首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2729篇
  免费   403篇
  国内免费   471篇
测绘学   106篇
大气科学   600篇
地球物理   560篇
地质学   1288篇
海洋学   441篇
天文学   9篇
综合类   174篇
自然地理   425篇
  2024年   12篇
  2023年   47篇
  2022年   98篇
  2021年   120篇
  2020年   95篇
  2019年   121篇
  2018年   84篇
  2017年   91篇
  2016年   96篇
  2015年   108篇
  2014年   147篇
  2013年   176篇
  2012年   122篇
  2011年   164篇
  2010年   141篇
  2009年   158篇
  2008年   203篇
  2007年   190篇
  2006年   213篇
  2005年   161篇
  2004年   163篇
  2003年   130篇
  2002年   116篇
  2001年   90篇
  2000年   78篇
  1999年   88篇
  1998年   81篇
  1997年   60篇
  1996年   51篇
  1995年   30篇
  1994年   26篇
  1993年   17篇
  1992年   21篇
  1991年   13篇
  1990年   12篇
  1989年   13篇
  1988年   14篇
  1987年   7篇
  1986年   1篇
  1985年   7篇
  1984年   9篇
  1983年   9篇
  1982年   13篇
  1981年   4篇
  1980年   1篇
  1978年   1篇
  1977年   1篇
排序方式: 共有3603条查询结果,搜索用时 0 毫秒
141.
The world’s largest coral reef ecosystem, the Great Barrier Reef (GBR), continues to be degraded from land-based pollution. Information about the source of pollutants is critical for catchment management to improve GBR water quality. We report here on an 11-year source to sea study of pollutant delivery in runoff from the Fitzroy River Basin (FRB), the largest GBR catchment. An innovative technique that relates land use to pollutant generation is presented. Study results indicate that maximum pollutant concentrations at basin and sub-catchment scales are closely related to the percentage area of croplands receiving heavy rain. However, grazing lands contribute the majority of the long-term average annual load of most common pollutants. Findings suggest improved land management targets, rather than water quality targets should be implemented to reduce GBR pollution. This study provides a substantial contribution to the knowledge base for the targeted management of pollution ‘hot-spots’ to improve GBR water quality.  相似文献   
142.
Storm events are major transporters of faecal microbial contaminants, but few studies have reported storm loads or concentration dynamics in relation to discharge or other pollutants, notably fine sediment. Episodically, high loads of faecal contamination during storm flows impact downstream uses of water bodies, particularly contact recreation and shellfish harvesting. We examined the storm dynamics of Escherichia coli, turbidity and discharge in the mixed land use Motueka catchment (2047 km2; 60% forest and 19% pasture) to gain insights into E. coli sources and transport. We also explored different approaches for calculating E. coli loads. Discharge and field turbidity were recorded continuously, and E. coli concentrations were sampled during events, over a 13‐month period near the mouth of the Motueka River. E. coli loads were estimated by interpolation, averaging estimators and by using linear regression with smearing correction of the log‐transformed variables: discharge, turbidity, and both turbidity and discharge. The annual E. coli load was dominated (~98%) by export during events. Comparison of monthly monitoring with the intensive storm monitoring campaign suggests that simple stratification of the sampling into storm and baseflow would greatly improve export estimates. E. coli peak concentrations always preceded discharge and turbidity peaks (which had similar timing). Turbidity can be a useful surrogate for faecal microbes in smaller catchments, but in the Motueka turbidity was no better for predicting E. coli concentration than discharge. Runoff from grazed pasture and direct deposition from livestock are probably the ultimate E. coli sources in the Motueka catchment. However, in‐channel stores seem to dominate E. coli dynamics during events and account for the typical feature of bacterial concentrations peaking ahead of discharge and turbidity. This study demonstrates the importance of storm events to faecal microbial loads and shows that E. coli concentration dynamics may contrast with those of turbidity. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   
143.
This paper presents a field study conducted in northwest Turkey and characterizes the NH3 concentration and emission measured in summer season from three chicken farms. The influence of environmental conditions on NH3 concentration and emission was also investigated in this study. Ammonia concentration, temperature, relative humidity and airflow rate were continuously recorded for four sequential days. The environmental conditions were measured using a multifunction temperature and humidity‐meter with a hot wire probe. Portable multiple gas detectors with electro‐chemical sensors were used to measure NH3 concentration. The NH3 emission rates for houses were calculated by multiplying simultaneously measured NH3 concentrations and air flow rates. The average daily mean (ADM) house concentrations of house 1 (H1), house 2 (H2), and house 3 (H3) were measured as 4.43, 3.71, and 6.20 ppm, respectively. NH3 concentration was inversely proportional to temperature (r = ?0.279), relative humidity (r = ?0.063) and airflow rate (r = ?0.554) for all monitored houses. The ADM house NH3 emission was 135 g/(h house) for H1, 255 g/(h house) for H2, and 117 g/(h house) for H3. The combined average emission rate in this study (0.26 g/(d bird)) was lower than the emission rate measured in chicken farms in the USA. However, our results were comparable to rates calculated in European studies because house design, ventilation system and bird diet applied in Turkish chicken farms are very similar to those employed in European countries. The NH3 emissions were significantly correlated to NH3 concentrations (r = 0.45, p ≤ 0.001) and airflow rates (r = 0.97, p ≤ 0.001). A clear diurnal pattern was obtained for NH3 concentrations rather than NH3 emissions at the end of the study.  相似文献   
144.
柱撑蛭石吸附去除废水中重金属离子的实验研究   总被引:2,自引:0,他引:2  
刘云  吴平霄  党志 《矿物岩石》2006,26(4):8-13
分别利用有机柱化剂十二烷基磺酸钠(SDS)和无机柱化剂聚羟基铝(HA)对蛭石进行柱撑制得十二烷基磺酸钠柱撑蛭石(SDS-V erm icu lite)和聚羟基铝柱撑蛭石(HA-V erm icu lite),并通过XRD、红外光谱、ZETA电位等表征手段对柱撑蛭石进行表征,同时针对柱撑蛭石对Cu2 ,Cd2 ,C r3 3种重金属离子的吸附进行研究,结果表明:吸附去除率受反应时间、重金属离子的初始浓度、pH值等因素的影响,经柱撑后的蛭石对重金属离子吸附的吸附性能比蛭石原矿要强。柱撑蛭石吸附3种重金属离子的动力学吸附过程可用E lov icb方程和双常数方程进行较好的拟合。  相似文献   
145.
为了弄清楚煤矸石回填地基对复垦区的环境效应,以开滦矿区吕家坨复垦新村为例,介绍了地基回填煤矸石工艺,分析了煤矸石化学成分及其含量;采用室内淋溶实验,对pH、总硬度(CaCO3)、F^-、SO4^2-、Pb、Cd和Cr^6+浓度等地下水环境质量指标进行分析,探讨了煤矸石回填地基对水环境的影响;讨论了煤矸石回填地基对大气质量的影响;参照《城市放射性废物管理办法》规定,评价了回填地基煤矸石的放射性对人体健康的危害。结果表明:通过现场振动压实试验确定了回填地基分层充填、分层振动压实参数及碾压趟数,确定按照煤矸石与土体积之比5:2进行回填,若回填一层厚500mm煤矸石,则要回填一层厚200mm粉煤灰或土,地表覆土厚度确定为0.5m;煤矸石中SiO2和Al2O3含量较高,属基性岩类,同时煤矸石含有炭、铝和CaO等物质,易发生水解和风化等现象;地基分层回填压实的复垦工艺减少了煤矸石中有害物质的释放量,降低了污染物的迁移速度,不会对水环境造成污染;地基回填煤矸石也显著减少粉尘和有害气体的排放量,降低对大气的污染;研究区煤矸石不属于放射性废物,不会影响人体健康,可作为建筑材料使用。  相似文献   
146.
锑矿区水体水环境锑污染及硫同位素示踪研究   总被引:4,自引:0,他引:4  
本文系统研究了贵州省半坡锑矿区水环境锑污染现状,用硫同位素指示采矿活动对矿区水体的影响,以查明锑矿区水环境中Sb的释放迁移过程和水污染程度。研究发现,矿区水体中Sb和SO42-含量分别高达1377μg/L和1926 mg/L;岔河下游近10 km处仍保持较高水平(182.5μg/L Sb和59.8 mg/L SO42-)。岔河水体中,δ34S-Sb、δ34S-SO42-和Sb-SO42-间均具显著正相关,相关系数分别为r=0.68(p<0.05)、r=0.89(p<0.01)、r=0.72(p<0.05)。表明岔河水体中,δ34S和SO42-能很好地指示矿业活动引起的Sb污染程度和扩散范围。根据同位素质量平衡原理估算,发现矿区下游水体中的硫主要来自矿山,表明矿区下游水体污染受采矿活动影响显著。  相似文献   
147.
Fuzzy process capability indices for quality control of irrigation water   总被引:1,自引:0,他引:1  
Water covers over 70% of the Earth surface and is a very important resource to people and the environment. Water pollution affects drinking water, rivers, lakes and oceans all over the world. This consequently harms human health and the natural environment. Water pollution can also affect the crops. So, water pollution is an important issue for humanity. Therefore, the control of irrigation water is a necessity. In this paper, a methodology based on process capability indices (PCIs) has been presented to control the levels of pH, dissolved oxygen (DO) and temperature (T) in dam’s water for irrigation. Fuzzy PCIs have been proposed for this aim. The fuzzy estimates of $ \hat C_p Water covers over 70% of the Earth surface and is a very important resource to people and the environment. Water pollution affects drinking water, rivers, lakes and oceans all over the world. This consequently harms human health and the natural environment. Water pollution can also affect the crops. So, water pollution is an important issue for humanity. Therefore, the control of irrigation water is a necessity. In this paper, a methodology based on process capability indices (PCIs) has been presented to control the levels of pH, dissolved oxygen (DO) and temperature (T) in dam’s water for irrigation. Fuzzy PCIs have been proposed for this aim. The fuzzy estimates of and are obtained for pH, DO, and T based on Buckley’s interval estimation approach and based on fuzzy specification limits. An application has been made for Kesikk?prü Dam in Ankara, Turkey. In this paper, Buckley’s approach is re-arranged to obtain a triangular fuzzy membership function because it cannot be obtained from Buckley’s approach in some situation.  相似文献   
148.
PROBLEMSOFTHEAQUATICENVIRONMENTANDCOUNTERMEASURESINRAPIDECONOMICDEVELOPMENTINTHEZHUJIANGRIVERDELTA¥WenYanmao(温琰茂);ChengGuopei...  相似文献   
149.
The shale gas boom in the United States spurred a shift in electricity generation from coal to natural gas. Natural gas combined cycle units emit half of the CO2 to produce the same energy as a coal unit; therefore, the market trend is credited for a reduction in GHG emissions from the US power sector. However, methane that escapes the natural gas supply chain may undercut these relative climate benefits. In 2016, Canada, the United States and Mexico pledged to reduce methane emissions from the oil and natural gas sector 40–45% from 2012 levels by 2025. This article reviews the science-policy landscape of methane measurement and mitigation relevant for meeting this pledge, including changes in US policy following the 2016 presidential election. Considerable policy incoherence exists in all three countries. Reliable inventories remain elusive; despite government and private sector research efforts, the magnitude of methane emissions remains in dispute. Meanwhile, mitigation efforts vary significantly. A framework that integrates science and policy would enable actors to more effectively inform, leverage and pursue advances in methane measurement and mitigation. The framework is applied to North America, but could apply to other geographic contexts.

Key policy insights

  • The oil and gas sector’s contribution to atmospheric methane concentrations is becoming an increasingly prominent issue in climate policy.

  • Efforts to measure and control fugitive methane emissions do not presently proceed within a coherent framework that integrates science and policy.

  • In 2016, the governments of Canada, Mexico and the United States pledged to reduce methane emissions from the oil and natural gas sector 40–45% from 2012 levels by 2025.

  • The 2016 presidential election in the United States has halted American progress at the federal level, suggesting a heavier reliance on industry and subnational efforts in that country.

  • Collectively or individually, the countries, individual agencies, or private stakeholders could use the proposed North American Methane Reduction framework to direct research, enhance monitoring and evaluate mitigation efforts, and improve the chances that continental methane reduction targets will be achieved.

  相似文献   
150.
商丘市的大雾多形成在大陆高压型、冷锋前暖区型、均压场型、(低压)倒槽型天气形势中。空气中的污染物不仅增加了水汽凝结核,那些可溶性气溶胶还降低了饱和水汽压,使空气中的水汽更容易饱和,因此被污染的空气更容易形成雾。而大雾天气抑制了近地面层污染物的垂直扩散,加剧了城市污染。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号