全文获取类型
收费全文 | 1015篇 |
免费 | 258篇 |
国内免费 | 230篇 |
专业分类
测绘学 | 73篇 |
大气科学 | 206篇 |
地球物理 | 284篇 |
地质学 | 571篇 |
海洋学 | 99篇 |
天文学 | 14篇 |
综合类 | 119篇 |
自然地理 | 137篇 |
出版年
2024年 | 5篇 |
2023年 | 18篇 |
2022年 | 89篇 |
2021年 | 80篇 |
2020年 | 61篇 |
2019年 | 60篇 |
2018年 | 82篇 |
2017年 | 72篇 |
2016年 | 75篇 |
2015年 | 60篇 |
2014年 | 74篇 |
2013年 | 61篇 |
2012年 | 50篇 |
2011年 | 64篇 |
2010年 | 61篇 |
2009年 | 47篇 |
2008年 | 52篇 |
2007年 | 41篇 |
2006年 | 45篇 |
2005年 | 46篇 |
2004年 | 32篇 |
2003年 | 33篇 |
2002年 | 14篇 |
2001年 | 18篇 |
2000年 | 24篇 |
1999年 | 23篇 |
1998年 | 32篇 |
1997年 | 26篇 |
1996年 | 28篇 |
1995年 | 20篇 |
1994年 | 21篇 |
1993年 | 16篇 |
1992年 | 23篇 |
1991年 | 9篇 |
1990年 | 8篇 |
1989年 | 5篇 |
1988年 | 12篇 |
1987年 | 1篇 |
1986年 | 7篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有1503条查询结果,搜索用时 46 毫秒
921.
Qingjie Han Jianjun Qu Kongtai Liao Shujuan Zhu Kecun Zhang Ruiping Zu Qinghe Niu 《Environmental Earth Sciences》2011,64(5):1375-1385
This article reported a wind tunnel test of sediment transport related to surface moisture content and wind velocity using
sands from tropical humid coastal area. A 1 mm-thick portion of surface sand was scraped using a self-made sediment sampler,
and the gravimetric moisture content was determined. Sand transport was measured via a standard vertical sand trap with a
60 cm height. The result shows that the sand transport profile above the wet surface can be expressed with an exponential
equation. In general, the influence of moisture content on sand transport profile mainly focuses on the bottom of the blowing
sand cloud. Meanwhile, with moisture content increased, total sand transport dropped, and a relatively larger proportion is
transported at greater heights. The vertical movement of particles on higher moisture surface (0.587% < M < 1.448%) is more sensitive to moisture content variation as compared to those on low wet surface (M < 0.587%), total sand transport rate tends to be rather low (0.99 g cm−1 s−1) when M > 1.448%. The total sand transport rate varying with moisture content is divided into three regions of differing gradient
at the moisture contents of 0.587 and 1.448%. The gradient of the curve reflected the different influences of the various
water forms in surface sediments. The higher moisture surface (M > 1.448%) merely functions as a transport plain for the saltation material. Surface moisture content was the dominant control
factor for saltation activity between the moisture contents of 0.587 and 1.448%, wind velocity could resume control saltation
after the surface dried to the extent (M < 0.587%). 相似文献
922.
923.
924.
Alpine soil infiltration process is an important part of the hydrological characteristics of alpine soil in permafrost. This research is carried out in the source region of the Yellow River where the permafrost is severely degraded, using various methods for choosing typical sample areas, and to experiment, study and simulate the soil water curve, soil saturated hydraulic conductivity, soil infiltration and soil moisture under different characteristics of degraded vegetation. The results indicate that the empirical equation θ=AS−B, proposed by Gradner and Visser, is very reliable in simulating the soil moisture curve; soil saturated hydraulic conductivity and soil infiltration are significantly different under different vegetation coverage: in the soil surface within 0–10 cm, the saturated hydraulic conductivity and infiltration intensity of Black Beach are the strongest; respectively, in soil layers below 30 cm, vegetation has almost no impacts on the saturated hydraulic conductivity, infiltration intensity and soil moisture content. Significant reduction of soil moisture occurs in soil surfaces with degraded vegetation. The more serious the degradation, the more water loss, and it can be up to 38.6% in the worst situation. Soil moisture of developed vegetation root systems in depths within 10–20 cm has the greatest impact on the soil environment, and the loss of moisture induces difficulty in the restoration of degraded meadows. Through a comparative study, the Kostiakov infiltration equation f(t) = at−b is more applicable for studies on the process of soil moisture infiltration of the alpine meadow in the source region of the Yellow River. 相似文献
925.
Baoyu Jiang Hugh D. Sinclair Yazhuo Niu Jinhai Yu 《International Journal of Earth Sciences》2014,103(1):23-40
The Early Paleozoic Wuyi-Yunkai fold belt of South China represents a major orogenic belt in East Asia; however, its formation is not well understood. This paper evaluates the origins of the Neoproterozoic–Early Paleozoic basin and the southern margin of the South China Block (SCB) based on a compilation of the spatiotemporal evolution of the Late Neoproterozoic–Early Paleozoic strata and their depositional environments. These data are combined with high-resolution structural, geochronologic, metamorphic and petrologic data that are published from the region. A new model of a retroarc thrust wedge/foreland basin system is proposed for the succession based on the following: (1) the nature of the basal unconformity, (2) the wedge-shaped basin-fill geometry bounded by the mountain belt to the south, (3) basinward propagation of the thrust-front with time and (4) the presence of a typical underfilled stratigraphic trinity of the foreland basin. The proposed model suggests two major cycles of evolution of the retroarc thrust wedge/foreland basin system (<725–<635 and ca. 564–446 Ma), which record two periods of northwestward subduction of the oceanic slab beneath the southern margin of the SCB. Terrane accretion is inferred to have taken place during the late stage of these cycles. 相似文献
926.
Landslide susceptibility assessment using object mapping units,decision tree,and support vector machine models in the Three Gorges of China 总被引:4,自引:3,他引:1
Due to the particular geographical location and complex geological conditions, the Three Gorges of China suffer from many landslide hazards that often result in tragic loss of life and economic devastation. To reduce the casualty and damages, an effective and accurate method of assessing landslide susceptibility is necessary. Object-based data mining methods were applied to a case study of landslide susceptibility assessment on the Guojiaba Town of the Three Gorges. The study area was partitioned into object mapping units derived from 30 m resolution Landsat TM images using multi-resolution segmentation algorithm based on the landslide factors of engineering rock group, homogeneity, and reservoir water level. Landslide locations were determined by interpretation of Landsat TM images and extensive field surveys. Eleven primary landslide-related factors were extracted from the topographic and geologic maps, and satellite images. Those factors were selected as independent variables using significance testing and correlation coefficient analysis, including slope, profile curvature, engineering rock group, slope structure, distance from faults, land cover, tasseled cap transformation wetness index, reservoir water level, homogeneity, and first and second principal components of the images. Decision tree and support vector machine (SVM) models with the optimal parameters were trained and then used to map landslide susceptibility, respectively. The analytical results were validated by comparing them with known landslides using the success rate and prediction rate curves and classification accuracy. The object-based SVM model has the highest correct rate of 89.36 % and a kappa coefficient of 0.8286 and outperforms the pixel-based SVM, object-based C5.0, and pixel-based SVM models. 相似文献
927.
How to evaluate reasonably the stability of a soil slope reinforced with piles (SSRP) still is an urgent problem. At present, the three-dimensional (3D) finite element strength reduction method has been used for the soil slope stability analysis. However, to accurately determine the global instability of soil slopes is the key to implementing the strength reduction finite element method. In this paper, the 3D finite element strength reduction algorithm (FESRA), based on Hill’s model theory, is proposed to assess the stability of SSRP and study on the relationship between the safety coefficients of SSRP and the displacements of slope mass. The results show that: (1) the relationship between the safety coefficients of SSRP and the displacements of slope mass agrees with the Hill’s model; (2) the proposed method (3D FESRA based on Hill’s model theory) in this study may take into account simultaneously the pile response and slope stability, and makes the results of SSRP stability analysis reasonable and reliable, which could be used as a reference for the evaluation of stability of the same type of slope; and (3) further study should be done to confirm whether the proposed method in this study is suitable for other types of slopes. 相似文献
928.
The role of rainfall in the thermal-moisture dynamics of the active layer at Beiluhe of Qinghai-Tibetan plateau 总被引:1,自引:1,他引:0
Zhi Wen Fujun Niu Qihao Yu Dayan Wang Wenjie Feng Jianfeng Zheng 《Environmental Earth Sciences》2014,71(3):1195-1204
The active layer in permafrost regions plays an important role in energy exchange between permafrost and atmosphere. Rainfall is one of the dominant factors affecting thermal-moisture dynamics of the active layer. To better understand the thermal-moisture dynamics and the interaction between rainfall and the active layer in-detail, in situ experiment was carried out and soil temperature, soil moisture and soil heat flux of the active layer were measured from 2007 to 2009. The observation data demonstrated that the volumetric soil water content of the active layer remained fairly constant during the winter and had a notable fluctuation resulted from evapotranspiration and rewetting from rainfall events in summer. The daily variation amplitude of soil temperature and soil heat flux in summer was bigger than that in winter. Soil moisture content increased and soil temperature decreased after rainfall. Rainfall in summer led to the change of surface energy balance and caused subsurface soil cooling. The convective heat transfer from water infiltration reduced the temperature gradient along depth and changed near-surface heat fluxes. The increase in rainfall may mitigate permafrost degradation on the Tibetan Plateau. 相似文献
929.
930.
基于诱导有序加权平均(IOWA)算子,将差分整合移动平均自回归(ARIMA)模型和Holt-Winters指数平滑模型进行组合,采用SBAS-InSAR监测值进行矿区地表沉降预测,并与各单一模型的预测结果进行对比分析。结果表明,基于IOWA算子的组合模型的预测精度较单一模型有明显提升,其中各点均方误差(MSE)平均值为1.458 mm,平均绝对误差(MAE)为2.175 mm,可用于矿山地表沉降监测预测。 相似文献