全文获取类型
收费全文 | 51605篇 |
免费 | 971篇 |
国内免费 | 525篇 |
专业分类
测绘学 | 1237篇 |
大气科学 | 4306篇 |
地球物理 | 10528篇 |
地质学 | 18265篇 |
海洋学 | 4232篇 |
天文学 | 11082篇 |
综合类 | 113篇 |
自然地理 | 3338篇 |
出版年
2020年 | 351篇 |
2019年 | 385篇 |
2018年 | 906篇 |
2017年 | 887篇 |
2016年 | 1154篇 |
2015年 | 788篇 |
2014年 | 1156篇 |
2013年 | 2425篇 |
2012年 | 1331篇 |
2011年 | 1861篇 |
2010年 | 1580篇 |
2009年 | 2264篇 |
2008年 | 1959篇 |
2007年 | 1839篇 |
2006年 | 1868篇 |
2005年 | 1543篇 |
2004年 | 1541篇 |
2003年 | 1527篇 |
2002年 | 1477篇 |
2001年 | 1321篇 |
2000年 | 1256篇 |
1999年 | 1060篇 |
1998年 | 1039篇 |
1997年 | 1049篇 |
1996年 | 856篇 |
1995年 | 875篇 |
1994年 | 784篇 |
1993年 | 691篇 |
1992年 | 671篇 |
1991年 | 657篇 |
1990年 | 664篇 |
1989年 | 603篇 |
1988年 | 544篇 |
1987年 | 708篇 |
1986年 | 627篇 |
1985年 | 751篇 |
1984年 | 882篇 |
1983年 | 870篇 |
1982年 | 772篇 |
1981年 | 747篇 |
1980年 | 708篇 |
1979年 | 657篇 |
1978年 | 702篇 |
1977年 | 607篇 |
1976年 | 568篇 |
1975年 | 585篇 |
1974年 | 602篇 |
1973年 | 591篇 |
1972年 | 381篇 |
1971年 | 359篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Oceanology - Field observations were applied to study the macrophytobenthos of the Abrau Peninsula of the Black Sea and the effect of the two most common and largest species of brown algae of the... 相似文献
2.
Kopylov A. I. Kosolapov D. B. Zabotkina E. A. Romanenko A. V. Sazhin A. F. 《Oceanology》2021,61(2):220-232
Oceanology - The distribution of heterotrophic bacteria, viruses, and heterotrophic nanoflagellates was studied in the shelf waters of the East Siberian Sea along the meridional transect from the... 相似文献
3.
Akinyemi G. A. Kolawole L. B. Dairo O. F. Willoughby A. A. Abdulrahim R. B. Rabiu A. B. 《Geomagnetism and Aeronomy》2021,61(4):647-657
Geomagnetism and Aeronomy - The total electron content (TEC) data obtained from the ground-based GPS receiver stations of the Nigerian GNSS network of stations (NIGNET) have been used in this study... 相似文献
4.
Maarten Wynants Aloyce Patrick Linus Munishi Kelvin Mtei Samuel Bodé Alex Taylor Geoffrey Millward Neil Roberts David Gilvear Patrick Ndakidemi Pascal Boeckx William H. Blake 《地球表面变化过程与地形》2021,46(15):3112-3126
Soil resources in parts of Tanzania are rapidly being depleted by increased rates of soil erosion and downstream sediment transport, threatening ecosystem health, water and livelihood security in the region. However, incomplete understanding to what effect the dynamics of soil erosion and sediment transport are responding to land-use changes and climatic variability are hindering the actions needed to future-proof Tanzanian land-use practices. Complementary environmental diagnostic tools were applied to reconstruct the rates and sources of sedimentation over time in three Tanzanian river systems that have experienced changing land use and climatic conditions. Detailed historical analysis of sediment deposits revealed drastic changes in sediment yield and source contributions. Quantitative sedimentation reconstruction using radionuclide dating showed a 20-fold increase in sediment yield over the past 120 years. The observed dramatic increase in sediment yield is most likely driven by increasing land-use pressures. Deforestation, cropland expansion and increasing grazing pressures resulted into accelerating rates of sheet erosion. A regime shift after years of progressive soil degradation and convergence of surface flows resulted into a highly incised landscape, where high amounts of eroded soil from throughout the catchment are rapidly transported downstream by strongly connected ephemeral drainage networks. By integrating complementary spatial and temporal evidence bases, this study demonstrated links between land-use change, increased soil erosion and downstream sedimentation. Such evidence can guide stakeholders and policy makers in the design of targeted management interventions to safeguard future soil health and water quality. 相似文献
5.
Codie Wilson Stephanie K. Kampf Sandra Ryan Tim Covino Lee H. MacDonald Hunter Gleason 《水文研究》2021,35(1):e13975
Wildfire increases the potential connectivity of runoff and sediment throughout watersheds due to greater bare soil, runoff and erosion as compared to pre-fire conditions. This research examines the connectivity of post-fire runoff and sediment from hillslopes (< 1.5 ha; n = 31) and catchments (< 1000 ha; n = 10) within two watersheds (< 1500 ha) burned by the 2012 High Park Fire in northcentral Colorado, USA. Our objectives were to: (1) identify sources and quantify magnitudes of post-fire runoff and erosion at nested hillslopes and watersheds for two rain storms with varied duration, intensity and antecedent precipitation; and (2) assess the factors affecting the magnitude and connectivity of runoff and sediment across spatial scales for these two rain storms. The two summer storms that are the focus of this research occurred during the third summer after burning. The first storm had low intensity rainfall over 11 hours (return interval <1–2 years), whereas the second event had high intensity rainfall over 1 hour (return interval <1–10 years). The lower intensity storm was preceded by high antecedent rainfall and led to low hillslope sediment yields and channel incision at most locations, whereas the high intensity storm led to infiltration-excess overland flow, high sediment yields, in-stream sediment deposition and channel substrate fining. For both storms, hillslope-to-stream sediment delivery ratios and area-normalised cross-sectional channel change increased with the percent of catchment that burned at high severity. For the high intensity storm, hillslope-to-stream sediment delivery ratios decreased with unconfined channel length (%). The findings quantify post-fire connectivity and sediment delivery from hillslopes and streams, and highlight how different types of storms can cause varying magnitues and spatial patterns of sediment transport and deposition from hillslopes through stream channel networks. 相似文献
6.
Multiscale simulation of fluvio‐deltaic stratigraphy was used to quantify the elements of the geometry and architectural arrangement of sub‐seismic‐scale fluvial‐to‐shelf sedimentary segments. We conducted numerical experiments of fluvio‐deltaic system evolution by simulating the accommodation‐to‐sediment‐supply (A/S) cycles of varying wavelength and amplitude with the objective to produce synthetic 3‐D stratigraphic records. Post‐processing routines were developed in order to investigate delta lobe architecture in relation to channel‐network evolution throughout A/S cycles, estimate net sediment accumulation rates in 3‐D space, and extract chronostratigraphically constrained lithosomes (or chronosomes) to quantify large‐scale connectivity, that is, the spatial distribution of high net‐to‐gross lithologies. Chronosomes formed under the conditions of channel‐belt aggradation are separated by laterally continuous abandonment surfaces associated with major avulsions and delta‐lobe switches. Chronosomes corresponding to periods in which sea level drops below the inherited shelf break, that is, the youngest portions of the late falling stage systems tract (FSST), form in the virtual absence of major avulsions, owing to the incision in their upstream parts, and thus display purely degradational architecture. Detailed investigation of chronosomes within the late FSST showed that their spatial continuity may be disrupted by higher‐frequency A/S cycles to produce “stranded” sand‐rich bodies encased in shales. Chronosomes formed during early and late falling stage (FSST) demonstrate the highest large‐scale connectivity in their proximal and distal areas, respectively. Lower‐amplitude base level changes, representative of greenhouse periods during which the shelf break is not exposed, increase the magnitude of delta‐lobe switching and favour the development of system‐wide abandonment surfaces, whose expression in real‐world stratigraphy is likely to reflect the intertwined effects of high‐frequency allogenic forcing and differential subsidence. 相似文献
7.
The Bonneville basin, located in north-western Utah, is a vast evaporite basin which is home to the world-renowned Bonneville Salt Flats international speedway and is a highly valued landscape undergoing rapid change and anthropogenic influence. Air quality, snowpack, the local hydrological system, and state tourism are all impacted by the nature of the surface sediments exposed in the Bonneville basin. Mapping the Bonneville basin over time with remote sensing methods provides insight into the dynamics and impacts of the changing surface landscape. Utilizing the Landsat-5 Thematic Mapper (TM) and Landsat-8 Operational Land Imager (OLI) sensors, a set of band math indices are empirically established to map the predominant halite, gypsum, and carbonates mineralogical zones of the Bonneville basin. Spectral comparisons of representative samples from the study area and image-derived spectra indicate the halite of the Bonneville basin is wet and that gypsum deposits are slightly mixed with halite. The established indices are assessed in four ways, all of which support the ability of the indices to accentuate the associated mineralogical endmembers. Two study areas within the Bonneville basin are investigated temporally from 1986, 1995, 2005, and 2016 and show changing patterns in mineral distribution that align with surface processes active through these timescales. These indices provide a resource for mapping mineralogy though time in evaporite basins globally with diverse applications for questions about land use and environmental change. 相似文献
8.
Simon D. Carrière Thomas Health Pascale F. M. Rakotomandrindra Chloé Ollivier Riva E. Rajaomahefasoa Heritiana A. Rakoto Jean Lapègue Yves E. Rakotoarison Marie Mangin Jean Kempf Théodore Razakamanana Konstantinos Chalikakis 《水文研究》2021,35(3):e14108
Madagascar has one of the highest poverty rates in the world and consequently the long-term monitoring of groundwater resources is not a priority for the authorities. However, groundwater is often the only sustainable resource that has a satisfactory quality to supply the population. This is especially true in the south-west of the country, which is a semi-arid region and a global change hot spot (intense land use and climate changes). In response to the lack of data, the Groundwater Resource Observatory for Southwestern Madagascar (GROSoM) was established to monitor piezometry and meteorology over the longer term as part of a humanitarian response. The first site was setup in 2014 in a catchment located over a carbonate plateau; in 2018, a second site was installed in an alluvial setting within a crystalline basement catchment and a third site will be installed in 2020 to monitor groundwater dynamics in a coastal setting. The three sites, located between Toliara and Taolagnaro cities, are complementary and representative of various hydrogeological systems in Southwestern Madagascar. Each site includes a weather station and between 3 and 6 piezometric probes. The monitoring data indicate a strong inter-annual variability in precipitation, which induces a strong variability in aquifers recharge. One of the driest years in 2016 seems to be consistent with strong El Niño – Southern Oscillation (ENSO) effects observed at the global scale, while years with higher recharge appear to be related to cyclones such as Fundi in 2015 and Eketsang in 2019. Preliminary results of cross-disciplinary studies demonstrated a link between groundwater and health issues (i.e., admissions to basic health centres). This observatory aims to produce long-term data and has two objectives: (i) strengthening the early warning system for humanitarian crises in Madagascar; (ii) contributing to a better understanding of the effects of climate change on groundwater resources in this semi-arid region. 相似文献
9.
Maarten Wynants Linus Munishi Kelvin Mtei Samuel Bodé Aloyce Patrick Alex Taylor David Gilvear Patrick Ndakidemi William H. Blake Pascal Boeckx 《地球表面变化过程与地形》2021,46(15):3096-3111
Water bodies in Tanzania are experiencing increased siltation, which is threatening water quality, ecosystem health, and livelihood security in the region. This phenomenon is caused by increasing rates of upstream soil erosion and downstream sediment transport. However, a lack of knowledge on the contributions from different catchment zones, land-use types, and dominant erosion processes, to the transported sediment is undermining the mitigation of soil degradation at the source of the problem. In this context, complementary sediment source tracing techniques were applied in three Tanzanian river systems to further the understanding of the complex dynamics of soil erosion and sediment transport in the region. Analysis of the geochemical and biochemical fingerprints revealed a highly complex and variable soil system that could be grouped in distinct classes. These soil classes were unmixed against riverine sediment fingerprints using the Bayesian MixSIAR model, yielding proportionate source contributions for each catchment. This sediment source tracing indicated that hillslope erosion on the open rangelands and maize croplands in the mid-zone contributed over 75% of the transported sediment load in all three river systems during the sampling time-period. By integrating geochemical and biochemical fingerprints in sediment source tracing techniques, this study demonstrated links between land use, soil erosion and downstream sediment transport in Tanzania. This evidence can guide land managers in designing targeted interventions that safeguard both soil health and water quality. 相似文献
10.
Kaizad F. Patel Ivan J. Fernandez Sarah J. Nelson Stephen A. Norton Cheryl J. Spencer 《水文研究》2021,35(5):e14147
The Bear Brook Watershed in Maine (BBWM) is a long-term research site established to study the response of forest ecosystem function to environmental disturbances of chronic acidic deposition and ecosystem nitrogen enrichment. Starting in 1989, the West Bear (treated) watershed received bimonthly applications of ammonium sulfate [(NH4)2SO4] fertilizer from above the canopy, whereas East Bear (reference) received ambient deposition. The treatments were stopped in 2016, marking the beginning of the recovery phase. Research at the site has focused on soils, streams, and vegetation. Here, we describe data collected over three decades at the BBWM—input and stream output nutrient fluxes, quantitative soil pits and soil chemistry, and soil temperature and moisture. 相似文献