全文获取类型
收费全文 | 41620篇 |
免费 | 3390篇 |
国内免费 | 5040篇 |
专业分类
测绘学 | 2356篇 |
大气科学 | 5575篇 |
地球物理 | 9334篇 |
地质学 | 19927篇 |
海洋学 | 3551篇 |
天文学 | 2391篇 |
综合类 | 3666篇 |
自然地理 | 3250篇 |
出版年
2024年 | 95篇 |
2023年 | 345篇 |
2022年 | 801篇 |
2021年 | 943篇 |
2020年 | 754篇 |
2019年 | 805篇 |
2018年 | 5419篇 |
2017年 | 4665篇 |
2016年 | 3403篇 |
2015年 | 1052篇 |
2014年 | 1085篇 |
2013年 | 1123篇 |
2012年 | 1912篇 |
2011年 | 3604篇 |
2010年 | 3011篇 |
2009年 | 3202篇 |
2008年 | 2638篇 |
2007年 | 3117篇 |
2006年 | 704篇 |
2005年 | 812篇 |
2004年 | 830篇 |
2003年 | 895篇 |
2002年 | 745篇 |
2001年 | 537篇 |
2000年 | 658篇 |
1999年 | 916篇 |
1998年 | 788篇 |
1997年 | 761篇 |
1996年 | 731篇 |
1995年 | 646篇 |
1994年 | 571篇 |
1993年 | 490篇 |
1992年 | 410篇 |
1991年 | 287篇 |
1990年 | 231篇 |
1989年 | 189篇 |
1988年 | 181篇 |
1987年 | 102篇 |
1986年 | 111篇 |
1985年 | 68篇 |
1984年 | 58篇 |
1983年 | 57篇 |
1982年 | 55篇 |
1981年 | 61篇 |
1980年 | 66篇 |
1979年 | 36篇 |
1978年 | 14篇 |
1976年 | 12篇 |
1974年 | 9篇 |
1958年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
There is a close relationship between groundwater level in a shallow aquifer and the surface ecological environment; hence, it is important to accurately simulate and predict the groundwater level in eco-environmental construction projects. The multiple linear regression (MLR) model is one of the most useful methods to predict groundwater level (depth); however, the predicted values by this model only reflect the mean distribution of the observations and cannot effectively fit the extreme distribution data (outliers). The study reported here builds a prediction model of groundwater-depth dynamics in a shallow aquifer using the quantile regression (QR) method on the basis of the observed data of groundwater depth and related factors. The proposed approach was applied to five sites in Tianjin city, north China, and the groundwater depth was calculated in different quantiles, from which the optimal quantile was screened out according to the box plot method and compared to the values predicted by the MLR model. The results showed that the related factors in the five sites did not follow the standard normal distribution and that there were outliers in the precipitation and last-month (initial state) groundwater-depth factors because the basic assumptions of the MLR model could not be achieved, thereby causing errors. Nevertheless, these conditions had no effect on the QR model, as it could more effectively describe the distribution of original data and had a higher precision in fitting the outliers. 相似文献
992.
Hanne M. L. Kvitsand Mette Myrmel Liv Fiksdal Stein W. Østerhus 《Hydrogeology Journal》2017,25(5):1257-1269
Two case studies were carried out in central Norway in order to assess the performance of bank filtration systems in cold-climate fluvial aquifers relying on recharge from humic-rich surface waters with moderate microbial contamination. Three municipal wells and two surface-water sources at operative bank filtration systems were monitored for naturally occurring bacteriophages, fecal indicators, natural organic matter (NOM) and physico-chemical water quality parameters during a 4-month period. Aquifer passage effectively reduced the microorganism and NOM concentrations at both study sites. Bacteriophages were detected in 13 of 16 (81%) surface-water samples and in 4 of 24 (17%) well-water samples, and underwent 3 ± 0.3 log10 reduction after 50–80-m filtration and 20–30 days of subsurface passage. NOM reductions (color: 74–97%; dissolved organic carbon: 54–80%; very hydrophobic acids: 70%) were similar to those achieved by conventional water-treatment processes and no further treatment was needed. Both groundwater dilution and sediment filtration contributed to the hygienic water quality improvements, but sediment filtration appeared to be the most important process with regard to microbial and NOM reductions. A strengths-weaknesses-opportunities-threats analysis showed that bank filtration technology has a high potential as a pretreatment method for the provision of hygienically safe drinking water in Norway. 相似文献
993.
Naegleria fowleri is a thermophilic free-living amoeba found worldwide in soils and warm freshwater. It is the causative agent of primary amebic meningoencephalitis, a nearly always fatal disease afflicting mainly children and young adults. Humans are exposed to the organism via swimming, bathing, or other recreational activity during which water is forcefully inhaled into the upper nasal passages. Although many studies have looked at the occurrence of N. fowleri in surface waters, limited information is available regarding its occurrence in groundwater and geothermally heated natural waters such as hot springs. This paper reviews the current literature related to the occurrence of N. fowleri in these waters and the methods employed for its detection. Case reports of potential groundwater exposures are also included. Despite increased interest in N. fowleri in recent years due to well-publicized cases linked to drinking water, many questions still remain unanswered. For instance, why the organism persists in some water sources and not in others is not well understood. The role of biofilms in groundwater wells and plumbing in individual buildings, and the potential for warming due to climate change to expand the occurrence of the organism into new regions, are still unclear. Additional research is needed to address these questions in order to better understand the ecology of N. fowleri and the conditions that result in greater risks to bathers. 相似文献
994.
Large-scale landslide dams can induce significant hazards to human lives by blocking the river flows and causing inundation upstream. They may trigger severe outburst flooding that may devastate downstream areas once failed. Thus, the advancement in understanding the formation of landslide dams is highly necessary. This paper presents 3D numerical investigations of the formation of landslide dams in open fluid channels via the discrete element method (DEM) coupled with computational fluid dynamics (CFD). By employing this model, the influence of flow velocity on granular depositional morphology has been clarified. As the grains settle downwards in the fluid channel, positive excess water pressures are generated at the bottom region, reducing the total forces acting on the granular mass. In the meantime, the particle sedimentations into the fluid channel with high impacting velocities can generate fluid streams to flow backwards and forwards. The coupled hydraulic effects of excess water pressure and fluid flow would entrain the solid grains to move long distances along the channel. For simulations using different flow velocities, the larger the flow velocity is, the further distance the grains can be transported to. In this process, the solid grains move as a series of surges, with decreasing deposit lengths for the successive surges. The granular flux into the fluid channel has very little influence on the depositional pattern of particles, while it affects the particle–fluid interactions significantly. The results obtained from the DEM-CFD coupled simulations can reasonably explain the mechanisms of granular transportation and deposition in the formation of landslide dams in narrow rivers. 相似文献
995.
Tapas R. Martha Priyom Roy Ritwik Mazumdar K. Babu Govindharaj K. Vinod Kumar 《Landslides》2017,14(2):697-704
High magnitude earthquakes trigger numerous landslides and their occurrences are mainly controlled by terrain parameters. We created an inventory of 15,551 landslides with a total area of 90.2 km2 triggered by the 2015 Mw 7.8 (Gorkha) and Mw 7.3 (Dolakha) earthquakes in Nepal, through interpretation of very high resolution satellite images (e.g. WorldView, Pleiades, Cartosat-1 and 2, Resourcesat-2). Our spatial analysis of landslide occurrences with ground acceleration, slope, lithology and surface defomation indicated ubiquitous control of steep slope on landslides with ground acceleration as the trigger. Spatial distribution of landslides shows increasing frequency away from the Gorkha earthquake epicentre up to 130 km towards east, dropping sharply thereafter, which is an abnormal phenomenon of coseismic landslides. Landslides are laterally concentrated in three zones which matches well with the seismic rupture evolution of Gorkha earthquake, as reported through teleseismic measurements. 相似文献
996.
997.
Stabilizing piles are widely used as an effective and economic reinforcement approach for slopes. Reasonable designs of pile reinforcement depend on the understanding of reinforcement mechanism of slopes. A series of centrifuge model tests were conducted on the pile-reinforced slopes and corresponding unreinforced slopes under self-weight and vertical loading conditions. The deformation of the slope was measured using image-based analysis and employed to investigate the pile reinforcement mechanism. The test results showed that the piles significantly reduced the deformation and changed the deformation distribution of the slope, and prevented the failure occurred in the unreinforced slope. The pile influence zone was determined according to the inflection points on the distribution curves of horizontal displacement, which comprehensively described the features of the pile–slope interaction and the characteristics of reinforced slopes. The concepts of anti-shear effect and compression effect were proposed to quantitatively describe the restriction features of the piles on the deformation of the slope, namely the reduction in the shear deformation and the increase in the compression deformation, respectively. The pile reinforcement effect mainly occurred in the pile influence zone and decreased with increasing distance from the piles. There was a dominated compression effect in the vicinities of the piles. The compression effect developed upwards in the slope with a transmission to the anti-shear effect. The anti-shear effect became significantly dominated near the slip surface and prevented the failure that occurred in the unreinforced slope. 相似文献
998.
Weidong Sun Jin-tuan Wang Li-peng Zhang Chan-chan Zhang He Li Ming-xing Ling Xing Ding Cong-ying Li Hua-ying Liang 《中国地球化学学报》2017,36(1):9-15
Copper is a moderately incompatible chalcophile element. Its behavior is strongly controlled by sulfides. The speciation of sulfur is controlled by oxygen fugacity. Therefore, porphyry Cu deposits are usually oxidized (with oxygen fugacities > ΔFMQ +2) (Mungall 2002; Sun et al. 2015). The problem is that while most of the magmas at convergent margins are highly oxidized, porphyry Cu deposits are very rare, suggesting that high oxygen fugacity alone is not sufficient. Partial melting of mantle peridotite even at very high oxygen fugacities forms arc magmas with initial Cu contents too low to form porphyry Cu deposits directly (Lee et al. 2012; Wilkinson 2013). Here we show that partial melting of subducted young oceanic slabs at high oxygen fugacity (>ΔFMQ +2) may form magmas with initial Cu contents up to >500 ppm, favorable for porphyry mineralization. Pre-enrichment of Cu through sulfide saturation and accumulation is not necessarily beneficial to porphyry Cu mineralization. In contrast, re-melting of porphyritic hydrothermal sulfide associated with iron oxides may have major contributions to porphyry deposits. Thick overriding continental crust reduces the “leakage” of hydrothermal fluids, thereby promoting porphyry mineralization. Nevertheless, it is also more difficult for ore forming fluids to penetrate the thick continental crust to reach the depths of 2–4 km where porphyry deposits form. 相似文献
999.
Nan Ma Zhaoliang Song Baoli Wang Fushun Wang Xiaomin Yang Xiaodong Zhang Qian Hao Yuntao Wu 《中国地球化学学报》2017,36(4):626-637
Rivers link terrestrial ecosystems and marine ecosystems, and they transport large amounts of substances into oceans each year, including several forms of silicon (Si), carbon (C), and other nutrients. However, river damming affects the water flow and biogeochemical cycles of Si, C, and other nutrients through biogeochemical interacting processes. In this review, we first summarize the current understanding of the effects of river damming on the processes of biogeochemical Si cycle, especially the source, composition, and recycling process of biogenic silica (BSi). Then, we introduce dam impacts on the cycles of C and some other nutrients. Dissolved silicon in rivers is mainly released from phytolith dissolution and silicate weathering. BSi in suspended matter or sediments in most rivers mainly consists of phytoliths and mainly originates from soil erosion. However, diatom growth and deposition in many reservoirs formed by river interception may significantly increase the contribution of diatom Si to total BSi, and thus significantly influence the biogeochemical Si, C, and nutrient cycles. Yet the turnover of phytoliths and diatoms in different rivers formed by river damming is still poorly quantified. Thus, they should be further investigated to enhance our understanding about the effects of river damming on global biogeochemical Si, C and nutrient cycles. 相似文献
1000.
River systems play an important role in the global carbon cycle. Rivers transport carbon to the ocean and also affect the carbon cycle in the coastal ocean. The flux from land to the ocean is thought to be a very important part of the land carbon budget. To investigate the effect of dam-building on dissolved organic carbon (DOC) in rivers, three reservoirs of different trophic states in the Wujiang basin, Guizhou Province, were sampled twice per month between May 2011 and May 2012. Temporal and spatial distributions of DOC in the reservoirs and their released waters were studied. It was found that different factors controlled DOC in river water, reservoir water, and released water. DOC in the rivers tended to be affected by primary production. For reservoirs, the main controlling factors of DOC concentration varied by trophic state. For the mesotrophic Hongjiadu Reservoir, the effect of primary production on DOC concentration was obvious. For the eutrophic Dongfengdu Reservoir and the hypereutrophic Wujiangdu Reservoir, primary production was not significant and DOC came instead from soil and plant litter. 相似文献