收费全文 | 8259篇 |
免费 | 1471篇 |
国内免费 | 2201篇 |
测绘学 | 542篇 |
大气科学 | 2300篇 |
地球物理 | 1966篇 |
地质学 | 4031篇 |
海洋学 | 1107篇 |
天文学 | 364篇 |
综合类 | 761篇 |
自然地理 | 860篇 |
2024年 | 13篇 |
2023年 | 108篇 |
2022年 | 340篇 |
2021年 | 381篇 |
2020年 | 316篇 |
2019年 | 406篇 |
2018年 | 411篇 |
2017年 | 364篇 |
2016年 | 460篇 |
2015年 | 404篇 |
2014年 | 461篇 |
2013年 | 509篇 |
2012年 | 410篇 |
2011年 | 434篇 |
2010年 | 401篇 |
2009年 | 486篇 |
2008年 | 369篇 |
2007年 | 367篇 |
2006年 | 320篇 |
2005年 | 311篇 |
2004年 | 264篇 |
2003年 | 294篇 |
2002年 | 240篇 |
2001年 | 262篇 |
2000年 | 323篇 |
1999年 | 455篇 |
1998年 | 352篇 |
1997年 | 354篇 |
1996年 | 362篇 |
1995年 | 298篇 |
1994年 | 249篇 |
1993年 | 224篇 |
1992年 | 175篇 |
1991年 | 146篇 |
1990年 | 115篇 |
1989年 | 122篇 |
1988年 | 103篇 |
1987年 | 57篇 |
1986年 | 59篇 |
1985年 | 42篇 |
1984年 | 38篇 |
1983年 | 18篇 |
1982年 | 18篇 |
1981年 | 24篇 |
1980年 | 16篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1976年 | 6篇 |
1973年 | 7篇 |
1958年 | 8篇 |
In many arid ecosystems, vegetation frequently occurs in high-cover patches interspersed in a matrix of low plant cover. However, theoretical explanations for shrub patch pattern dynamics along climate gradients remain unclear on a large scale. This context aimed to assess the variance of the Reaumuria soongorica patch structure along the precipitation gradient and the factors that affect patch structure formation in the middle and lower Heihe River Basin (HRB). Field investigations on vegetation patterns and heterogeneity in soil properties were conducted during 2014 and 2015. The results showed that patch height, size and plant-to-patch distance were smaller in high precipitation habitats than in low precipitation sites. Climate, soil and vegetation explained 82.5% of the variance in patch structure. Spatially, R. soongorica shifted from a clumped to a random pattern on the landscape towards the MAP gradient, and heterogeneity in the surface soil properties (the ratio of biological soil crust (BSC) to bare gravels (BG)) determined the R. soongorica population distribution pattern in the middle and lower HRB. A conceptual model, which integrated water availability and plant facilitation and competition effects, was revealed that R. soongorica changed from a flexible water use strategy in high precipitation regions to a consistent water use strategy in low precipitation areas. Our study provides a comprehensive quantification of the variance in shrub patch structure along a precipitation gradient and may improve our understanding of vegetation pattern dynamics in the Gobi Desert under future climate change.
相似文献The factors affecting permeability change under repeated mining of coal seams are important study aspects that need to be explored. This study combined various stress variation characteristics of protective seam mining and simplified the stress path of repeated mining in protective seam mines. Based on the results from the bespoke gas flow and displacement testing apparatus, seepage tests for simulated repetitive mining were carried out. The results simulated the actual behavior very well. With any drastic increase in the mining influence, the axial deviation stress in the stress path increased, and the greater the difference in coal permeability during the unloading and stress recovery stage, the more substantial the increase in permeability. The change in coal permeability was significantly influenced by the severity of simulated repeated mining cycles. When the mining stress exceeded a critical value, the permeability of the coal sample increased with the increase in the number of loading and unloading cycles, but the reverse was true when the mining stress was lower than the critical value. The effective sensitivity of seepage to the applied stress decreased with an increase in the number of stress cycles. With a decrease in the deviation stress, that is, with lower severity of mining influence, the effective sensitivity of coal seepage to stress gradually decreased.
相似文献Exploring the spatial relationships between various geological features and mineralization is not only conducive to understanding the genesis of ore deposits but can also help to guide mineral exploration by providing predictive mineral maps. However, most current methods assume spatially constant determinants of mineralization and therefore have limited applicability to detecting possible spatially non-stationary relationships between the geological features and the mineralization. In this paper, the spatial variation between the distribution of mineralization and its determining factors is described for a case study in the Dingjiashan Pb–Zn deposit, China. A local regression modeling technique, geological weighted regression (GWR), was leveraged to study the spatial non-stationarity in the 3D geological space. First, ordinary least-squares (OLS) regression was applied, the redundancy and significance of the controlling factors were tested, and the spatial dependency in Zn and Pb ore grade measurements was confirmed. Second, GWR models with different kernel functions in 3D space were applied, and their results were compared to the OLS model. The results show a superior performance of GWR compared with OLS and a significant spatial non-stationarity in the determinants of ore grade. Third, a non-stationarity test was performed. The stationarity index and the Monte Carlo stationarity test demonstrate the non-stationarity of all the variables throughout the area. Finally, the influences of the degree of non-stationary of all controlling factors on mineralization are discussed. The existence of significant non-stationarity of mineral ore determinants in 3D space opens up an exciting avenue for research into the prediction of underground ore bodies.
相似文献