全文获取类型
收费全文 | 416篇 |
免费 | 24篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 26篇 |
大气科学 | 22篇 |
地球物理 | 138篇 |
地质学 | 213篇 |
海洋学 | 14篇 |
天文学 | 15篇 |
综合类 | 5篇 |
自然地理 | 20篇 |
出版年
2023年 | 2篇 |
2022年 | 10篇 |
2021年 | 14篇 |
2020年 | 30篇 |
2019年 | 22篇 |
2018年 | 44篇 |
2017年 | 43篇 |
2016年 | 46篇 |
2015年 | 28篇 |
2014年 | 32篇 |
2013年 | 41篇 |
2012年 | 33篇 |
2011年 | 29篇 |
2010年 | 20篇 |
2009年 | 13篇 |
2008年 | 9篇 |
2007年 | 7篇 |
2006年 | 11篇 |
2004年 | 3篇 |
2003年 | 2篇 |
2002年 | 3篇 |
2000年 | 2篇 |
1996年 | 1篇 |
1993年 | 1篇 |
1991年 | 1篇 |
1989年 | 3篇 |
1988年 | 1篇 |
1982年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有453条查询结果,搜索用时 15 毫秒
111.
Hojat Mirani Moghadam Rahim Bagheri Gholam Hossein Karami Hadi Jafari 《Ground water》2020,58(5):771-776
A systematic study of the chemo-isotopic characteristics and origin of the groundwater was carried out at six major qanats in the hyper-arid Gonabad area, eastern Iran. These qanats as a sustainable groundwater extraction technology have a long history, supporting human life for more than a thousand years in this region. The Gonabad qanats are characterized by outlet electrical conductivity (EC) values of 750 to 3900 µS/cm and HCO3-Na-Mg and Cl-Na water types. The Gonabad meteoric water line (GnMWL) was drawn at the local scale as δ2H = 6.32×δ18O + 8.35 (with R2 = 0.90). It has a lower slope and intercept than the global meteoric water line due to different water vapor sources and isotope kinetic fractionation effects during precipitation in this arid region. The altitude effects on isotopic content of precipitation data were derived as δ18O = (−0.0031 × H(m.a.s.l))−1.3). The δ2H and δ18O isotopes signatures demonstrate a meteoric origin of the groundwater of these qanats. The shift of the qanat's water samples from the local meteoric water line (LMWL) in a dry period with higher temperatures is most probably due to evaporation during the infiltration process and water movement in qanat gallery. Based on the isotopic results and mass balance calculations, the qanats are locally recharged from an area between 2000 to 2400 m.a.s.l of nearby carbonate formations and coarse alluvial sediments. The dissolution of evaporate interlayers in Neogene deposits deteriorates the groundwater quality, especially in Baidokht qanat. 相似文献
112.
Headcut, known as knickpoint migration too, is developed due to sudden change in channel bed followed by bed scour and erosion which progressing upstream. The results are the downstream morphological change and transporting massive sediment to the downstream reservoir. Most of the past studies focus on non-cohesive soils, although many problems occur because of cohesive soils. In this study, 10 different samples of cohesive soils in long term consolidation with different composition of silt and clay were tested under different circumstances of waterfall height and flow velocity to investigate the neck migration rate and the sediment yield. Tests were continued to reach a constant migration rate. One of the effective phenomena in all tests was tensional cracks on soil surface. The size and number of these cracks have inverse relation with percent of clay. Because of these cracks, massive erosion occurs at the beginning of all tests. By reducing percent of clay, headcut, waterfall height and sediment yield were increased and by reducing waterfall height and flow velocity these parameters were reduced. In lower percent of clay, headcut erosion will occur quickly with more slants. Caving phenomenon was not observed in any tests and massive erosion rate was more quickly. 相似文献
113.
Adel Asnaashari Amir Ahmad Dehghani Ali Akbar Akhtari Hossein Bonakdari 《Water Resources》2018,45(4):565-577
Channel expansions are common in both natural and artificial open channels. With increasing cross-sectional dimensions in an expansion, the flow decelerates. Due to separation of flow and subsequent eddy formation, a significant head loss is occurred along the transition. This study presents the results of experimental investigations on subcritical flow along the expansive transition of rectangular to trapezoidal channels. Also, a numerical simulation was developed using the finite volume method with Reynolds Stress turbulent model. Water surface profiles and velocity distributions of flow through the transition were measured experimentally and compared with the numerical results. Also, hydraulic efficiency of the transition and coefficient of energy head loss were calculated. The results show that with increasing the upstream Froude number, hydraulic efficiency of the transition and coefficient of energy head loss are decreased and increased, respectively. The results also showed the ability of numerical simulation for simulating the flow separation zones and bed shear stress along the transition for different inlet discharges and inflow Froude numbers. 相似文献
114.
The effect of fully submerged boulders on the flow structure in channels has been studied by some researchers. However, many natural streams have bed material with boulders that are not fully submerged under water. In many natural streams, boulders cover between 1% and 10% of the area of the stream reach. The effect of non-submerged boulders on the velocity profile and flow characteristics is very important for assessing riverbed deformation. The objectives of this paper are to find the pattern of velocity distribution around a non-submerged boulder and to compare it with the classical studies on flow resistance and Reynolds stress distribution in open channels. Also, by considering the variation in the Reynolds stress distribution at different locations around a non-submerged boulder, the effect of a non-submerged boulder on the estimation of shear velocity and resistance to flow has been investigated. Results indicates that inside the scour hole caused by a non-submerged boulder in a river velocity distributions are irregular. However, velocity distributions are regular outside the scour hole. The presence of the boulder causes a considerable deviation of the Reynolds shear stress from the classic distribution, showing a non-specific distribution with negative values. The classical methods for calculating shear velocity are not suitable because these methods do not give detailed velocity and Reynolds stress distributions in natural rivers with a lot of boulders. Thus, the effect of a non-submerged boulder on the estimation of the resistance to flow by considering the variations in velocity and Reynolds stress distributions at different locations around a non-submerged boulder is important and needs to be studied in a natural river instead of just in laboratory flumes. The negative values in Reynolds stress distribution around a boulder indicate that the classical methods are unable to predict resistance to flow, and also show strong turbulence inside the scour hole where the complex flow conditions present ambiguous Reynolds stress distributions. In the current study, to obtain a reasonable estimation of parameters in natural rivers, the classical method has been modified by considering velocity and Reynolds stress distributions through the boundary layer method. 相似文献
115.
Lewis Matt J. Palmer Tamsin Hashemi Resa Robins Peter Saulter Andrew Brown Jenny Lewis Huw Neill Simon 《Ocean Dynamics》2019,69(3):367-384
Ocean Dynamics - The combined hazard of large waves occurring at an extreme high water could increase the risk of coastal flooding. Wave-tide interaction processes are known to modulate the wave... 相似文献
116.
117.
Hossein Ghaffarian Hervé Piégay Diego Lopez Nicolas Rivière Bruce MacVicar Aurelie Antonio Emmanuel Mignot 《地球表面变化过程与地形》2020,45(10):2219-2234
Wood in rivers plays a major role both ecologically and morphologically. In recent decades, due to human activities in the river channels and along the riparian zone, wood obstruction and jamming has exacerbated flooding hazards and infrastructure damage. Therefore, it is necessary to quantify the wood flux and discharge in rivers to improve wood hazard management. Among the various methods for monitoring the wood flux in a river, the streamside videography technique is effective given its high temporal and spatial resolution. Previous work monitored the wood discharge (m3/s) using this technique in the Ain River (France) during three floods (MacVicar and Piégay, 2012), and the same method is implemented on the Isère River (France) to obtain the statistics of wood discharge for two floods. Comparison between the two sites supports the generalization of both the monitoring technique and the link between wood discharge and flood characteristics. We first show that the maximum wood discharge is observed at bankfull discharge, and we confirm the three stage model proposed by MacVicar and Piégay (2012). Additionally, transverse distributions of the number of wood pieces and corresponding wood length appear to be similar for different flood magnitudes on each site. As a technical contribution, the use of the same technique on two sites allows for recommendations on key decisions related to the location and implementation of the equipment. Both statistical and technical contributions can be used by decision makers to implement this monitoring technique, acquire the wood transport parameters, and evaluate the potential wood hazards at local scale or along a river. © 2020 John Wiley & Sons, Ltd. 相似文献
118.
In this study, the Maastrichtian-Danian boundary was measured and sampled in two stratigraphic sections, the north and south flanks of the Dehnow anticline in Coastal Fars, Southern Iran. This boundary was also investigated in the drilled exploratory well-1 in the same region. The lithology of the Maastrichtian-Danian deposits consists of glauconitic, phosphatic argillaceous limestones and marlstones. 30 genera and 77 species of planktonic foraminifera have been determined. The recognized biozones are the Gansserina gansseri Interval Zone, and the Contusotruncana contusa Interval Zone, which indicates latest Campanian to middle Maastrichtian age for the upper part of the Gurpi Formation. In addition, the Eoglobigerina edita (P1) Partial Range Zone, Praemurica uncinata (P2) Lowest Occurrence Zone, Morozovella angulata (P3) Lowest Occurrence Zone and Globanomalina psudomenardii (P4) Taxon Range Zone represent a Danian to Thanetian age for the lower part of the Pabdeh Formation. From the absence of the Abathomphalus mayaroensis Interval Zone, Pseudoguembelina hariaensis Interval Zone, Pseudotextularia elegans Interval Zone, Plummerita hantkeninoides Interval Zone, Guembelitria cretacea (P0) Partial Range Zone and Parvularugoglobigerina eugubina (Pα) Total Range Zone, it can be deduced that there is a paraconformity across the Maastrichtian–Danian boundary in the studied area, this hiatus encompassing the late Maastrichtian and the earliest Danian. Danian deposits from the study area contain reworked glauconitized macrofossils, planktonic and benthic foraminifera of the Cretaceous. The obtained surface gamma-ray spectrometry logs resemble the exploratory well-1 gamma-ray wireline log. 相似文献
119.
120.
Land use and land cover classification over a large area in Iran based on single date analysis of satellite imagery 总被引:3,自引:0,他引:3
Hossein Saadat Jan AdamowskiRobert Bonnell Forood SharifiMohammad Namdar Sasan Ale-Ebrahim 《ISPRS Journal of Photogrammetry and Remote Sensing》2011,66(5):608-619
Accelerated soil erosion, high sediment yields, floods and debris flow are serious problems in many areas of Iran, and in particular in the Golestan dam watershed, which is the area that was investigated in this study. Accurate land use and land cover (LULC) maps can be effective tools to help soil erosion control efforts. The principal objective of this research was to propose a new protocol for LULC classification for large areas based on readily available ancillary information and analysis of three single date Landsat ETM+ images, and to demonstrate that successful mapping depends on more than just analysis of reflectance values. In this research, it was found that incorporating climatic and topographic conditions helped delineate what was otherwise overlapping information. This study determined that a late summer Landsat ETM+ image yields the best results with an overall accuracy of 95%, while a spring image yields the poorest accuracy (82%). A summer image yields an intermediate accuracy of 92%. In future studies where funding is limited to obtaining one image, late summer images would be most suitable for LULC mapping. The analysis as presented in this paper could also be done with satellite images taken at different times of the season. It may be, particularly for other climatic zones, that there is a better time of season for image acquisition that would present more information. 相似文献