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161.
A study on two closed salt lake basins, Tal Chapar and Parihara in the eastern margin of the Thar Desert, Rajasthan, was carried
out to unravel late Quaternary geomorphic evolution of these saline lakes. Both lakes are elliptical in shape bordered by
stabilised dunes, and are oriented in a NE-SW direction, i.e., in the direction of the prevailing summer monsoon wind. Both
lakes have been formed in the wind-shadow zones of isolated hills of Precambrian quartzite. Our study indicates that the late
Quaternary sediments in the lakes began with the cyclic deposition of laminated fine silt layers (0.5 m thick), rich in organic
matter, alternating with ripple cross-bedded sand layers (each ∼1.5–2 m thick). Sand layers that are moderately sorted are
separated by laminated silt-clay layers with gypsum/calcite and this unit occurs in the upper most 4 m sequence in deeper
sections. The presence of gypsum crystals within the laminated sediments suggests a high concentration of Ca in the inflowing
water.
At Parihara Lake the organic carbon-rich sediments at 95 cm depth was dated to 7,375 + 155/−150 year BP. At Tal Chapar radiocarbon
dates of 7,190 + 155/−150 and 9,903 + 360/−350 was obtained from the sediments rich in organic carbon occurring at a depth
of 1.35 m and 1.80 m, respectively. The study reveals strong hydrologic oscillations during the past ∼14,000 year BP (13,090 + 310/−300 year
BP).
Quaternary geomorphic processes, especially the strong aeolian processes during dry climatic phases, played a major role in
the formation of the lake basins, as well as the fringing linear dunes. Geochemical and mineralogical analyses of the lacustrine
sediments, supported by radiocarbon dates indicate the existence of an ephemeral lake earlier than ∼13,000 year BP as sediments
began to be deposited in a lacustrine environment implying sustained runoff in the catchments. A freshwater lake formed between
9,000 year and 7,000 year BP. The lake dried periodically and this strong fluctuating regime continued until about ∼7,000 year
BP. Mid-Holocene was wet and this was possibly due to higher winter rains A saline lake existed between 6,000 year and 1,300 year
BP and finally present day semi arid conditions set in since 1,200 year BP. Remnants of a habitation site (hearth and charred
bones) on stabilised dune at Devani near Tal Chapar were dated to 240 ± 120 year, while that at Gopalpura was dated to 335 ± 90 year.
These historical sites on stabilised dunes were, according to the local accounts, settlements of people who used the lake
brine for manufacturing salt. 相似文献
162.
This paper includes an investigation of the influence of the soil–structure interaction (SSI) on the fundamental period of buildings. The behaviour of both the soil and the structure is assumed to be elastic. The soil‐foundation system is modelled using translational and rotational discrete springs. Analysis is first conducted for one‐storey buildings. It shows that the influence of the SSI on the fundamental frequency of building depends on the soil–structure relative rigidity Kss. Analysis is then extended for multi‐storey buildings. It allows the generalization of the soil–structure relative rigidity Ks to such complex structures. Charts are proposed for taking into account the influence of the SSI in the calculation of the fundamental frequency of a wide range of buildings. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
163.
基于MEM1谱分析的水文时间序列隐含周期特性研究 总被引:13,自引:1,他引:13
水文时间序列的隐含周期(本文将简单周期、复合周期及近似周期等统称为隐含周期)的识别、判定是一个重要而又较为困难的问题,相对成熟和有效的做法是对其进行频谱分析。建立在最大熵原理(POME)基础之上的MEM1谱分析,克服了传统谱分析方法的诸多不足,具有频谱光滑、分辨率高等独特优势。为此,以黄河花园口(秦厂)测站年径流系列、月径流系列和年最大洪峰流量序列隐含周期特性的研究为例,探讨了MEM1谱分析在水文时间序列隐含周期特性中的应用。 相似文献
164.
165.
广西两系杂交稻制种安全期气候分析 总被引:1,自引:0,他引:1
根据两系杂交水稻制种对气象条件的要求 ,利用广西 80多站的气象资料 ,统计分析了广西各地两系杂交水稻制种的安全期 ,提出了广西杂交水稻安全制种应注意的一些气候问题 相似文献
166.
167.
168.
中国西北地区季节性积雪的性质与结构 总被引:17,自引:2,他引:17
中国内陆地区积雪分布十分广泛。根据西北地区大陆性气候条件下形成的“干寒型”积雪的特征 ,对中国天山和阿尔泰山山区的季节性积雪进行了观测与分析。结果表明 ,该区最大积雪深度达 15 2cm(1997) ,积雪层一般由新雪 (或表层凝结霜 )、细粒雪、中粒雪、粗粒雪、松散深霜、聚合深霜层和薄融冻冰层组成。与“湿暖型”积雪相比 ,“干寒型”积雪的性质具有密度小 (新雪的最小密度为 0 .0 4 g/cm3 )、含水率少 (隆冬期 <1% )、温度梯度大(最大可达 - 0 .5 2℃ /cm)、深霜发育层厚等特点 ,并且变质作用以热量交换和雪层压力变质作用为主。据中国科学院天山积雪与雪崩研究站 (43°2 0N ,84°2 9E ,海拔 1776m)的观测资料 ,中国内陆干旱区冬季积雪期雪面太阳辐射通量以负平衡为主 ,新雪雪面反射率达 96 % ,短波辐射在干寒型积雪中的穿透厚度达 2 8cm。春季积雪消融期 ,深霜层厚度可占整个积雪层厚度的 80 %。随着气温的升高 ,雪粒间的键链首先融化 ,使积雪变得松散 ,内聚力、抗压、抗拉和抗剪强度降低 ,积雪含水率也随之增大 ,整个积雪层趋于接近 0℃的等温现象 ,因此 ,春季天山、阿尔泰山等山地全层性湿雪崩频繁发生 相似文献
169.
The “greatest lake period” means that the lakes are in the stage of their maximum areas. As the paleo lake shorelines are
widely distributed in the lake basins on the Tibetan Plateau, the lake areas during the “greatest lake period” may be inferred
by the last highest lake shorelines. They are several, even tens times larger than that at present. According to the analyses
of tens of lakes on the Plateau, most dating data fell into the range of 40-25 ka BP, some lasted to 20 ka BP. It was corresponded
to the stage 3 of marine isotope and interstitial of last glaciation. The occurrence of maximum areas of lakes marked the
very humid period on the Plateau and was also related to the stronger summer monsoon during that period. 相似文献
170.