全文获取类型
收费全文 | 360篇 |
免费 | 29篇 |
国内免费 | 18篇 |
专业分类
林业 | 30篇 |
农学 | 7篇 |
基础科学 | 17篇 |
177篇 | |
综合类 | 84篇 |
农作物 | 6篇 |
水产渔业 | 49篇 |
畜牧兽医 | 5篇 |
园艺 | 4篇 |
植物保护 | 28篇 |
出版年
2024年 | 2篇 |
2023年 | 7篇 |
2022年 | 7篇 |
2021年 | 11篇 |
2020年 | 7篇 |
2019年 | 16篇 |
2018年 | 10篇 |
2017年 | 19篇 |
2016年 | 17篇 |
2015年 | 12篇 |
2014年 | 25篇 |
2013年 | 25篇 |
2012年 | 27篇 |
2011年 | 27篇 |
2010年 | 25篇 |
2009年 | 23篇 |
2008年 | 19篇 |
2007年 | 21篇 |
2006年 | 15篇 |
2005年 | 16篇 |
2004年 | 10篇 |
2003年 | 10篇 |
2002年 | 9篇 |
2001年 | 11篇 |
2000年 | 8篇 |
1999年 | 7篇 |
1998年 | 4篇 |
1997年 | 4篇 |
1996年 | 1篇 |
1995年 | 7篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1991年 | 1篇 |
排序方式: 共有407条查询结果,搜索用时 0 毫秒
281.
282.
复合坡度下雨水高效集蓄利用模式研究 总被引:1,自引:0,他引:1
通过复合坡面人工模拟降雨径流冲刷试验,研究了2种雨强下4种下垫面模式的坡面径流调控效应,从产流时间、侵蚀量、径流量以及泥沙含量等方面分析了径流调控的机理。结果表明:帕特草+PAM是一种理想的措施,其坡面径流量和侵蚀量均最小,较裸地对照(CK)分别提高了93.02%和98.13%,其次是帕特草+地孔,PAM+地孔;单纯在裸地上打地孔的坡面径流调控效果较裸地对照效果不明显。坡度对不同下垫面复合措施的影响甚微,而降雨强度对其影响却极为显著。坡面雨水的集蓄利用复合措施与水土保持综合治理模式改变了坡面的小地形状况,增加了植物(作物)覆盖度,加大了降水的入渗速率和数量,对黄土高原的水资源有效利用具有理论指导作用。 相似文献
283.
284.
《Land Degradation u0026amp; Development》2017,28(4):1386-1397
Planning effective landscape interventions is an important tool to fight against land degradation and requires knowledge on spatial distribution of runoff. The objective of this paper was to test models that predict temporal and spatial distribution of runoff. The selected models were parameter‐efficient semi‐distributed watershed model (PED‐WM), Hydrologiska Byrans Vattenbalansavdelning integrated hydrological modeling system (HBV‐IHMS), and Soil and Water Assessment Tool (SWAT). We choose 7‐km2 Awramba watershed in the Lake Tana basin with detailed hydrological information for testing these models. Discharge at the outlet, rainfall, and distributed information on infiltration rates, water table, and extent of the saturated area were collected from 2013 to 2015. The maximum saturated area was 6·5% of the watershed. Infiltration rates exceeded rainfall intensities 91% of the time. Hence, saturation excess runoff was the main runoff mechanism. Models were calibrated for the rainy seasons in 2013 and 2014 and validated for 2015. For daily flow validation, the PED‐WM model (Nash–Sutcliff efficiency, NSE = 0·61) outperformed HBV‐IHMS (NSE = 0·51) and SWAT (NSE = 0·48). Performance on monthly time step was similar. Difference in model behavior depended on runoff mechanism. In PED‐WM, saturation excess is the main direct runoff process and could predict the maximum extent of the saturated area closely at 6·9%. HBV‐IHMS model runoff simulation depended on soil moisture status and evapotranspiration, and hence was able to simulate saturation excess flow but not the extent of the saturated area. SWAT, where infiltration excess is the main runoff mechanism, could only predict the monthly discharges well. This study shows that prevailing runoff mechanisms and distribution of runoff source areas should be used for proper model selection. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
285.
喀斯特石漠化小流域土壤渗滤水化学特征及影响因素研究 总被引:1,自引:0,他引:1
对贵州省清镇王家寨喀斯特小流域9个样地的土壤渗滤水进行了系统监测。结果表明,随着石漠化退化程度的加剧,黑色石灰土地区土壤渗滤水中的HCO3-,Ca2+,方解石饱和指数(SIC)和黄壤地区土壤渗滤水中的Mg2+离子浓度均呈升高趋势。初步认为,植被和土壤的双重退化是控制石漠化发育过程中渗滤水化学组成的主要原因,当植被系统的退化先于土壤系统时,水—土反应强度的影响更大,渗滤水化学组成表现为HCO3-和Ca2+含量随石漠化的发育而减少;当土壤系统的退化先于植被系统时,水—土反应时间的影响更大,渗滤水的化学组成则表现为HCO3-和Ca2+含量随石漠化的发育而增加。 相似文献
286.
F. Soto-Varela M. L. RodrÍguez-Blanco M. T. Taboada-Castro J. L. Oropeza-Mota 《Communications in Soil Science and Plant Analysis》2015,46(4):332-338
Pollution assessment in a given area depends mostly on the precise determination of the so-called background values. In this work, enrichment factors (EFs) for iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) in suspended solids (SS) of a stream draining a rural catchment (NW Spain) were determined to assess whether the observed concentrations of these metals represent natural or contaminated levels. Aluminum was used as a normalizer element. Four approaches were used for defining background values used in the EF analyses: a local background value for metals concentrations in SS, a local background value for metals concentrations in C-horizon soils, mean crust values, and mean shale values. The most satisfactory result was obtained when using local background values for metals concentrations of SS, suggesting that on average the Fe, Mn, Cu, and Zn concentrations measured represent natural levels. However, some signs of enrichment were observed (16–28 percent of the samples with EF > 1.5). 相似文献
287.
Robert Mollenhauer Shannon K. Brewer Joshuah S. Perkin Dusty Swedberg Maeghen Wedgeworth Zachary D. Steffensmeier 《水产资源保护:海洋与淡水生态系统》2021,31(11):3215-3227
- Dam construction threatens global aquatic biodiversity by fragmenting stream networks and altering flow regimes. The negative effects of dams are exacerbated by increased drought periods and associated water withdrawals, especially in semi-arid regions. Stream fishes are particularly threatened owing to their mobile nature and requirement for multiple habitats to complete their life cycles. An understanding of relationships with fragmentation and flow regimes, particularly as coarse-scale (e.g. catchment) constraints on species distributions, is essential for stream fish conservation strategies.
- Prairie chub (Macrhybopsis australis) is a small-bodied minnow (Cyprinidae) with poorly understood ecology endemic to the North American Great Plains. Suspected declines in abundance and extirpations have resulted in conservation interest for prairie chub at state and federal levels. Prairie chub is thought to share its reproductive strategy with pelagic-broadcast spawning minnows (pelagophils). Freshwater pelagic-broadcast spawning fishes have been disproportionately affected by fragmentation and streamflow alteration globally.
- Relationships of prairie chub occurrence with coarse-scale fragmentation and streamflow metrics were examined in the upper Red River catchment. Occurrence probability was modelled using existing survey data, while accounting for variable detection. The modelled relationships were used to project the distribution of prairie chub in both a wet and dry climatic period.
- The probability of prairie chub occurrence was essentially zero at sites with higher densities of upstream dams, but increased sharply with increases in flow magnitude, downstream open mainstem, and flood duration. The projected distribution of prairie chub was broader than indicated by naïve occurrence, but similar in both climatic periods. The occurrence relationships are consistent with the hypotheses of pelagic broadcast spawning and represent coarse-scale constraints that are useful for identifying areas of the stream network with higher potential for finer-scale prairie chub conservation and recovery efforts. In addition to informing pelagophil conservation, the relationships are also applicable to pelagic-broadcast spawning fishes in marine environments.
288.
Wondwossen Gebretsadik 《林业研究》2014,25(1):97-102
Removal of trees and shrubs from hillsides exposes a site to erosion that threatens soil aggregation and stability. The present study aimed at evaluating the performance of five indigenous tree species in rehabilitation of degraded hillsides of Kuriftu Lake Catchment and the role of water harvesting structures. Adaptability varied by tree species and water harvesting structures significantly augmented seedling es-tablishment for some tree species. Height ofAcacia abyssinicaplanted on steeper slopes (18%-27%) without infiltration pits was lower than for conspecifics planted with infiltration pits.Dodonaea angustifolia wasproved to be best adapted to the siteand showed no need for water harvesting regardless of planting position across the degraded hillside. Planting ofAcacia seyalshould be restricted to gentler slopes (0-17%) with infiltration pits: tree height declined significantly on steep slopes without infiltration pits.Olea africanaperformed better on gentle slopes with pits but also grew well on steeper slopes with pits.Euclea schim-periwasproved to be least effective of the species evaluated in this study. 相似文献
289.
Sarah M. Larocque Colin Lake Jonathan D. Midwood Vivian M. Nguyen Gabriel Blouin‐Demers Steven J. Cooke 《水产资源保护:海洋与淡水生态系统》2020,30(9):1783-1790
- Although it is sometimes difficult for researchers to ensure that their work is used by resource managers to make informed decisions, an example where this knowledge–action gap has been breached is in research published in Aquatic Conservation: Marine and Freshwater Ecosystems (AQC) – among other journals – that has assisted fisheries managers in identifying strategies for reducing freshwater turtle bycatch in commercial hoop net fisheries in Ontario, Canada.
- Research published in AQC has provided evidence towards a simple and effective method for preventing turtle bycatch mortality in hoop nets, which could be adopted by the fishers. Other research published in AQC evaluated the effect of bycatch mortality on the probability of persistence of turtle populations with population viability analyses, and outlined the need to minimize bycatch mortality to prevent local extirpation. Nine other papers have been published on freshwater turtle bycatch in Ontario, furthering our knowledge on this issue including seasonality and temperature effects on catches, other net modifications, post‐release effects and assisted recovery, and the perspectives of fishers.
- The research results were presented to local resource managers with further discussions involving industry and stakeholders to minimize turtle bycatch mortality. Over several years, researchers have provided information to resource managers; however, when an incident of high turtle mortality caught the public eye, the research was readily available and changes in regulations were quick to occur.
- Reasonably good communication among researchers, resource managers, industry, stakeholders, and the broader public allowed the rapid implementation of regulations to mitigate freshwater turtle bycatch mortality and bridged the knowledge–action gap between researchers and resource managers.
- Both articles published in AQC had practical conservation impacts and were influential in providing local resource managers with feasible solutions, and the impetus to change regulations. These impacts extended to other jurisdictions and their monitoring programmes, where methods to reduce turtle bycatch mortality were also implemented.
290.