全文获取类型
收费全文 | 248929篇 |
免费 | 5253篇 |
国内免费 | 3247篇 |
专业分类
测绘学 | 6759篇 |
大气科学 | 18782篇 |
地球物理 | 52131篇 |
地质学 | 85803篇 |
海洋学 | 20994篇 |
天文学 | 54199篇 |
综合类 | 976篇 |
自然地理 | 17785篇 |
出版年
2021年 | 2154篇 |
2020年 | 2562篇 |
2019年 | 2816篇 |
2018年 | 2987篇 |
2017年 | 2679篇 |
2016年 | 5407篇 |
2015年 | 4163篇 |
2014年 | 6871篇 |
2013年 | 14170篇 |
2012年 | 6261篇 |
2011年 | 7415篇 |
2010年 | 6461篇 |
2009年 | 9125篇 |
2008年 | 8003篇 |
2007年 | 7397篇 |
2006年 | 9602篇 |
2005年 | 7621篇 |
2004年 | 7526篇 |
2003年 | 7012篇 |
2002年 | 6642篇 |
2001年 | 5942篇 |
2000年 | 5906篇 |
1999年 | 4797篇 |
1998年 | 4850篇 |
1997年 | 4619篇 |
1996年 | 4194篇 |
1995年 | 4298篇 |
1994年 | 3980篇 |
1993年 | 3729篇 |
1992年 | 3478篇 |
1991年 | 3520篇 |
1990年 | 3619篇 |
1989年 | 3316篇 |
1988年 | 3156篇 |
1987年 | 3709篇 |
1986年 | 3246篇 |
1985年 | 4107篇 |
1984年 | 4617篇 |
1983年 | 4296篇 |
1982年 | 4214篇 |
1981年 | 3831篇 |
1980年 | 3584篇 |
1979年 | 3430篇 |
1978年 | 3428篇 |
1977年 | 3218篇 |
1976年 | 2964篇 |
1975年 | 2902篇 |
1974年 | 2866篇 |
1973年 | 3061篇 |
1972年 | 1992篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Colin P. R. McCarter Stephen D. Sebestyen Susan L. Eggert Randall K. Kolka Carl P. J. Mitchell 《水文研究》2020,34(26):5354-5369
Sustainable fuels legislation and volatility in energy prices have put additional pressures on the forestry sector to intensify the harvesting of biomass for “advanced biofuel” production. To better understand how residual biomass removal after harvest affects forest hydrology in relatively low slope terrain, a Before-After-Control-Impact (BACI) study was conducted in the USDA Forest Service's Marcell Experimental Forest, Minnesota, USA. Hydrological measurements were made from 2010–2013 on a forested hillslope that was divided into three treatment blocks, where one block was harvested and residual biomass removed (Biomass Removed), the second was harvested and residual biomass left (Biomass Left), and the last block was left as an Unharvested Control. The pre-harvest period (2 years) was 2010–11 and post-harvest (2 years) was 2012–13. Water table elevation at the upslope and downslope position, subsurface runoff, and soil moisture were measured between May–November. Mixed effect statistical models were used to compare both the before-after and “control” treatment ratios (ratios between harvested hillslopes and the Unharvested Control hillslope). Subsurface runoff significantly increased (p < .05) at both harvested hillslopes but to a greater degree on the Biomass Left hillslope. Greater subsurface runoff volumes at both harvested hillslopes were driven by substantial increases during fall, with additional significant increases during summer on the Biomass Left hillslope. The hydrological connectivity, inferred from event runoff ratios, increased due to harvesting at both hillslopes but only significantly on the Biomass Left hillslope. The winter harvest minimized soil disturbance, resulting in no change to the effective hydraulic conductivity distribution with depth. Thus, the observed hydrological changes were driven by increased effective precipitation and decreased evapotranspiration, increasing the duration that both harvested hillslopes were hydrologically active. The harvesting of residual biomass appears to lessen hydrological connectivity relative to leaving residual biomass on the hillslope, potentially decreasing downstream hydrological impacts of similar forestry operations. 相似文献
92.
Charles C. Rhoades Timothy S. Fegel Timothy P. Covino Kathleen A. Dwire Kelly Elder 《水文研究》2021,35(3):e14089
Springs are the point of origin for most headwater streams and are important regulators of their chemical composition. We analysed solute concentrations of water emerging from 57 springs within the 3 km2 Fool Creek catchment at the Fraser Experimental Forest and considered sources of spatial variation among them and their influence on the chemical composition of downstream water. On average, calcium and acid neutralizing capacity (bicarbonate-ANC) comprised 50 and 90% of the cation and anion charge respectively, in both spring and stream water. Variation in inorganic chemical composition among springs reflected distinct groundwater sources and catchment geology. Springs emerging through glacial deposits in the upper portion of the catchment were the most dilute and similar to snowmelt, whereas lower elevation springs were more concentrated in cations and ANC. Water emerging from a handful of springs in a geologically faulted portion of the catchment were more concentrated than all others and had a predominant effect on downstream ion concentrations. Chemical similarity indicated that these springs were linked along surface and subsurface flowpaths. This survey shows that springwater chemistry is influenced at nested spatial scales including broad geologic conditions, elevational and spatial attributes and isolated local features. Our results highlight the role of overlapping factors on solute export from headwater catchments. 相似文献
93.
Slyuta E. N. Vysochkin V. V. Ivanov V. V. Makovchuk V. Yu. Nazarov A. I. Pogonin V. I. Roskina E. A. Safronov V. V. Tatsiy L. P. 《Solar System Research》2021,55(5):437-445
Solar System Research - The ionization-type cosmic dust detector METEOR-L is being developed for the lunar orbiter Luna-26 and is designed to study the distribution of meteoric bodies in space by... 相似文献
94.
Various network model creation algorithms have been introduced to demonstrate a better approximation of the actual walking pattern and to provide a better wayfinding guide. However, it is under‐investigated which algorithm creates the most appropriate indoor navigation network model in the context of wayfinding applications. Due to the lack of discussion, some studies unconsciously extended an algorithm designed for creating an outdoor navigation network model to indoor space applications. This is problematic because indoor space has different spatial contexts from outdoor space, such as non‐linear space and no‐designated walking space. Our solution is to select five well‐known algorithms that have been introduced, to reproduce the algorithm for the automated construction of the indoor navigation network model, and to evaluate the applicability of algorithms for indoor wayfinding applications. This article compares the quality of wayfinding results from the output of the indoor navigation network model against two criteria: route efficiency (i.e., length) and route simplicity (i.e., number of directions). Our statistical analysis illustrates that the visibility graph algorithm is the most appropriate for indoor wayfinding applications. 相似文献
95.
Astronomy Letters - The sensitivity of future far-infrared telescopes, such as Millimetron, will be limited by the confusion noise produced by distant galaxies. We have constructed a model of the... 相似文献
96.
Sequence stratigraphy for clastic continental margins predicts the development of sand-rich turbidite deposits during specific times in relation to base-level cycles. It is now widely understood that deltas can extend to the shelf-edge forced by high sediment flux and/or base level, providing a direct connection to transfer sediment and sand to the slope and basin floor even during high base level periods. Herein, we build a stratigraphic forward model for the last 120 kyr of the fluvio-deltaic to deep-water Brazos system (USA) where sediment partitioning along an Icehouse continental margin can be evaluated. The reduced-complexity stratigraphic forward model employs geologically constrained input parameters and mass balance. The modelled architecture is consistent with the location of depositional units previously mapped in the shelf. Sand bypasses the shelf and upper slope between 35 to 15 kyr before present and only about 20%–30% of all the sediment and sand supplied to the system is transferred to deep water. Several scenarios based on the initial Brazos model investigate the relationships between base level and deep-water sand ratio (DWSR). DWSR is defined as the relative amount of sand transferred to the deep-water portions of the system subdivided by the total sand input to the model. Linear correlations between DWSR and base level change rates or base level are very poor. Short-term variability due to local processes (for example avulsions) is superimposed to the long-term trends and mask the base level signal. DWSR for an entire base-level cycle is mainly controlled by the proportion of time the delta stays docked at the shelf-edge. Stratigraphic forward models are useful to complement field observations and quantify how different processes control stratigraphy, which is important for making predictions in areas with limited information. 相似文献
97.
Bondur V. G. Vorobyev V. E. Murynin A. B. 《Izvestiya Atmospheric and Oceanic Physics》2020,56(9):887-897
Izvestiya, Atmospheric and Oceanic Physics - A method for retrieving sea wave spectra from space images of high spatial resolution has been developed for various conditions of wave generation based... 相似文献
98.
Garetsky R. G. Gribik Ya. G. Litvinovskaya R. P. Savchuk A. L. Mardosevich M. A. Khripach V. A. 《Doklady Earth Sciences》2020,494(1):688-690
Doklady Earth Sciences - The content of plant hormones (brassinosteroids) in oil samples from a number of oilfields in Belarus is determined using an enzyme immunoassay method. Brassinosteroids of... 相似文献
99.
Nemirovskaya I. A. Zavyalov P. O. Konovalov B. V. Khramtsova A. V. 《Doklady Earth Sciences》2020,492(1):387-391
Doklady Earth Sciences - The results of studying the content and composition of aliphatic hydrocarbons (AHCs) and polycyclic aromatic hydrocarbons (PAHs) in the suspended particulate matter of the... 相似文献
100.
Doklady Earth Sciences - Based on investigation of Proterozoic mafic dike swarms of the Siberian Craton, we inferred how the geochemical and isotopic characteristics of dike swarms of dolerites of... 相似文献