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21.
Two sedimentary cores with pollen, charcoal and radiocarbon data are presented. These records document the Late‐glacial and Holocene dry forest vegetation, fire and environmental history of the southern Cauca Valley in Colombia (1020 m). Core Quilichao‐1 (640 cm; 3° 6′N, 76° 31′W) represents the periods of 13 150–7720 14C yr BP and, following a hiatus, from 2880 14C yr BP to modern. Core La Teta‐2 (250 cm; 3° 5′N, 76° 32′W) provides a continuous record from 8700 14C yr BP to modern. Around 13 150 14C yr BP core Quilichao‐1 shows an active Late‐glacial drainage system and presence of dry forest. From 11 465 to 10 520 14C yr BP dry forest consists mainly of Crotalaria, Moraceae/Urticaceae, Melastomataceae/Combretaceae, Piper and low stature trees, such as Acalypha, Alchornea, Cecropia and Celtis. At higher elevation Andean forest comprising Alnus, Hedyosmum, Quercus and Myrica was common. After 10 520 14C yr BP the floral composition of dry forest changed, with extensive open grass vegetation indicative of dry climatic conditions. This event may coincide with the change to cool and dry conditions in the second part of the El Abra stadial, an equivalent to the Younger Dryas. From 8850 14C yr BP the record from La Teta indicates dry climatic conditions relative to the present, these prevailing up to 2880 14C yr BP at Quilichao and to 2720 14C yr BP at La Teta. Severe dryness reached maxima at 7500 14C yr BP and 4300 14C yr BP, when dry forest reached maximum expansion. Dry forest was gradually replaced by grassy vegetation, reaching maximum expansion around 2300 14C yr BP. After 2300 14C yr BP grassy vegetation remains abundant. Presence of crop taxa (a.o. Zea mays), disturbance indicators (Cecropia) and an increase in charcoal point to the presence of pre‐Columbian people since 2300 14C yr BP. After 950 14C yr BP, expansion of secondary forest taxa may indicate depopulation and abandonment of previously cultivated land. After 400 14C yr BP, possibly related to the Spanish conquest, secondary forest expanded and charcoal concentrations increased, possibly indicating further reduction of cultivated land. During the past century, Heliotropium and Didymopanax became abundant in an increasingly degraded landscape. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
22.
Rainfall, throughfall, stemflow and spring were studied in a secondary forest during a wet season from April to August in
2006. Some of the chemicals in throughfall, stemflow and spring were increased in contrast with incident rainfall. Specifically,
Cl−, HCO3
−, Na+ and Ca2+ were leached negatively in throughfall, but K+ and Mg2+ were leached positively. In stemflow, Cl− and Na+ were leached negatively, the others were leached positively and their concentrations were higher than those in throughfall.
Total carbon, organic carbon and inorganic carbon in throughfall and stemflow were increased as rainfall went through the
secondary forest. The concentration of free CO2 in rainfall was lower than both, throughfall and stemflow; the relationship between total acidity and free CO2 was linear. pH of throughfall and stemflow , such as maximum, minimum and mean, were lower than that of rainfall and the
extent of pH in spring was changed minimally. We came to a conclusion that rainfall via the secondary forest can lead to further
erosion, accelerate the biogeochemical cycle in epikarst zone, enhance the effective state of alkali elements in the soil,
supply vegetation with more nutrients and advance vegetation’s growth and succession, which are reasonably sufficient to form
a stable karst ecosystem. 相似文献
23.
Alfred Ngomanda Katharina Neumann Astrid Schweizer Jean Maley 《Quaternary Research》2009,71(3):307-318
The third millennium BP crisis of the central African rainforest is not sufficiently understood. The low resolution of most pollen profiles and a large plateau of the calibration curve aggravate the exact dating of the event, and its causal climatic parameters are debated. We present a high-resolution pollen profile from the swamp site Nyabessan in the southern Cameroonian rainforest, covering the period 3100-2300 cal yr BP. Between 3100 and 2500 cal yr BP, the climate was favourable for a regional evergreen forest with Caesalpiniaceae and Lophira and a local Raphia swamp forest. Around 2500/2400 cal yr BP, a significant decrease of mature forest and swamp forest taxa and an increase of pioneers indicate that the rain forest was seriously disturbed and replaced by secondary formations. The dominance of Trema orientalis, a pioneer well adapted to seasonal desiccation, points to a much more accentuated seasonality after 2500 cal yr BP, which seems to be linked to a southwards shift of the ITCZ during the northern hemisphere winter months. We propose that the rain forest crisis between 2500 and 2200 cal BP created favourable conditions for farming and paved the way for a major expansion of Bantu speaking populations. 相似文献
24.
ABSTRACTThroughfall drop size distributions (DSDs) are important for plant–soil interactions. This is the first known study to quantify differences in throughfall DSDs with the presence and absence of foliage. Employing a disdrometer, three parameters solely representing throughfall drip were measured and calculated: maximum drop diameter (DMAX), median volume diameter of drops (D50DR) and relative volume percentage of drops (pDR). Beneath Liriodendron tulipifera L. in Maryland (USA), DMAX, D50DR and pDR were substantially larger when the canopy was unfoliated. In fact, the presence or absence of foliage was one of the primary factors affecting all three throughfall DSDs along with air temperature, according to the boosted regression tree analysis. Experimental results were attributed to differing physical properties of intercepted water between foliated and unfoliated periods and differential water behavior on leaves and bark. Future work should examine the effects of concentrated drip points on the development of throughfall-induced hot spots.
Editor M.C. Acreman; Associate editor F. Hattermann 相似文献
25.
《水文科学杂志》2012,57(15):1909-1917
ABSTRACTThis study was conducted to characterize the hysteresis of suspended sediment concentration (SSC) at different timescales for the Educational and Research Forest Watershed of Tarbiat Modares University, Iran. Hysteresis loops were developed between flow discharge and SSC using data for (i) the whole period, (ii) seasonal, (iii) monthly and (iv) storm-event timescales. The data were collected on an hourly basis at daily and storm-event scales from October 2007 to July 2008. The SSC hysteresis for all the events in the whole study period showed a clockwise pattern, with a flushing behaviour pointing to sudden changes in sediment sources. The results further reveal that the study watershed behaves differently from the viewpoint of hysteresis patterns, with different conditions of dilution and flushing between SSC and flow discharge at different timescales. These result from the complex varying effect of governing conditions on the watershed. 相似文献
26.
27.
Teresa Margarita González‐Martínez Guadalupe Williams‐Linera Friso Holwerda 《水文研究》2017,31(5):1174-1183
Stemflow (Sf) measurements in tropical rain and montane forests dominated by large trees rarely include the understory and small trees. In this study, contributions of lower (1‐ to 2‐m height) and upper (>2‐m height and <5‐cm diameter at breast height [DBH]) woody understory, small trees (5 < DBH < 10 cm), and canopy trees (>10‐cm DBH) to Sf per unit ground area (Sfa) of a Mexican lower montane cloud forest were quantified for 32 days with rainfall (P) during the 2014 wet season. Rainfall, stemflow yield (Sfy), vegetation height, density, and basal area were measured. Subsequently, stemflow funneling ratios (SFRs) were calculated, and three common methods to scale up Sfy from individual trees to the stand level (tree‐Sfy correlation, P‐Sfy correlation, and mean‐Sfy extrapolation) were used to calculate Sfa. Understory woody plants, small trees, and upper canopy trees represented 96%, 2%, and 2%, respectively, of the total density. Upper canopy trees had the lowest SFRs (1.6 ± 0.5 Standard Error (SE) on average), although the lower understory had the highest (36.1 ± 6.4). Small trees and upper understory presented similar SFRs (22.9 ± 5.4 and 20.2 ± 3.9, respectively). Different Sf scaling methods generally yielded similar results. Overall Sfa during the study period was 22.7 mm (4.5% of rainfall), to which the understory contributed 70.1% (15.9 mm), small trees 10.6% (2.4 mm), and upper canopy trees 19.3% (4.4 mm). Our results strongly suggest that for humid tropical forests with dense understory of woody plants and small trees, Sf of these groups should be measured to avoid an underestimation of overall Sf at the stand level. 相似文献
28.
29.
In 1988, four states in the northeastern USA commissioned a study to address land use changes in the Northern Forest, 26 million acres of temperate and boreal forest extending from Maine to eastern New York State. Against a backdrop of economic destabilization and concerns regarding social and ecological implications of a real estate boom, the sustained deliberative dialogue catalyzed by this study has come to rely heavily on the ambiguous concept of “working forest.” To clarify political and environmental dynamics in the region, we analyzed how people respond to and seek to capitalize on the interpretive flexibility of the term working forest. We combine an analysis of socio-political discourses of working forest based on a structured literature review with an assessment of local peoples’ definitions of working forest based on a survey conducted in a pair of contrasting New York State communities. The first study site represents an amenity-oriented community (i.e. a place where the forest supports a service economy including recreation and tourism) and the other study site represents a timber-dependent community. By linking data from community-level analysis to data derived from a general analysis of forest politics, we seek to develop a more robust perspective. By comparing discourses across differently structured communities, we investigate how local forest politics are mediated by local economic development processes. Our study empirically illustrates contested and geographically uneven processes of social construction of environment and rural development in a region confronting pressures of globalization. Results indicate that timber harvesting is a heavily privileged management objective, as a logic of ‘the forest that pays is the forest that stays’ dominates. Environmental politics in the region, and perhaps more generally, increasingly conforms to a form of pragmatism in which economic opportunities structure conservation planning and investment. 相似文献
30.
Effects of forest harvesting on the occurrence of landslides and debris flows in steep terrain of central Japan 总被引:2,自引:0,他引:2
Landslides and debris flows associated with forest harvesting can cause much destruction and the influence of the timing of harvesting on these mass wasting processes therefore needs to be assessed in order to protect aquatic ecosystems and develop improved strategies for disaster prevention. We examined the effects of forest harvesting on the frequency of landslides and debris flows in the Sanko catchment (central Japan) using nine aerial photo periods covering 1964 to 2003. These photographs showed a mosaic of different forest ages attributable to the rotational management in this area since 1912. Geology and slope gradient are rather uniformly distributed in the Sanko catchment, facilitating assessment of forest harvesting effects on mass wasting without complication of other factors. Trends of new landslides and debris flows correspond to changes in slope stability explained by root strength decay and recovery; the direct impact of clearcutting on landslide occurrence was greatest in forest stands that were clearcut 1 to 10 yr earlier with progressively lesser impacts continuing up to 25 yr after harvesting. Sediment supply rate from landslides in forests clearcut 1 to 10 yr earlier was about 10‐fold higher than in control sites. Total landslide volume in forest stands clearcut 0 to 25 yr earlier was 5·8 × 103 m3 km?2 compared with 1·3 × 103 m3 km?2 in clearcuts >25 yr, indicating a fourfold increase compared with control sites during the period when harvesting affected slope stability. Because landslide scars continue to produce sediment after initial failure, sediment supply from landslides continues for 45 yr in the Sanko catchment. To estimate the effect of forest harvesting and subsequent regeneration on the occurrence of mass wasting in other regions, changes in root strength caused by decay and recovery of roots should be investigated for various species and environmental conditions. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献