全文获取类型
收费全文 | 9571篇 |
免费 | 2470篇 |
国内免费 | 3355篇 |
专业分类
测绘学 | 2236篇 |
大气科学 | 1292篇 |
地球物理 | 1877篇 |
地质学 | 6289篇 |
海洋学 | 1690篇 |
天文学 | 97篇 |
综合类 | 769篇 |
自然地理 | 1146篇 |
出版年
2024年 | 113篇 |
2023年 | 267篇 |
2022年 | 627篇 |
2021年 | 804篇 |
2020年 | 603篇 |
2019年 | 747篇 |
2018年 | 651篇 |
2017年 | 674篇 |
2016年 | 576篇 |
2015年 | 723篇 |
2014年 | 695篇 |
2013年 | 809篇 |
2012年 | 887篇 |
2011年 | 864篇 |
2010年 | 813篇 |
2009年 | 794篇 |
2008年 | 804篇 |
2007年 | 739篇 |
2006年 | 680篇 |
2005年 | 609篇 |
2004年 | 457篇 |
2003年 | 299篇 |
2002年 | 318篇 |
2001年 | 290篇 |
2000年 | 226篇 |
1999年 | 86篇 |
1998年 | 58篇 |
1997年 | 33篇 |
1996年 | 17篇 |
1995年 | 13篇 |
1994年 | 14篇 |
1993年 | 11篇 |
1992年 | 18篇 |
1991年 | 4篇 |
1990年 | 14篇 |
1989年 | 5篇 |
1988年 | 9篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1979年 | 8篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1965年 | 2篇 |
1963年 | 1篇 |
1957年 | 4篇 |
1954年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
241.
Understanding the degree of sea/saltwater intrusion in coastal regions is of great significance to treating the intrusion and improving the environment. Based on the character analysis of the sea/saltwater intrusion, five factors were selected in the fuzzy-synthetical evaluation approach to form the index system, so as to evaluate the degree of sea/saltwater intrusion in southern Laizhou Bay. The results show that the sea/saltwater intrusion is stronger in the middle and northern areas and weaker on the sides and in southern area; currently, the intrusion is relatively serious, and the intrusion area has covered about 68.2% of the areas under study, among which the heavily intruded area is over 50%. Based on the factors analysis of the occurrence and development of sea/saltwater intrusion, the thesis proposes treatment measures. 相似文献
242.
Double packer equipment for hydraulic test can be used to measure pressure of test zone directly, and it is frequently used to perform many kinds of hydraulic tests and take groundwater sample from borehole. The test method of this equipment mainly includes the test design, implementation, interpretation and synthetic analysis. By adopting the double packer equipment for hydraulic test, the parameter distribution of rock permeability along borehole can be acquired, as well as the connectivity, water conductivity and water bearing capacity of the disclosed structure and the chemical characteristics of the deep groundwater. It is a necessary method for the research and evaluation of the complex hypotonicity terrace site selection under geological conditions. This method is not only suitable for the geological disposal of high level radioactive waste, but also can be used in the site selection of underground facilities such as storage of petroleum and carbon dioxide. Meanwhile, it has a good application prospect in other hydrogeological investigation fields. 相似文献
243.
为研究褐菖鲉(Sebastiscus marmoratus)发育早期氨基酸和脂肪酸的组成及变化规律,采用GC/MS法等方法分析了褐菖鲉的受精卵、初产仔鱼、前仔鱼期和后仔鱼期四个阶段的氨基酸和脂肪酸组成特点及其含量变化。结果表明:(1)总氨基酸含量从受精卵至初产仔鱼显著下降、前仔鱼期又显著回升、至后仔鱼期再次呈显著下降(P0.05);(2)总游离氨基酸量以初产仔鱼最高,是受精卵的10倍,各期的游离氨基酸占总氨基酸比值为1.4%—20.0%;(3)受精卵中以DHA、C16:0两者实际含量最高,分别为104.88 mg/g和68.74 mg/g,C18:1n-9和EPA次之,分别为41.23 mg/g和27.23 mg/g;(4)褐菖鲉内源性营养阶段被选择性消耗的主要脂肪酸依次为C16:0C18:1EPADHAARA,就脂肪酸的利用率而言MUFASFAPUFA;(5)褐菖鲉仔鱼在开口后的外源营养阶段对脂肪酸的利用率为PUFASFAMUFA,其中DHA相对EPA和ARA被选择性消耗。研究表明,褐菖鲉受精卵的游离氨基酸/总氨基酸比值符合海水鱼类沉性卵的特征,褐菖鲉在早期发育不同阶段对脂肪酸消耗具有不同的选择性,C16:1、C16:0、C18:0和C18:1是褐菖鲉早期发育的主要能源物质,在褐菖鲉开口饵料中添加一定水平的异亮氨酸、亮氨酸、酪氨酸等氨基酸及高度不饱和脂肪酸DHA等是十分必要的。 相似文献
244.
245.
246.
以扎龙湿地龙泡子为研究对象,利用58个实测水深数据和季相最接近的Quick Bird数据,建立湖泊水深的反演模型。探索性地建立了单波段和多波段组合的线性(多元)回归模型、指数模型、二次多项式模型、微分模型和对数模型等;通过对比模型的决定系数R2,比较模型精度;线性模型、对数模型、指数模型和幂指数模型的R2小于0.5,而二次多项式模型和多元线性回归模型的R2大于0.5,精度相对较高;筛选出拟合度较高的模型,用20个实测验证样本,采用相对误差和均方根误差进行模型精度评价;最后,利用精度较高的模型,进行龙泡子水深反演计算。水深反演结果表明,用选出的模型反演得到的龙泡子水深基本一致,为170~200 cm,即使有稀疏的水草覆盖,依然可以表现出水深渐变的趋势。以蓝、绿、红和近红外波段多光谱遥感反射率为自变量,建立的线性湖泡水深反演模型y=123.990-3.332B1+183.859B2-237.133B3-37.143B4(y为水深;B1、B2、B3和B4分别为蓝、绿、红和近红外波段的水体反射率),能较好地反演扎龙湿地湖泡的水深。 相似文献
247.
248.
LIU Qun HOU Zhishuai WEN Haishen LI Jifang HE Feng WANG Jinhuan GUAN Biao WANG Qinglong 《中国海洋大学学报(英文版)》2016,15(4):667-675
The goal of the study was to examine the effect of stocking density on the water quality of culture area, as well as the growth, body composition and cortisol content of rainbow trout(Oncorhynchus mykiss). Pen-reared trout were stocked in densities of 40, 60, 80 fish individuals m-3(4.6, 6.6, 8.6 kg m-3, SD1, SD2 and SD3 groups, respectively) for 300 days. Compared to the water from SD1 and SD2, that from SD3 exhibited significantly higher NH4+-N content and COD(chemical-oxygen-demand), and a significant reduction of dissolved oxygen in day 180(40.6 kg m-3). Stocking density was significantly associated with body weight, standard length, VSI(viscerosomatic index), CF(condition factor) and FC(food coefficient) in group SD3, particularly in day 240 and day 300(45 or 49.3 kg m-3). Increased crude fat and decreased crude protein were displayed in high density group when the density reached to 36 kg m-3. As a cumulative effect of density-related stress, VSI, CF, FC, moisture, and crude protein content varied over time in each density group(SD1, SD2, and SD3). In summary, trout exhibited a better growth performance in low density(26.3 kg m-3) than those reared in high densities(36 and 45 kg m-3). The results indicate that rainbow trout(114.44 g ± 6.21 g, 19.69 cm ± 0.31 cm) initially stocked in 6.6 or 8.6 kg m-3 should be lightened to less than 36 kg m-3 after an intensive rearing for 240 days. 相似文献
249.
250.