全文获取类型
收费全文 | 1716篇 |
免费 | 215篇 |
国内免费 | 91篇 |
专业分类
测绘学 | 19篇 |
大气科学 | 10篇 |
地球物理 | 423篇 |
地质学 | 1463篇 |
海洋学 | 15篇 |
天文学 | 1篇 |
综合类 | 47篇 |
自然地理 | 44篇 |
出版年
2023年 | 5篇 |
2022年 | 24篇 |
2021年 | 33篇 |
2020年 | 34篇 |
2019年 | 40篇 |
2018年 | 26篇 |
2017年 | 38篇 |
2016年 | 62篇 |
2015年 | 66篇 |
2014年 | 81篇 |
2013年 | 54篇 |
2012年 | 38篇 |
2011年 | 52篇 |
2010年 | 62篇 |
2009年 | 149篇 |
2008年 | 202篇 |
2007年 | 170篇 |
2006年 | 189篇 |
2005年 | 118篇 |
2004年 | 105篇 |
2003年 | 65篇 |
2002年 | 57篇 |
2001年 | 44篇 |
2000年 | 40篇 |
1999年 | 38篇 |
1998年 | 36篇 |
1997年 | 32篇 |
1996年 | 29篇 |
1995年 | 37篇 |
1994年 | 26篇 |
1993年 | 23篇 |
1992年 | 14篇 |
1991年 | 1篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 7篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 6篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有2022条查询结果,搜索用时 640 毫秒
341.
Forecasting of groundwater level in hard rock region using artificial neural network 总被引:2,自引:0,他引:2
In hardrock terrain where seasonal streams are not perennial source of freshwater, increase in ground water exploitation has
already resulted here in declining ground water levels and deteriorating its’ quality. The aquifer system has shown signs
of depletion and quality contamination. Thus, to secure water for the future, water resource estimation and management has
urgently become the need of the hour. In order to manage groundwater resources, it is vital to have a tool to predict the
aquifer response for a given stress (abstraction and recharge). Artificial neural network (ANN) has surfaced as a proven and
potential methodology to forecast the groundwater levels. In this paper, Feed-Forward Network based ANN model is used as a
method to predict the groundwater levels. The models are trained with the inputs collected from field and then used as prediction
tool for various scenarios of stress on aquifer. Such predictions help in developing better strategies for sustainable development
of groundwater resources. 相似文献
342.
An extension to the DRASTIC model is proposed in order to assess aquifer vulnerability to pollution. In contrast to the DRASTIC
model, which considers the unsaturated and saturated zones together and computes a global intrinsic vulnerability index, the
suggested approach discriminates between the aquifer vertical vulnerability (a concept related to the pollutant percolation)
and the groundwater susceptibility (a concept that depends on the behaviour and uses of the groundwater). This approach is
applied to the Haouz aquifer (Morocco) that supplies water to the Marrakech area. This aquifer is widely overexploited and
there is evidence that the groundwater quality is threatened by various sources of pollution. Evaluation of the vertical vulnerability
indicates that the aquifer mainly presents a moderate-to-weak vertical vulnerability. The zones potentially most favourable
to pollutant percolation are mainly located in Central Haouz, along or near the surface wadis. The aquifer susceptibility
is high in places located near the N’Fis, Baaja and Issil wadis. Everywhere else, low-to-moderate susceptibility is observed.
This new approach therefore enables areas of vertical vulnerability and areas of susceptibility to be delineated separately.
As a result, it constitutes a valuable decision-making tool for optimising the management of aquifer water resources and land-use
planning. 相似文献
343.
The occurrence and development of riparian forests, which were mainly dominated by mesophytes species related closely with
surface water. Since there was no water discharged to the lower reaches of Tarim River in the past three decade years, the
riparian forests degrade severely. The groundwater table, the saline content of the groundwater, as well as the content of
free proline, soluble sugars, plant endogenous hormones (abscisic acid (ABA), and cytokinins (CTK)) of the leaves and relative
rates of sap flow of the Populus euphratica Oliv. (arbor species), Tamarix ramosissima Ldb. (bush species), and Apocynum venetum L. (herb species) were monitored and analyzed at the lower reaches of the Tarim River in the study area where five positions
on a transect were fixed at 100 m intervals along a sampling direction from riverbank to the sand dunes before and after water
release. The physiological responses and acclimation strategies of three species to variations in water and salinity stress
were discussed. It was found that A. venetum population recovered to groundwater table ranging from −1.73 to −3.56 m, and when exposed to saline content of the groundwater
ranging from 36.59 to 93.48 m mol/L; P. euphratica appeared to be more sensitive to the elevation of groundwater table than the A. venetum and T. ramosissima at groundwater table ranging from −5.08 to −5.80 m, and when exposed to saline content of the groundwater ranging from 42.17
to 49.55 m mol/L. T. ramosissima tended to be the best candidate species for reclamation in this hyper-arid area because it responded to groundwater table
ranging from −1.73 to −7.05 m, and when exposed to saline content of the groundwater ranging from 36.59 to 93.48 m mol/L.
These results explained the distribution patterns of desert vegetation in the lower reaches of the Tarim River. Understanding
the relationships among ecological factors variables, physiological response and acclimation strategies of plant individuals
could provide guidance to sustainable management, reclamation and development of this and similar regions. 相似文献
344.
345.
Evaluation on Groundwater Resources of Medium Salinity in Tarim Basin and Development and Demonstration of Under-mulch-drip Irrigation Technology for Cotton
下载免费PDF全文
![点击此处可从《地下水科学与工程》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The groundwater of medium salinity refers to the groundwater with a mineralization degree of 2-7 g/L that can be directly or indirectly used for irrigation. This paper determined the distribution area of the groundwater with medium salinity in Tarim Basin, estimated the total amount of recharge resources and exploitable quantity of the groundwater of medium salinity. The irrigation water quality assessment was conducted on the groundwater of medium salinity by using multiple factors and methods. The salt tolerance parameters and irrigation water mineralization control parameters of the cotton in under-mulch-drip irrigation were determined. The under-mulch-drip irritation technology for cotton in groundwater of medium salinity was developed and applied successfully to the demonstration area. The social, economic and ecological effects of this technology as well as the application prospect were analysed. This paper also puts forward two questions that need to be further studied 相似文献
346.
本文收集整理了距2010年4月14日玉树7.1级地震震中700km范围内的全部13个地下流体观测井孔的温度观测资料,统计分析了2008年5月以来各井温度当月超过3倍均方差的数据个数(以下简称月超差个数).结果发现,在玉树地震前6个月,13个观测井中有10个井观测数据出现月超差个数升高现象,有4个观测井月超差个数出现了同震升高和震后升高现象.2008、2009年大柴旦发生的2次6级以上地震前,也有类似现象.此异常现象可能反映了玉树地震前场兆和源兆的共同作用过程. 相似文献
347.
348.
349.
Thirty years(1984–2014) of groundwater science teaching and research in China: A dissertation-based bibliometric survey
下载免费PDF全文
![点击此处可从《地下水科学与工程》网站下载免费的PDF全文](/ch/ext_images/free.gif)
ZHAI Yuan-zheng JIANG Shi-jie TENG Yan-guo WANG Jin-sheng GU Hong-biao XIE Liang YIN Zhi-hua 《地下水科学与工程》2015,3(3):222-237
In order to track the footprint of groundwater science teaching and research in China during the past years, a bibliometric analysis was conducted based on the database of China National Knowledge Infrastructure (CNKI), which covered the time span from 1984 to present and included more than 2.6 million master and doctoral dissertations from hundreds of institutions of Mainland China. The bibliometric analysis summarized output, geographical, and institutional patterns, as well as research directions and hotspots in groundwater studies in China in the period of 1984-2014. A total of 1 396 groundwater-related dissertations including 1 161 master dissertations and 235 doctoral dissertations, contributed by 128 institutions distributed in 53 cities nationwide, were searched out in the database. It can be seen obviously that, the groundwater science teaching and research in China has experienced a notable growth in the past three decades especially during 2000-2014. Groundwater modeling, resource, and exploitation were the top three major subject categories; China University of Geosciences (Beijing), Jilin University and Chang’an University were the top three productive institutions together accounting for more than one third of the total dissertations and 50% of the doctoral dissertations, which further prompted the cities (Beijing, Changchun, and Xi’an) to become the top three productive cities. The dissertations generally covered all the international research topics, which indicated that hydrogeologists in China have tracked the international frontier closely during the past decades. The keywords analysis revealed that, (1) numerical modeling was still the hottest topic and PHREEQC, MODFLOW, GMS, and FEFLOW were the four most used softwares; (2) the topics related with groundwater pollution and quality developed steadily and rapidly; (3) environmental isotopes and GIS were frequently used tools for hydrogeological condition analysis, and spatial data processing respectively; and (4) the unsaturated zone as an integral part of aquifers attracted more and more attentions from hydrogeologists focus on saturated zone. 相似文献
350.
Analysis of borehole flow logs is a valuable technique for identifying the presence of fractures in the subsurface and estimating properties such as fracture connectivity, transmissivity and storativity. However, such estimation requires the development of analytical and/or numerical modeling tools that are well adapted to the complexity of the problem. In this paper, we present a new semi-analytical formulation for cross-borehole flow in fractured media that links transient vertical-flow velocities measured in one or a series of observation wells during hydraulic forcing to the transmissivity and storativity of the fractures intersected by these wells. In comparison with existing models, our approach presents major improvements in terms of computational expense and potential adaptation to a variety of fracture and experimental configurations. After derivation of the formulation, we demonstrate its application in the context of sensitivity analysis for a relatively simple two-fracture synthetic problem, as well as for field-data analysis to investigate fracture connectivity and estimate fracture hydraulic properties. These applications provide important insights regarding (i) the strong sensitivity of fracture property estimates to the overall connectivity of the system; and (ii) the non-uniqueness of the corresponding inverse problem for realistic fracture configurations. 相似文献