearing J.F. Boyle P.G. Appleby A.W. Mackay R.J. Flower 《Journal of Paleolimnology》1998,20(2):163-173
Mineral magnetic measurements of six 210Pb-dated surface cores from different basins of Lake Baikal, Siberia, show temporal records controlled by a range of internal and external processes. With the exception of sediments on the Academician Ridge, there is clear evidence for widespread reductive diagenesis effects on the ferrimagnetic component coupled with neo-formation of paramagnetic iron minerals. Greigite formation, bacterial magnetosome accumulation and turbidite layers may affect the properties of some sediment levels. Concentrations of canted antiferromagnetic minerals (eg. haematite) appear to increase from the 19th century onwards. These minerals are less affected by dissolution processes and probably represent detrital minerals delivered by catchment fluvial processes. The magnetic evidence for recent atmospheric pollution by fossil-fuel combustion processes is weak in all the cores, and supports the findings from studies of spherical carbonaceous particles (SCPs) and heavy metals that pollution is largely restricted to the southern basin. Correlations between recent sediments based on magnetic data may be insecure over long distances or between basins. 相似文献
53.
从生态学和地理学的角度提出“水土流失带”的概念,及其在城市边缘地区的分布,剖析了它的空间结构,物质组成结构,指出其域界范围,宽度、“环状”;分布,垂直结构特征及物质组成结构的多元性。 相似文献
54.
55.
A record of accelerated erosion in the recent sediments of Blelham Tarn in the English Lake district 总被引:1,自引:0,他引:1
K. D. van der Post F. Oldfield E. Y. Haworth P. R. J. Crooks P. G. Appleby 《Journal of Paleolimnology》1997,18(2):103-120
Two frozen cores from Blelham Tarn were subsampled and measured using mineral magnetic, loss-on-ignition (LOI), radiometric, granulometric and diatom analyses. A detailed chronology was established using varves, radioisotopes and diatoms. This has enabled an accurately dated reconstruction of sedimentation over the past forty years. Despite a large increase in lake productivity, evidence suggests that the observed exponential increase in sedimentation rates can be attributed to erosion within the catchment. The predominant sediment source has been identified as surface soil. A comparison between the trend of accelerated sedimentation and the record of increased sheep stocking density for the area within which the most of the catchment lies, as well as observations of contemporary surface processes within the catchment, both suggest that much of the recent erosion is a direct response to increased pressure from sheep grazing. 相似文献
56.
Marcelo Pablo Chartier Csar Mario Rostagno Fidel Alejandro Roig 《Geomorphology》2009,106(3-4):344-351
Soil erosion is an important process of land degradation in many rangelands and a significant driver of desertification in the world's drylands. Dendrogeomorphology is an alternative to traditional methods for determining soil erosion rate. Specifically, the vertical distance between the upper portion of exposed roots and the actual soil surface can be used as a bioindicator of erosion since plant establishment. In this study, we determined (i) the soil erosion rate from exposed roots of the dwarf shrub Margyricarpus pinnatus [Lam.] Kuntze in two ecological sites in the northeastern rangelands of Patagonia and (ii) the relationship between shrub age and upper root diameter. We selected two ecological sites, a pediment-like plateau and a flank pediment, where the dominant soils were Xeric Haplocalcids and Xeric Calciargids, respectively. The soil erosion rates in the pediment-like plateau and in the flank pediment were 2.4 and 3.1 mm yr− 1, respectively. Data clearly indicate a high rate of soil erosion during the mean 8-year life span of the dwarf shrubs in degraded patches, which represent ~ 10% of surface cover in the study area. Simple linear regression analysis yielded a highly significant predictive model for age estimation of M. pinnatus plants using the upper root diameter as a predictor variable. The measurement of ground lowering against datable exposed roots represents a simple method for the determination of soil erosion rates. In combination with other soil surface features, it was used to infer the episodic nature of soil erosion. This approach could be particularly useful for monitoring the effects of land management practices on recent soil erosion and for the establishment of records in regions where historical data regarding this process are scarce or absent. 相似文献
57.
Land degradation: A global perspective 总被引:1,自引:0,他引:1
Arthur Conacher 《New Zealand geographer》2009,65(2):91-94
Land degradation has a long history and is on the increase. It has adverse consequences for people's economies, health and well-being, and for ecosystems, and it is causally linked to population growth and inappropriate land-use practices. It is an urban as well as a rural problem. Solutions require an integrated, geographical approach. Otherwise, some problems may be ignored and others created inadvertently. 相似文献
58.
《The Professional geographer》1988,40(1):116-131
Books reviewed in this article: Boundary Control and Legal Priniciples . Curtis M. Brown , Walter G. Robillard and Donald A. Wilson . Lost Initiatives: Canada's Forest Industries, Forest Policy, and Forest Conservation . R. Peter Gillis and Thomas R. Roach . The Navajo Atlas: Resources, People, and History of the Dine Bikeyah . James M. Goodman . The City and the Sign . M. Gottdiener AND Alexandros Ph . Lagopoulos , EDS. The Comfortable House: North American Suburban Architecture 1890–1930 . Alan Gowans . Arabic-Islamic Cities. Building and Planning Principles . Besim Salim Hakim . Contemporary Climatology . Ann Henderson -Sellers and Peter J. Robinson . A Trace of Desert Waters . Samuel G. Houghton . Geography, Resources and Environment. Vol. 1: Selected Writings of Gilbert F. White. Vol. 2: Themes from the Work of Gilbert F. White . Robert W. Kates AND Ian Burton , ED. Spatial Dimensions of Unemployment and Underemployment (A Case Study of Rural Punjab) . Gopal Krishan . Maps of the Holy Land: Cartobibliography of Printed Maps, 1475–1900 . Compiled by Eran Laor , assisted by Shoshana Klein . The Geography of Third World Cities . Stella Lowder . A Perspective on U.S. Farm Problems and Agricultural Policy . Lance Mc Kinzie , Timothy G. Baker AND Wallace E. Tyner . Soil Erosion and Its Control. R.P.C . Morgan , ED Geopolitics . Patrick O'Sullivan . Landscape, Meanings and Values . Edmund C. Penning -Rowsell AND David Lowenthal , EDS. 相似文献
59.
In order to monitor the pattern, distribution, and trend of land use/cover change (LUCC) and its impacts on soil erosion, it is highly appropriate to adopt Remote Sensing (RS) data and Geographic Information System (GIS) to analyze, assess, simulate, and predict the spatial and temporal evolution dynamics. In this paper, multi-temporal Landsat TM/ETM+ remotely sensed data are used to generate land cover maps by image classification, and the Cellular Automata Markov (CA_Markov) model is employed to simulate the evolution and trend of landscape pattern change. Furthermore, the Revised Universal Soil Loss Equation (RUSLE) is used to evaluate the situation of soil erosion in the case study mining area. The trend of soil erosion is analyzed according to total/average amount of soil erosion, and the rainfall (R), cover management (C), and support practice (P) factors in RUSLE relevant to soil erosion are determined. The change trends of soil erosion and the relationship between land cover types and soil erosion amount are analyzed. The results demonstrate that the CA_Markov model is suitable to simulate and predict LUCC trends with good efficiency and accuracy, and RUSLE can calculate the total soil erosion effectively. In the study area, there was minimal erosion grade and this is expected to continue to decline in the next few years, according to our prediction results. 相似文献
60.
Capacity of soil loss control in the Loess Plateau based on soil erosion control degree 总被引:4,自引:0,他引:4
Haidong Gao Zhanbin Li Lianlian Jia Peng Li Guoce Xu Zongping Ren Guowei Pang Binhua Zhao 《地理学报(英文版)》2016,26(4):457-472
The capacity of soil and water conservation measures, defined as the maximum quantity of suitable soil and water conservation measures contained in a region, were determined for the Loess Plateau based on zones suitable for establishing terraced fields, forestland and grassland with the support of geographic information system (GIS) software. The minimum possible soil erosion modulus and actual soil erosion modulus in 2010 were calculated using the revised universal soil loss equation (RUSLE), and the ratio of the minimum possible soil erosion modulus under the capacity of soil and water conservation measures to the actual soil erosion modulus was defined as the soil erosion control degree. The control potential of soil erosion and water loss in the Loess Plateau was studied using this concept. Results showed that the actual soil erosion modulus was 3355 t?km-2?a-1, the minimum possible soil erosion modulus was 1921 t?km-2?a-1, and the soil erosion control degree was 0.57 (medium level) in the Loess Plateau in 2010. In terms of zoning, the control degree was relatively high in the river valley-plain area, soil-rocky mountainous area, and windy-sandy area, but relatively low in the soil-rocky hilly-forested area, hilly-gully area and plateau-gully area. The rate of erosion areas with a soil erosion modulus of less than 1000 t?km-2?a-1 increased from 50.48% to 57.71%, forest and grass coverage rose from 56.74% to 69.15%, rate of terraced fields increased from 4.36% to 19.03%, and per capita grain available rose from 418 kg?a-1 to 459 kg?a-1 under the capacity of soil and water conservation measures compared with actual conditions. These research results are of some guiding significance for soil and water loss control in the Loess Plateau. 相似文献