全文获取类型
收费全文 | 1735篇 |
免费 | 71篇 |
国内免费 | 18篇 |
专业分类
测绘学 | 41篇 |
大气科学 | 124篇 |
地球物理 | 503篇 |
地质学 | 570篇 |
海洋学 | 171篇 |
天文学 | 281篇 |
综合类 | 6篇 |
自然地理 | 128篇 |
出版年
2021年 | 22篇 |
2020年 | 25篇 |
2019年 | 24篇 |
2018年 | 42篇 |
2017年 | 34篇 |
2016年 | 56篇 |
2015年 | 38篇 |
2014年 | 43篇 |
2013年 | 83篇 |
2012年 | 45篇 |
2011年 | 62篇 |
2010年 | 78篇 |
2009年 | 76篇 |
2008年 | 67篇 |
2007年 | 83篇 |
2006年 | 61篇 |
2005年 | 55篇 |
2004年 | 52篇 |
2003年 | 52篇 |
2002年 | 50篇 |
2001年 | 26篇 |
2000年 | 33篇 |
1999年 | 28篇 |
1998年 | 27篇 |
1997年 | 28篇 |
1996年 | 26篇 |
1995年 | 29篇 |
1994年 | 31篇 |
1993年 | 14篇 |
1992年 | 31篇 |
1991年 | 29篇 |
1990年 | 39篇 |
1989年 | 25篇 |
1988年 | 22篇 |
1987年 | 17篇 |
1986年 | 17篇 |
1985年 | 28篇 |
1984年 | 35篇 |
1983年 | 26篇 |
1982年 | 19篇 |
1981年 | 30篇 |
1980年 | 25篇 |
1979年 | 21篇 |
1978年 | 21篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1975年 | 13篇 |
1974年 | 14篇 |
1973年 | 15篇 |
1971年 | 11篇 |
排序方式: 共有1824条查询结果,搜索用时 15 毫秒
101.
102.
AbstractMeasurements by free fall instruments, in the San Diego Trough, the Florida Current, and the central Pacific, reveal the detailed structure of the vertical component of the oceanic temperature gradient. The temperature changes are concentrated into regions on the order of a meter thick wherein the measured gradients are often more than ten times the average gradient. The horizontal extent of the regions of high gradient is greater than 750 meters in the seasonal thermocline off San Diego, but is only a few hundred meters at depths greater than 400 meters.Fine scale measurements show that the layers of high gradient consist of even finer fluctuations in gradient which are only a few centimeters thick. Time scales of the thinnest of these regions of high gradient are of the order of five minutes. The data also yields an estimate of the entropy generation. According to the results of an idealized model relating entropy generation to the turbulent heat transport, only 240 to 700 ergs per cm.2 per sec were transported in a 25 meter vertical section measured in the San Diego Trough. This value compared with 3600 ergs per cm.2 per sec estimated from the mean gradient and an eddy coefficient of 1 cm.2 per sec. 相似文献
103.
104.
An ensemble of stochastic daily rainfall projections has been generated for 30 stations across south‐eastern Australia using the downscaling nonhomogeneous hidden Markov model, which was driven by atmospheric predictors from four climate models for three IPCC emissions scenarios (A1B, A2, and B1) and for two periods (2046–2065 and 2081–2100). The results indicate that the annual rainfall is projected to decrease for both periods for all scenarios and climate models, with the exception of a few scenarios of no statistically significant changes. However, there is a seasonal difference: two downscaled GCMs consistently project a decline of summer rainfall, and two an increase. In contrast, all four downscaled GCMs show a decrease of winter rainfall. Because winter rainfall accounts for two‐thirds of the annual rainfall and produces the majority of streamflow for this region, this decrease in winter rainfall would cause additional water availability concerns in the southern Murray–Darling basin, given that water shortage is already a critical problem in the region. In addition, the annual maximum daily rainfall is projected to intensify in the future, particularly by the end of the 21st century; the maximum length of consecutive dry days is projected to increase, and correspondingly, the maximum length of consecutive wet days is projected to decrease. These changes in daily sequencing, combined with fewer events of reduced amount, could lead to drier catchment soil profiles and further reduce runoff potential and, hence, also have streamflow and water availability implications. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
105.
106.
Jari plantation is the largest pulp growing operation in the humid tropics, and occupies about 1000 sqkm previously covered by tropical forest. A study of five sites at Jari in 1980 indicated declining soil fertility, and low productivity of pulpwood. Low pulp production at Jari was caused by several factors, one of which appeared to be low soil fertility. The sams sites were re-evaluated in 1987. There was no discernable trend in either nutrients or productivity since 1980.Although productivity and nutrient stocks may have stabilized, low profitability caused by low pulp production resulted in the sale of Jari in 1982, for a $ 720 million loss. Despite growth rates at Jari which were too low to produce a profit on original investment, another plantation is planned for the eastern Amazon which is 10 times the size of Jari. 相似文献
107.
108.
Jaidong Ko Nancy E. Brown Alexandra Navrotsky Charles T. Prewitt Tibor Gasparik 《Physics and Chemistry of Minerals》1989,16(8):727-733
The phase boundary between MnTiO3 I (ilmenite structure) and MnTiO3 II (lithium niobate structure) has been determined by analysis of quench products from reversal experiments in a cubic anvil
apparatus at 1073–1673 K and 43–75 kbar using mixtures of MnTiO3 I and II as starting materials. Tight brackets of the boundary give P(kbar)=121.2−0.045 T(K). Thermodynamic analysis of this boundary gives ΔHo=5300±1000 J·mol−1, ΔSo = 1.98 ±1J·K−1· mol−1. The enthalpy of transformation obtained directly by transposed-temperature-drop calorimetry is 8359 ±2575 J·mol−1. Possible topologies of the phase relations among the ilmenite, lithium niobate, and perovskite polymorphs are constrained
using the above data and the observed (reversible with hysteresis) transformation of II to III at 298 K and 20–30 kbar (Ross
et al. 1989). The observed II–III transition is likely to lie on a metastable extension of the II–III boundary into the ilmenite
field. However the reversed I–II boundary, with its negative dP/ dT does represent stable equilibrium between ilmenite and lithium niobate, as opposed to the lithium niobate being a quench
product of perovskite. We suggest a topology in which the perovskite occurs stably at low T and high P with a triple point (I, II, III) at or below 1073 K near 70 kbar. The I–II boundary would have a negative P-T slope while the II–III and I–III boundaries would be positive, implying that entropy decreases in the order lithium niobate,
ilmenite, perovskite. The inferred positive slope of the ilmenite-perovskite transition in MnTiO3 is different from the negative slopes in silicates and germanates. These thermochemical parameters are discussed in terms
of crystal structure and lattice vibrations. 相似文献
109.
110.
Guy R. Cochrane Brian T. R. Lewis Kevin J. McClain 《Pure and Applied Geophysics》1988,128(3-4):767-800
Seismic reflection and refraction data off Washington and Oregon are used to determine the style of sediment deformation and to infer the physical properties of accreted sediments on the lower slope. Onshore-offshore seismic refraction data off Washington are used to determine the location of the trench, or where the plate bending starts.We find that off Washington the subduction zone is characterized by a trench whose physiographic expression is buried under several kilometers of sediments and is tens of kilometers landward of the lower slope, which is accreting seaward as the result of the offscraping of sediments.Seismic reflection data support previous observations that offscraping occurs along seaward and landward dipping thrust faults. Refraction data indicate that a sediment package thrust up along a seaward dipping fault (off Washington) was not measurably changed in velocity with respect to a Cascadia basin section. However a package uplifted by thrusting along a landward dipping fault (off Oregon) did have increased velocity. It is suggested that the increased velocities off Oregon could be the result of erosion and exposure of more deeply buried and compacted sediments, rather than the result of dewatering due to tectonic stress. Off Washington the sensitivity of velocity to porosity and resolution of the seismic method does not preclude dewatering due to tectonic stress, but it does limit the degree of dewatering.In the deeper parts of the lower slope section off Washington and Oregon velocities as high as 3 to 4 km/sec are found. Heat flow data indicate that the temperatures in this high velocity regime are greater than 100°C. It is hypothesized that lithification related to clay diagenesis may be partly responsible for the high velocities, rather than simply compaction. It also appears that the high velocity sediments are subducted while the unlithified low velocity sediments are offscraped. 相似文献