全文获取类型
收费全文 | 5547篇 |
免费 | 1084篇 |
国内免费 | 1170篇 |
专业分类
测绘学 | 745篇 |
大气科学 | 953篇 |
地球物理 | 2099篇 |
地质学 | 2034篇 |
海洋学 | 623篇 |
天文学 | 258篇 |
综合类 | 494篇 |
自然地理 | 595篇 |
出版年
2024年 | 21篇 |
2023年 | 59篇 |
2022年 | 148篇 |
2021年 | 234篇 |
2020年 | 273篇 |
2019年 | 294篇 |
2018年 | 226篇 |
2017年 | 274篇 |
2016年 | 291篇 |
2015年 | 288篇 |
2014年 | 329篇 |
2013年 | 418篇 |
2012年 | 362篇 |
2011年 | 356篇 |
2010年 | 302篇 |
2009年 | 296篇 |
2008年 | 363篇 |
2007年 | 377篇 |
2006年 | 336篇 |
2005年 | 303篇 |
2004年 | 239篇 |
2003年 | 243篇 |
2002年 | 239篇 |
2001年 | 170篇 |
2000年 | 168篇 |
1999年 | 180篇 |
1998年 | 173篇 |
1997年 | 153篇 |
1996年 | 129篇 |
1995年 | 104篇 |
1994年 | 102篇 |
1993年 | 76篇 |
1992年 | 53篇 |
1991年 | 59篇 |
1990年 | 38篇 |
1989年 | 22篇 |
1988年 | 23篇 |
1987年 | 18篇 |
1986年 | 10篇 |
1985年 | 10篇 |
1984年 | 8篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 3篇 |
1971年 | 1篇 |
1954年 | 4篇 |
排序方式: 共有7801条查询结果,搜索用时 218 毫秒
941.
942.
943.
全球海平面变化对古地理、古生物、古气候演化以及能源矿产分布具有重要的控制作用。然而,目前学界对深时海平面变化的驱动机制尚不清楚,部分归因于缺乏高精度全球海平面变化的恢复。文章回顾了全球海平面变化研究的起始与发展过程,归纳出五种类型的深时海平面变化重建方法和技术,即地层学、沉积学、洋盆动力学、同位素地球化学方法和大数据技术。并总结了上述研究方法的原理、优势和不足,并以白垩纪全球海平面重建为实例,讨论了当前深时全球海平面重建的难点和争议点,并对未来深时全球海平面变化重建进行了展望。 相似文献
944.
High resolution(939 samples)total organic carbon content(TOC)analyses were conducted on the Shuanghe Section of^152.6 m in the Changning area,Sichuan Basin.The sampling section was divided into two units considering the distinct-different deposit environment and sediments accumulation rate.The lower part(Unit 1)and the peer part(Unit 2)with high resolution sample spacing(0.08–0.4 m)enables the identification of the precession cycle in two sedimentary sequences with distinct different sedimentary accumulation rate.MTM Power spectral analyses on untuned TOC series reveals significant peaks exceeding above the 95%confidence level and shows that both Unit 1 and Unit 2 have recorded Milankovitch cycles of 405 kyr long eccentricity,short eccentricity,obliquity and precession.The floating astronomical time scale(ATS)was constructed on the Shuanghe Section in the Early Silurian(~439.673–444.681 Ma),and which was calibrated by 405 kyr long eccentricity cycles.The total duration of the Wufeng and Longmaxi shales is 5.01 Myr.The floating ATS used for estimating the duration of the graptolite zones and each stage in the study interval.Finally,we postulated two models that could verify the linkage between orbital cycle and organic accumulation.To make sure whether productivity or preservation is the main factor that under long eccentricity control,the phase correlation between the obtained filtered signal and the theoretical orbital solution should be made clear in the further research. 相似文献
945.
This study investigated the effect of urbanization on runoff from the On-Cheon Stream watershed in Pusan, Korea. This watershed has been experiencing considerable urbanization since the 1960s. There are two gauging stations in the watershed. For one of the stations there are recent flow data and for the other flow data were observed in the past. A linear reservoir model was chosen and runoff was analysed for several flood events. The linear reservoir model has been found to generate flood hydrographs accurately for both gauging stations, and its applicability to the study area has also been established. Using two methods of computing effective rainfall or rainfall excess (ϕ-index and constant percentage method), the results of runoff analyses were investigated. The ϕ-index method yielded better results than the constant percentage method. A comparison of hydrographs observed in the past with the simulation results at the Ie-Sup bridge site revealed that the peak discharge increased and the mean lag time of the study area decreased owing to urbanization over the past two decades. It is also possible to evaluate the effect of urbanization quantitatively. © 1998 John Wiley & Sons, Ltd. 相似文献
946.
时域反射仪(Time Domain Reflectometry)可用于室内和田间快速、 准确、 自动测定土壤含水量, 是目前应用最广泛的土壤含水量测定方法之一。适宜的土壤含水量标定曲线(即土壤表观介电常数和土壤含水量之间的关系)是TDR准确测定土壤含水量的关键。目前文献中存在大量的土壤含水量标定曲线, 但尚未有研究对这些标定曲线进行系统的验证和分析评价。因此, 它们的准确性和适用范围尚不明晰, 严重影响到与土壤含水量测定相关的研究。通过查阅大量国内外文献, 收集整理了一系列土壤含水量标定曲线的经验公式(19个)和半经验半物理模型(5个), 并利用大量的文献实测数据对其进行综合评价。同时运用均方根差(RMSE), 平均误差(AD), 纳什效率系数(NSE)等三个指标对比分析和评价这些标定曲线的准确性和可靠性。研究结果表明: 经验公式中Topp、 Roth(1992)2、 Jacobosen、 Yoshikawa2、 Alharathi模型和半经验模型中Malicki1公式及其修订模型综合性能较好。研究成果可为利用TDR准确测定土壤含水量及土壤含水量标定曲线的选择提供参考和指导。 相似文献
947.
948.
949.
A morphometric comparison of valleys has been made for the Ben Ohau Range in the central Southern Alps of New Zealand. The range is undergoing rapid tectonic transport and uplift. The humid north of the range is a glacial trough-and-arête landscape, with a temperate glacial climate. The dry south has rounded divides and plateau remnants dissected by fluvial valleys. Assuming that space–time substitution allows today's spatial valley-form transition to represent evolutionary stages in valley development, the tectonic history allows time constraints to be placed on the rate of transition to an alpine glacial landscape. Morphometric change has been quantified using hypsometric curves, and distance–elevation plots of cirque and valley-floor altitudes. Ancestral fluvial valleys have less concave long profiles but are stepped at altitude owing to the presence of high-level cirques and remnant plateau surfaces, and possess a low proportion of land area at low elevation. Increasing glacial influence is manifest as smoother, more deeply concave long profiles and U-shaped cross-profiles associated with a higher proportion of the land area at lower elevation. The full morphological transition has involved up to 2.4 km of vertical denudation over the 4 Ma lifetime of the mountain range, of which 80 per cent would have occurred by preglacial fluvial erosion. Combining the trajectory of tectonic transport with reconstructed glaciation limits and climatic history, it is indicated that about 200 ka of temperate glacial erosion produces recognizable trough-and areête topography. Mean and modal relief increase where glacial activity is confined to cirques, but decrease when trough incision by ice becomes established as a dominant process in the landscape. © 1997 by John Wiley & Sons, Ltd. 相似文献
950.
The disruption of a transportation network can have a high social and economic impact on the welfare of a society, as it can significantly affect the daily routines of a community. Although many studies have focused on the estimation of physical risk in the components that compose these networks, only a limited number have analyzed their interconnections and impact in the traffic flow. The present study analyzes how earthquake damage can disrupt the road network in an urban environment, and how this will influence the ability of the population to travel. Traffic due to daily commutes is modeled for different layouts of the network, corresponding to possible disruptions caused by earthquake damage. The duration and length of each trip were calculated both for the undamaged network conditions and for the disrupted network. The increase in the median duration and length of each trip allows estimating the economic loss for each event due to drivers' delay. By combining the probability of a specific road being blocked with its number of users, the average number of affected vehicles was estimated, and the most critical segments identified. The methodology was applied to a case study concerning the road network of the area around the Italian city of Messina in Sicily. The results were calculated for both a repetition of the well-known historical event of 1908 and a set of simulated earthquakes consistent with the national probabilistic seismic hazard model of Italy. 相似文献