全文获取类型
收费全文 | 1591篇 |
免费 | 176篇 |
国内免费 | 231篇 |
专业分类
测绘学 | 22篇 |
大气科学 | 211篇 |
地球物理 | 413篇 |
地质学 | 686篇 |
海洋学 | 356篇 |
天文学 | 5篇 |
综合类 | 52篇 |
自然地理 | 253篇 |
出版年
2024年 | 7篇 |
2023年 | 15篇 |
2022年 | 38篇 |
2021年 | 39篇 |
2020年 | 50篇 |
2019年 | 56篇 |
2018年 | 24篇 |
2017年 | 91篇 |
2016年 | 67篇 |
2015年 | 87篇 |
2014年 | 92篇 |
2013年 | 87篇 |
2012年 | 57篇 |
2011年 | 101篇 |
2010年 | 68篇 |
2009年 | 127篇 |
2008年 | 120篇 |
2007年 | 103篇 |
2006年 | 106篇 |
2005年 | 77篇 |
2004年 | 70篇 |
2003年 | 49篇 |
2002年 | 65篇 |
2001年 | 42篇 |
2000年 | 54篇 |
1999年 | 51篇 |
1998年 | 44篇 |
1997年 | 35篇 |
1996年 | 36篇 |
1995年 | 37篇 |
1994年 | 16篇 |
1993年 | 22篇 |
1992年 | 15篇 |
1991年 | 16篇 |
1990年 | 7篇 |
1989年 | 9篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有1998条查询结果,搜索用时 47 毫秒
331.
R. Althaus S. Klump A. Onnis R. Kipfer R. Purtschert F. Stauffer W. Kinzelbach 《Journal of Hydrology》2009,370(1-4):64-72
The Grenchen aquifer system in the Swiss Plateau was extensively investigated in order to determine the extent of groundwater contamination and to assess the natural attenuation capacity. Environmental tracer data were applied to estimate groundwater travel times, mixing ratios, and evaluate groundwater origin. Recharge is basically possible in two distinct topographical areas, the immediate vicinity of the town of Grenchen and the elevated plateau of the first Jura Mountain ridge. Groundwater dating was performed with the 3H/3He dating method and supplemented by 85Kr measurements. Stable isotope data (δ18O, δ2H) and dissolved noble gas concentrations allow the determination of the recharge temperature, which is correlated to the recharge elevation. Noble gas temperatures (NGT) decrease in the direction of groundwater flow and range from 10 to 13 °C in the upstream area of the town to 7–9 °C in the downstream river plain. This trend could suggest the admixture of water from the underlying limestone aquifer recharged under cooler infiltration conditions, e.g. at higher recharge elevations. However, it is shown in this study that the difference in NGT does not require such a recharge. Rather, increasing air temperatures over the last 40 years and the urban heat island effect could possibly explain most of the observed temperature shift. Furthermore, it is concluded that the downstream river plain is hydrologically disconnected from the upstream town area. Consequently most water from the town area is drained by the creek Witibach and recharge in the river plain is higher than previously assumed. 相似文献
332.
N. Aarab D.M. Pampanin A. Nvdal K.B.
ysd L. Gastaldi R.K. Bechmann 《Marine pollution bulletin》2008,57(6-12):569
Histopathological characteristics of specific organs express condition, and represent time-integrated impacts on the organism stemming from alterations at lower levels of biological organisation. As integrative parameters, histochemical investigations have proved to be sensitive tools to detect effects of chemical compounds.The objective of this study was to determine changes in the tissues of mussels collected at a PAH contaminated site compared to a reference site using histopathological and histochemical parameters: lipofuscin (LF) accumulation in mussel digestive gland, and lysosomal membrane stability (LMS), and using additional information provided by body burden analysis to compare the sensitivity of these parameters.The histochemical measurements for both LF and LMS gave a clear indication of a high level of stress in animals from the PAH contaminated site. This LF accumulation in lysosomes is the result of peroxidation of autophagocytosed proteins associated with protein aggregates and oxidatively damaged organelles. These measurements were able to detect the effects of PAHs, and showed a strong relationship with the body burden results. 相似文献
333.
Rehan Sadiq Solomon Tesfamariam 《Stochastic Environmental Research and Risk Assessment (SERRA)》2008,22(4):495-505
Environmental indices (EI) constitute a common communication tool that is often used to describe the overall status of environmental
systems (air, water and soil). EI development entails the use of mathematical operators to aggregate various non-commensurate
input parameters in a logical manner. The ordered weighted averaging (OWA) operator is a general mean type operator that provides
flexibility in the aggregation process such that the aggregated value is bounded between minimum and maximum values of the
input parameters. This flexibility of the OWA operator is realized through the concept of orness, which is a surrogate for
decision maker’s attitude. The type of input parameters also affects the choice of aggregation operators. If the input parameters
are linguistic or fuzzy, the aggregation through OWA operators is not possible, and the use of fuzzy arithmetic is warranted.
The concept of fuzzy number OWA (FN-OWA) operators is explored to handle situations in which one or more input parameter has
fuzzy (or linguistic) values. The proposed approach is demonstrated using data provided in an earlier study by Swamee and
Tyagi (ASCE J Environ Eng 126(5):451–455, 2000) for establishing water quality indices. Multiple hypothetical scenarios are also generated to highlight the utility and
sensitivity of the proposed approach. 相似文献
334.
335.
The construction of multiple dams and barrages in many Indian River basins over the last few decades significantly reduced river flow to the sea and affected the sediment regime. More reservoir construction is planned through the proposed National River Linking Project (NRLP), which will transfer massive amounts of water from the North to the South of India. The impacts of these developments on fertile and ecologically sensitive deltaic environments are poorly understood and quantified at present. In this paper an attempt is made to identify, locate and quantify coastal erosion and deposition processes in one of the major river basins in India—the Krishna—using a time series of Landsat images for 1977, 1990 and 2001 with a spatial resolution ranging from 57.0 m to 28.5 m. The dynamics of these processes are analyzed together with the time series of river flow, sediment discharge and sediment storage in the basin. Comparisons are made with similar processes identified and quantified earlier in the delta of a neighboring similarly large river basin—the Godavari. The results suggest that coastal erosion in the Krishna Delta progressed over the last 25 years at the average rate of 77.6 ha yr− 1, dominating the entire delta coastline and exceeding the deposition rate threefold. The retreat of the Krishna Delta may be explained primarily by the reduced river inflow to the delta (which is three times less at present than 50 years ago) and the associated reduction of sediment load. Both are invariably related to upstream reservoir storage development. 相似文献
336.
This paper raises fundamental questions about the sole use of paleolimnological techniques to identify sediment sources and
develop catchment management plans. The concept of an integrated lake: catchment framework was established 30 years ago, yet
paleolimnologists occasionally fail to appreciate the dynamics of the contributing catchment. This is especially critical
when the predominant source of sediment accumulating in a lake is allochthonous. In this paper we argue that a detailed appraisal
of catchment sources and investigation of historical documentary evidence is needed to identify and evaluate the relative
significance of sediment sources. We used such an approach at Aqualate Mere, Shropshire, UK. Mineral magnetic and radionculide
signatures of potential catchment sources and accumulating lake sediments were compared in an attempt to match the sources
to sediments deposited in the Mere. Dated lake sediments indicate there has been an increase in sedimentation rate and the
relative amount of minerogenic material delivered to the Mere over the last 200 years. In contrast to a previous study at
the same site, there is no evidence to attribute this increase to an overspill from a nearby canal. Other catchment disturbances
include landscaping in parkland surrounding the Mere in the early nineteenth century and drainage systems installed to improve
catchment agriculture over the last ca. 150 years. Both activities may explain the change in sedimentation rates and types,
independent of the hypothesized canal origin. Although our results exclude the canal as a major sediment source, identifying
the contribution of other potential catchment sources remains problematic. 137Cs inventories for the lake are similar to those recorded at a local reference site, suggesting little influx of 137Cs-bearing topsoil, yet 137Cs activities remain high in the upper 20–30 cm of the lake sediment profile, indicating a topsoil origin. Combined radionuclide
and mineral magnetic signatures proved to be relatively poor discriminators of potential sources, and the high atmospheric
pollution load from the West Midland conurbation has probably altered recent lake sediment signatures. Although further research
is required to identify the origins of recent (last ca. 200 years) minerogenic sediment inputs to the Mere, we suggest that
the combined lake: catchment approach offers a more rigorous method for understanding the impact of catchment disturbance
than analysis of the paleolimnological record alone. 相似文献
337.
Mark B. Edlund Laura D. Triplett Mark D. Tomasek Kathy Bartilson 《Journal of Paleolimnology》2009,41(4):679-689
Paleolimnological studies show that phosphorus (P) loads to the federally protected St. Croix River, a tributary of the Upper
Mississippi River, have increased about threefold over the last century. Ongoing management efforts to protect and restore
the river hinge on the question of whether the increased nutrient load results from point-source discharges or nonpoint runoff
from agricultural intensification and urban expansion. Here we determine the historical contribution of point source phosphorus
(P) loads to the St. Croix watershed from 1900–2000 A.D. Historical point source loads were estimated based on discharge volumes,
demographics, industrial sources, wastewater technologies, and facility discharge records, where available. Sewering in the
basin began in 1905, and since that time, there have been as many as 169 permitted point source dischargers basinwide, including
municipal, industrial, and agricultural facilities. Early wastewater management typically discharged untreated sewage; technological
advances had secondary treatment in place at most facilities by the 1960s–1970s and much of the municipal population was served
by tertiary treatment by the 1990s. Peak nutrient discharges from point sources occurred in the 1960s–1970s. Detergent phosphorus
bans instituted in the late 1970s for Minnesota and Wisconsin, greater use of land and groundwater effluent disposal, and
improvements in treatment technology brought about decreases in P loads in the 1980s and 1990s. Point-source discharges were
compared to historical total phosphorus loads estimated in a whole-basin phosphorus mass balance to calculate the historical
contribution of point sources, anthropogenic nonpoint sources, and natural or background sources. We estimated 1990s point
source loads at 48 t P yr−1, which represents about 10% of the total phosphorus load (459 t P yr−1, flow-corrected to 412 t P yr−1) to the basin. Without further controls on nutrient inputs to the St. Croix River, annual flow-corrected P loads are projected
to increase to 498 t P yr−1 by the 2020s with point source phosphorus loading contributions at 65 t P yr−1 or 13% of the total load. However, if we exclude background P loads to the St. Croix (166 t P yr−1), recent nutrient loads are primarily from anthropogenic nonpoint sources. Point sources also contribute over 19% of the
current and future phosphorus load that can be attributed to human activities in the watershed. Interstate and federal efforts
to decrease P loading to the St. Croix River by 20% will need to target both point and nonpoint sources.
This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this
special issue of the Journal of Paleolimnology. D.R. Engstrom served as guest editor of the special issue. 相似文献
338.
下击暴流是雷暴天气产生的近地面短时强风,具有和大气边界层近地风显著不同的风场特征,其风荷载数值模拟方法也和常规风荷载模拟方法不同。在将下击暴流风速表达为时变的平均风速和调制的非平稳脉动风速之和的前提下,本文采用基于改进的OBV模型(Oseguera and Bowles/Vicroy(OBV))的方法来模拟平均风速,其平均风速直接由改进的OBV模型表达,而不同于已往研究中将平均风速表达为竖向风剖面与时间函数的乘积,并采用结合快速傅里叶变化的谐波叠加法来模拟调制的非平稳脉动风速。改进的OBV模型考虑了下击暴流的风暴中心移动和强度演化的影响,更准确地描述了下击暴流的实际物理过程。通过实例仿真,说明结果准确合理。 相似文献
339.
Environmental decision-making under uncertainty using intuitionistic fuzzy analytic hierarchy process (IF-AHP) 总被引:1,自引:0,他引:1
Rehan Sadiq Solomon Tesfamariam 《Stochastic Environmental Research and Risk Assessment (SERRA)》2009,23(1):75-91
Analytic hierarchy process (AHP) is a utility theory based decision-making technique, which works on a premise that the decision-making
of complex problems can be handled by structuring them into simple and comprehensible hierarchical structures. However, AHP
involves human subjective evaluation, which introduces vagueness that necessitates the use of decision-making under uncertainty. The vagueness is commonly handled through fuzzy sets theory, by assigning degree of membership. But, the environmental decision-making
problem becomes more involved if there is an uncertainty in assigning the membership function (or degree of belief) to fuzzy pairwise comparisons, which is referred to as ambiguity (non-specificity). In this paper, the concept of intuitionistic fuzzy set is applied to AHP, called IF-AHP to handle both vagueness and ambiguity
related uncertainties in the environmental decision-making process. The proposed IF-AHP methodology is demonstrated with an
illustrative example to select best drilling fluid (mud) for drilling operations under multiple environmental criteria. 相似文献
340.
In this paper, an analytical framework to evaluate the along-wind-induced dynamic responses of a transmission tower is presented. Two analytical models and a new method are developed: (1) a higher mode generalized force spectrum (GFS) model of the transmission tower is deduced; (2) an analytical model that includes the contributions of the higher modes is further derived as a rational algebraic formula to estimate the structural displacement response; and (3) a new approach, applying load with displacement (ALD) instead of force, to solve the internal force of transmission tower is given. Unlike conventional methods, the ALD method can avoid calculating equivalent static wind loads (ESWLs). Finally, a transmission tower structure is used as a numerical example to verify the feasibility and accuracy of the ALD method. 相似文献