全文获取类型
收费全文 | 2508篇 |
免费 | 268篇 |
国内免费 | 88篇 |
专业分类
电工技术 | 22篇 |
技术理论 | 1篇 |
综合类 | 172篇 |
化学工业 | 91篇 |
金属工艺 | 6篇 |
机械仪表 | 3篇 |
建筑科学 | 404篇 |
矿业工程 | 36篇 |
能源动力 | 80篇 |
轻工业 | 55篇 |
水利工程 | 1775篇 |
石油天然气 | 38篇 |
无线电 | 22篇 |
一般工业技术 | 34篇 |
冶金工业 | 4篇 |
原子能技术 | 5篇 |
自动化技术 | 116篇 |
出版年
2024年 | 13篇 |
2023年 | 66篇 |
2022年 | 113篇 |
2021年 | 147篇 |
2020年 | 167篇 |
2019年 | 106篇 |
2018年 | 146篇 |
2017年 | 80篇 |
2016年 | 118篇 |
2015年 | 93篇 |
2014年 | 156篇 |
2013年 | 173篇 |
2012年 | 198篇 |
2011年 | 205篇 |
2010年 | 165篇 |
2009年 | 158篇 |
2008年 | 122篇 |
2007年 | 140篇 |
2006年 | 95篇 |
2005年 | 68篇 |
2004年 | 58篇 |
2003年 | 43篇 |
2002年 | 41篇 |
2001年 | 30篇 |
2000年 | 16篇 |
1999年 | 18篇 |
1998年 | 18篇 |
1997年 | 11篇 |
1996年 | 15篇 |
1995年 | 13篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 7篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 8篇 |
1988年 | 2篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 13篇 |
1983年 | 8篇 |
1982年 | 2篇 |
1980年 | 2篇 |
排序方式: 共有2864条查询结果,搜索用时 15 毫秒
11.
Lake-Level Change and Water Balance Analysis at Lake Qinghai,West China during Recent Decades 总被引:7,自引:0,他引:7
Xiao-Yan Li He-Ye Xu Yong-Liang Sun Deng-Shan Zhang Zhi-Peng Yang 《Water Resources Management》2007,21(9):1505-1516
Lake Qinghai, the largest saline lake with an area of 4,260 km2 (2000) and average depth of 21 m (1985) in West China, has experienced severe decline in water level in recent decades. This
study aimed to investigate water balance of the lake and identify the causes for the decline in lake level. There was a 3.35-m
decline in water level with an average decreasing rate of 8.0 cm year−1 between 1959 and 2000. The lake water balance showed that mean annual precipitation between 1959 and 2000 over the lake was
357 ± 10 mm, evaporation was 924 ± 10 mm, surface runoff water inflow was 348 ± 21 mm, groundwater inflow was 138 mm ± 9 and
the change in lake level was −80 ± 31 mm. The variation of lake level was highly positively correlated to surface runoff and
precipitation and negatively to evaporation, the correlation coefficients were 0.89, 0.81 and −0.66, respectively. Water consumption
by human activities accounts for 1% of the evaporation loss of the lake, implying that water consumption by human activities
has little effect on lake level decline. Most dramatic decline in lake level occurred in the warm and dry years, and moderate
decline in the cold and dry years, and relatively slight decline in the warm and wet years, therefore, the trend of cold/warm
and dry climate in recent decades may be the main reasons for the decline in lake level. 相似文献
12.
指出湖滨带是连接湖泊水体和陆地的一个功能过渡界面区。它作为生物的重要栖息地,具有净化水质、缓解波浪等多种功能。因此,对湖滨带实行生态恢复建设,应予以高度的重视。笔者通过具体工程的实践,阐述了湖滨带生态恢复建设工程的特点、施工阶段的监理工作,以及对监理人员的素质要求,可供类似工程借鉴、参考。 相似文献
13.
双龙湖是浅水型城市湖泊,一度为重富营养化湖泊。综合治理后水体仍处于中-富营养化。探求富营养化限制性因素对于防治双龙湖富营养化、维护双龙湖水质至关重要。本文通过双龙湖藻类增长潜力试验研究,确定双龙湖富营养化限制性因素是磷,这对于采取适宜管理措施修复水体、保持水质具有重要意义,为湖泊治理、水环境保持提供科学依据。 相似文献
14.
Baak K. Taeli 《Lakes & Reservoirs: Research and Management》2006,11(3):149-168
Lake Mogan, located in the steppes of central Anatolia, is a vital Turkish ecological system, in terms of its water resources, flora and fauna. The lake also is a unique recreational area for Ankara, the capital of Turkey. However, the lake is currently threatened by severe pollution, being under the risk of serious degradation because of (i) uncontrolled urbanization; (ii) point and non‐point source pollution; (iii) inefficient sewerage systems; (iv) sediment transportation and pollution via influent creeks to the lake; (v) uncontrolled soil erosion in its drainage basin; (vi) inappropriate flood control measures; and (vii) severe degradation of the natural wetlands within its drainage basin. This study evaluates the influence of its influent creeks on the water quality of Lake Mogan, mainly because the creeks are believed to be responsible for the major pollutant load reaching the lake, and because the extensive database documenting this situation has not previously been critically evaluated. Accordingly, this study reviews (i) change in the water quality of Lake Mogan between 1993 and 2002; (ii) the water quality classification of the major influent creeks feeding Lake Mogan; and (iii) the influence of the influent creeks on the water quality of the lake between 1993 and 2004. 相似文献
15.
随着经济发展、城市化率的提高,加之滨湖开发,骆马湖的水质呈现一定的恶化趋势,生态系统遭受破坏,水体富营养化发展有越来越严重的趋势。通过对骆马湖营养化程度和污染原因的分析,提出了一些对策措施和解决办法,以期实现湖泊的生态、环境与经济的协调发展。 相似文献
16.
Jennifer G. Winter M. Catherine Eimers Peter J. Dillon Lem D. Scott Wolfgang A. Scheider Campbell C. Willox 《Journal of Great Lakes research》2007,33(2):381-396
Total phosphorus (TP) inputs to Lake Simcoe have led to hypolimnetic dissolved oxygen (DO) depletion and loss of cold water fish habitat. Since 1990, efforts have been made to reduce the total TP input to the lake below a defined target of 75 t/year, which was predicted to lead to reductions in spring TP concentration and improvements in end-of-summer hypolimnetic DO concentrations. The total TP load to the lake during the most recent period of record (1998/99-2003/04) ranged from 53 to 76 t/yr and averaged 67 t/yr, compared to an average of 114 t/yr estimated between 1990/91 and 1997/98 (range 85-157 t/yr). Reductions in TP loads from the catchment via tributary discharge (∼26 t) accounted for the majority of the decrease in total load between the two time periods. Total P concentrations decreased significantly in four out of six long-term monitored tributaries; however, concentrations in all six tributaries remain above the level recommended to avoid nuisance plant growth (30 μg/L). Although TP loads to the lake are currently below the target 75 t/yr, excessive growths of filamentous algae and macrophytes continue to be a problem in the nearshore zone. End-of-summer minimum hypolimnetic DO concentrations (average 4.3 mg/L, 1998/99-2003/04) remain substantially below the level (7 mg/L) that is considered protective of lake trout. Efforts to reduce TP loads to the lake therefore need to continue. 相似文献
17.
Spectral slope (S), describing the exponential decrease of the absorption spectrum over a given wavelength range, is an important parameter in the study of of chromophoric dissolved organic matter (CDOM) dynamics, and also an essential input parameter in remote sensing models. Furthermore, S is often used as a proxy for CDOM composition, including the ratio of fulvic to humic acids and molecular weight. The relative broad range in S values reported in the literature can be explained by the different spectral ranges and fitting methods used. A single exponential model is used to fit the S values for 17 investigations involving 458 samples in Lake Taihu from January to October in 2004. The average S value was 15.18 ± 1.39 μm−1 for the range of 280–500 nm, which fell within the range reported in the literature. The frequency distribution of S value basically obeyed a normal distribution. Significant differences in S values between summer and other seasons showed that phytoplankton degradation was one of the important sources of CDOM in summer, whereas CDOM mainly came from the river input in other seasons. Furthermore, the estimated S value decreased with increasing wavelength range used in regression. The maximum and minimum values derived from the regression were 17.89 ± 1.25 μm−1 and 13.62 ± 2.11 μm−1 for the wavelength ranges of 280–380 nm and 400–500 nm, respectively, a decrease of 23.9%. S values significantly decreased with the increase of CDOM absorption coefficients. CDOM absorption coefficients could be more appropriately estimated from exponential model introducing the variation of S with absorption coefficients, making them useful for a remote sensing bio-optical model of Lake Taihu. DOC-specific absorption coefficient a*(λ) and the parameter M describing molecular size of the humic molecules could also be used as a proxy for the sources and types of CDOM. A general relationship was found between S and a*(λ), and M values. S increased with the decrease of DOC-specific absorption coefficient and the increase of M corresponding to the decrease of molecular weight. 相似文献
18.
An Interval Fuzzy Multiobjective Watershed Management Model for the Lake Qionghai Watershed, China 总被引:4,自引:0,他引:4
Lijing Wang Wei Meng Huaicheng Guo Zhenxing Zhang Yong Liu Yingying Fan 《Water Resources Management》2006,20(5):701-721
Integrated watershed management is required to ensure the reasonable use of resources and reconcile interactions among natural and human systems. In the present study, an interval fuzzy multiobjective programming (IFMOP) method was used to solve an integrated watershed management problem. Based on system analysis, an IFMOP model suitable for a lake watershed system {IFMOPLWS} was developed and applied to the Lake Qionghai watershed in China. Scenario analysis and an interactive approach were used in the solution process. In this manner, various system components were incorporated into one framework for holistic consideration and optimization. Integrality and uncertainty, as well as the multiobjective and dynamic characteristics of the watershed system, were well addressed. Using two scenarios, two planning schemes were generated. Agriculture, tourism, macroeconomics, cropland use, water supply, forest coverage, soil erosion, and water pollution were fully interpreted and compared to identify a preferable planning alternative for local agencies. This study showed that the IFMOPLWS is a powerful tool for integrated watershed management planning and can provide a solid base for sustainable watershed management. 相似文献
19.
John D. Fitzsimons Bill Williston Georgina Williston Lisa Brown Abdel El-Shaarawi Lenore Vandenbyllaardt Dale Honeyfeld Don Tillitt Martha Wolgamood Scott B. Brown 《Journal of Great Lakes research》2007,33(1):93-103
Alewives (Alosa pseudoharengus), the major prey fish for Lake Ontario, contain thiaminase. They are associated with development of a thiamine deficiency in salmonines which greatly increases the potential for developing an early mortality syndrome (EMS). To assess the possible effects of thiamine deficiency on salmonine reproduction we measured egg thiamine concentrations for five species of Lake Ontario salmonines. From this we estimated the proportion of families susceptible to EMS based on whether they were below the ED20, the egg thiamine concentration associated with 20% mortality due to EMS. The ED20s were 1.52, 2.63, and 2.99 nmol/g egg for Chinook salmon (Oncorhynchus tshawytscha), lake trout (Salvelinus namaycush), and coho salmon (Oncorhynchus kisutch), respectively. Based on the proportion of fish having egg thiamine concentrations falling below the ED20, the risk of developing EMS in Lake Ontario was highest for lake trout, followed by coho (O. kisutch), and Chinook salmon, with the least risk for rainbow trout (O. mykiss). For lake trout from western Lake Ontario, mean egg thiamine concentration showed significant annual variability during 1994 to 2003, when the proportion of lake trout at risk of developing EMS based on ED20 ranged between 77 and 100%. Variation in the annual mean egg thiamine concentration for western Lake Ontario lake trout was positively related (p < 0.001, r2 = 0.94) with indices of annual adult alewife biomass. While suggesting the possible involvement of density-dependent changes in alewives, the changes are small relative to egg thiamine concentrations when alewife are not part of the diet and are of insufficient magnitude to allow for natural reproduction by lake trout. 相似文献
20.