全文获取类型
收费全文 | 25463篇 |
免费 | 4683篇 |
国内免费 | 5024篇 |
专业分类
测绘学 | 1024篇 |
大气科学 | 2630篇 |
地球物理 | 6226篇 |
地质学 | 12741篇 |
海洋学 | 4984篇 |
天文学 | 71篇 |
综合类 | 1706篇 |
自然地理 | 5788篇 |
出版年
2024年 | 134篇 |
2023年 | 350篇 |
2022年 | 846篇 |
2021年 | 1076篇 |
2020年 | 1034篇 |
2019年 | 1222篇 |
2018年 | 1015篇 |
2017年 | 1160篇 |
2016年 | 1098篇 |
2015年 | 1250篇 |
2014年 | 1599篇 |
2013年 | 1851篇 |
2012年 | 1536篇 |
2011年 | 1715篇 |
2010年 | 1478篇 |
2009年 | 1587篇 |
2008年 | 1653篇 |
2007年 | 1761篇 |
2006年 | 1713篇 |
2005年 | 1438篇 |
2004年 | 1361篇 |
2003年 | 1201篇 |
2002年 | 989篇 |
2001年 | 887篇 |
2000年 | 813篇 |
1999年 | 718篇 |
1998年 | 604篇 |
1997年 | 538篇 |
1996年 | 461篇 |
1995年 | 409篇 |
1994年 | 389篇 |
1993年 | 319篇 |
1992年 | 241篇 |
1991年 | 178篇 |
1990年 | 129篇 |
1989年 | 132篇 |
1988年 | 75篇 |
1987年 | 48篇 |
1986年 | 31篇 |
1985年 | 39篇 |
1984年 | 18篇 |
1983年 | 14篇 |
1982年 | 7篇 |
1981年 | 16篇 |
1980年 | 10篇 |
1979年 | 5篇 |
1978年 | 11篇 |
1977年 | 4篇 |
1973年 | 2篇 |
1971年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
在云南省西北部德钦县古水一带深切的澜沧江河谷中,发现了一套第四纪湖相沉积物,以纹层状粘土、粉砂质粘土和粉砂的互层组合为特征,构成了第三和第四级阶地的基座.笔者对该湖相剖面进行了U系、孢粉和若干地球化学指标的分析测试,并以这些结果讨论了古湖的形成时代和成因,以及古湖堆积时期的古植被和古气候.剖面顶部、中部和中下部粘土的U系法年龄测定结果分别为52.3±3.4 ka、64.2±5.6 ka和81.9±6.5 ka,说明古水古湖形成于晚更新世早中期的末次间冰期晚期和末次冰期的早冰阶与间冰阶早期,很可能是大型冰川(如下游的明永冰川等)或巨量的冰水沉积物堰塞了澜沧江河谷而造成的结果. 相似文献
52.
Improper design, faulty planning, mismanagement and incorrect operation of irrigation schemes are the principle reasons for
the deterioration of groundwater quality in a large number of countries, in particular in semi-arid and arid regions. The
aim of this study is to determine the dimensions of groundwater quality after surface irrigation was begun in the semi-arid
Harran Plain. Physical and chemical parameters of the groundwater including pH, temperature, electrical conductivity (EC),
sodium, potassium, calcium, magnesium, chloride, bicarbonate, sulphate, nitrate, nitrite, ammonium, total phosphorus, total
organic carbon and turbidity were determined monthly during the 2006 water year. The quality of the groundwater in the study
area was assessed hydrochemically in order to determine its suitability for human consumption and agricultural purposes. In
the general plain, the EC values measured were considerably above the guide level of 650 μS/cm, while nitrate in particular
was found in almost all groundwater samples to be significantly above the maximum admissible concentration of 50 mg/l for
the quality of water intended for human consumption as per the international and national standards. Total hardness reveals
that a majority of the groundwater samples fall in the very hard water category. Interpretation of analytical data shows that
Ca–HCO3 and Ca–SO4 are the dominant hydrochemical facies in the study area. 相似文献
53.
The Krishni–Yamuna interstream area is a micro-watershed in the Central Ganga Plain and a highly fertile track of Western
Uttar Pradesh. The Sugarcane and wheat are the major crops of the area. Aquifers of Quaternary age form the major source of
Irrigation and municipal water supplies. A detailed hydrogeological investigation was carried out in the study area with an
objective to assess aquifer framework, groundwater quality and its resource potential. The hydrogeological cross section reveals
occurrence of alternate layers of clay and sand. Aquifer broadly behaves as a single bodied aquifer down to the depth of 100 m bgl
(metre below ground level) as the clay layers laterally pinch out. The depth to water in the area varies between 5 and 16.5 m bgl.
The general groundwater flow direction is from NE to SW with few local variations. An attempt has been made to evaluate groundwater
resources of the area. The water budget method focuses on the various components contributing to groundwater flow and groundwater
storage changes. Changes in ground water storage can be attributed to rainfall recharge, irrigation return flow and ground
water inflow to the basin minus baseflow (ground water discharge to streams or springs), evapotranspiration from ground water,
pumping and ground water outflow from the basin. The recharge is obtained in the study area using Water table fluctuation
and Tritium methods. The results of water balance study show that the total recharge in to the interstream region is of the
order of 185.25 million m3 and discharge from the study area is of the order of 203.24 million m3, leaving a deficit balance of −17.99 million m3. Therefore, the present status of groundwater development in the present study area has acquired the declining trend. Thus,
the hydrogeological analysis and water balance studies shows that the groundwater development has attained a critical state
in the region. 相似文献
54.
Heavy metals in sediments of the Tecate River, Mexico 总被引:1,自引:0,他引:1
F. T. Wakida D. Lara-Ruiz J. Temores-Peña J. G. Rodriguez-Ventura C. Diaz E. Garcia-Flores 《Environmental Geology》2008,54(3):637-642
Ten sites along the Tecate River, Mexico were sampled to evaluate the cadmium, lead, nickel and chromium concentrations in
sediments. The result shows contamination for cadmium in most of the sites, where two sites were class 4 (polluted to strongly
polluted) according to geoaccumulation index proposed by Muller. Two sites were found polluted for all the heavy metals analyzed
(Cr, Cd, Pb and Ni), indicating the effect of anthropogenic activities. A correlation between Ni and Cd concentration had
been found indicating a common source. These metals are usually used in electroplating industry. The results of this study
can be used for decision makers to prioritize measures to control the pollution for these metals. 相似文献
55.
Response of groundwater chemistry to water deliveries in the lower reaches of Tarim River,Northwest China 总被引:2,自引:0,他引:2
Yongjin Chen Kefa Zhou Yaning Chen Weihong Li Jiazhen Liu Tao Wang 《Environmental Geology》2008,53(6):1365-1373
In this paper, we analysed the monitored data from nine groundwater-monitoring transects in the lower reaches of Tarim River
during the five times of stream water deliveries to the river transect where the stream flow ceased. The results showed that
the groundwater depth in the lower reaches of Tarim River rose from −9.30 m before the conveyances to −8.17 and −6.50 m after
the first and second conveyances, −5.81 and −6.00 m after the third and fourth the conveyance, and −4.73 m after the fifth.
The horizontal extent of groundwater recharge was gradually enlarged along both sides of the channel of conveyance, i.e.,
from 250 m in width after the first conveyance to 1,050 m away from the channel after the fourth delivery. With the rising
groundwater level, the concentrations of major anions Cl−, SO42− and cations Ca2+, Mg2+, Na+, as well as total dissolved solids (TDS) in groundwater underwent a significant change. The spatial variations in groundwater
chemistry indicated that the groundwater chemistry at the transect near Daxihaizi Reservoir changed earlier than that farther
from it. In the same transect, the chemical variations were earlier in the monitoring well close to watercourse than that
farther away from the stream. In general, the concentration of the major ions and TDS at each monitoring well increased remarkably
when the water delivery started, and decreased with the continued water delivery, and then increased once again at the end
of the study period. Hence, the whole study period may be divided into three stages: the initial stage, the intermediate stage
and the later stage. According to the three stages of groundwater chemistry reaction to water delivery and the relationships
between groundwater chemical properties and groundwater depths, we educe that under the situation of water delivery, the optimum
groundwater depth in the lower reaches of the Tarim River should be −5 m. 相似文献
56.
Natural radioactivity in sediment of Wei River,China 总被引:1,自引:0,他引:1
The concentrations of natural radionuclides in sediment of Wei River of China were measured using γ-ray spectrometry with
the aim of estimating the radiation hazard as establishing a database for radioactivity levels of river sediment of China.
The activity concentrations of 226Ra, 232Th and 40K in sediment samples ranged from 10.4 to 39.9 Bq kg−1, 15.3 to 54.8 Bq kg−1 and 514.8 to 1,175.5 Bq kg−1, respectively. The concentrations of these radionuclides were compared with the typical world values and the average activities
of Chinese soil and Shaanxi soil. Radium equivalent activity (Raeq), external hazard index (H
ex) and representative level index (I
γr) were calculated for the samples to assess the radiation hazards arising due to the use of these sediment samples in the
construction of dwellings. All the sediment samples have Raeq lower than the limit of 370 Bq kg−1, H
ex less than unity and I
γr close to 1 Bq kg−1. The overall mean outdoor terrestrial gamma dose rate is 64.8 nGy h−1 and the corresponding outdoor annual effective dose is 0.079 mSv. None of the studied location is considered a radiological
risk and sediment can be safely used in construction. 相似文献
57.
A modified DRASTIC model for Siting Confined Animal Feeding Operations in Williams County,Ohio, USA 总被引:1,自引:0,他引:1
Three of DRASTIC’s parameters (Depth to Water, Soil Media, and Topography) were modified and another parameter was added (land
use/land cover) to the model to determine the potential impact on groundwater from Confined Animal Feeding Operations (CAFO)
manure lagoon settings and manure application as fertilizer. Williams County is a mostly agricultural county located in northwest
Ohio, USA. It currently has three CAFOs, all dairy, with the possibility of the construction of a multi-million chicken egg
CAFO in the near future. A Geographic Information System (GIS) was utilized to modify the Ohio Department of Natural Resources
(ODNR) DRASTIC map for the county to fully assess the county-wide pollution potential of CAFOs. The CAFO DRASTIC map indicates
that almost half of Williams County has elevated groundwater pollution potential. The rest of the county, primarily the southeast
corner, has lower CAFO groundwater pollution potential. Future CAFO development within the county should focus on the southeastern
portion of the county where the groundwater table is deeper, and the aquifer is composed of shale substrate with low hydraulic
conductivity. The CAFO DRASTIC results are intended to be used as a screening tool and are not to replace site-specific hydrogeologic
investigations. 相似文献
58.
Thomas B. Boving Mark H. Stolt Janelle Augenstern Brian Brosnan 《Environmental Geology》2008,55(3):571-582
The control of polluted surface runoff and the assessment of possible impacts on groundwater is a concern at the local and
regional scale. On this background, a study investigates possible impacts of organic and inorganic pollutants (including bacteria)
originating from a permeable asphalt parking lot on the water quality immediately beneath it. The functioning of the permeable
pavement, including clogging and restricted vertical percolation, was also evaluated. Four nested sample ports (shallow and
deep) were installed below low- and high-traffic areas, including one port outside the parking lot. At least initially there
was a good hydraulic connection between the parking surface and the shallow sample ports. The presence of a geotextile layer
at the base of the parking lot structure, however, was identified in lab tests as one factor restricting vertical percolation
to the deeper ports. Clogging of the permeable surface was most pronounced in heavy traffic areas and below snow pile storage
areas. Corroborated by high electric conductivity and chloride measurements, sand brought in by cars during winter was the
principal cause for clogging. No bacteria or BOD were found in percolating water. Polycyclic aromatic hydrocarbons (PAH) were
present at concentrations near minimum detection limit. Nutrients (nitrate and phosphate) were being leached into the ground
via the permeable parking lot surface at annual flux rates of 0.45–0.84 g/m2/year. A multi-species tracer test demonstrated a retention capacity of the permeable parking lot structure of >90% for metals
and 27% for nutrients, respectively. 相似文献
59.
Three discriminant function models are raised and cross-compared in order to distinguish geochemical patterns characteristic
for the Drava River floodplain sediments. Based on data representing total element concentrations in samples collected from
alluvium (A), terrace (T), and unconsolidated bedrock (B) at the border of a floodplain, four element clusters emerged accounting
for discrimination between the referred groups of sediments. The most prominent is contaminant/carbonate cluster characteristic
for alluvium. The other two are: silicate cluster typical for unconsolidated geological substrate (Neogene sedimentary rocks);
and naturally dispersed heavy metal cluster separating terrace from the former two groups. Models introducing depth intervals
and single profiles as grouping criteria reveal identical sediment-heavy metal matrices. The second important issue of this
paper is possibility of reclassification of samples originally assigned to one of the a priori defined groups of sediments,
based on established geochemical pattern. The mapped geological units can be reconsidered by the post hoc assignments to a
different group if geological border between alluvium and terrace or between terrace and bedrock can not be established geologically
with absolute certainty. 相似文献
60.
Concentrations of trace elements and heavy metals (Al, As, Ba, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sb, Se, Sr, V and Zn) in
the Danjiangkou Reservoir, the water source area of the Middle Route of China’s interbasin South to North Water Transfer Project,
were analyzed using an Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES) and compared with the national and
international standards for drinking water. The results indicated that concentrations of As, Pb, Sb and Se in the Reservoir
exceeded the standards and they would pose health risk for residents in the region and the water receiving areas of the interbasin
water transfer project. Spatial and temporal variability of the trace elements and heavy metals in the Reservoir implies their
mixed sources of natural processing and anthropogenic activities in the upper drainage of the Reservoir. The research results
would help develop water resource management and conservation strategy for the interbasin water transfer project. 相似文献