首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   846篇
  免费   79篇
  国内免费   30篇
安全科学   13篇
废物处理   9篇
环保管理   738篇
综合类   103篇
基础理论   49篇
污染及防治   5篇
评价与监测   22篇
社会与环境   10篇
灾害及防治   6篇
  2023年   15篇
  2021年   8篇
  2020年   7篇
  2019年   11篇
  2018年   19篇
  2017年   33篇
  2016年   20篇
  2015年   18篇
  2014年   17篇
  2013年   30篇
  2012年   27篇
  2011年   29篇
  2010年   32篇
  2009年   24篇
  2008年   21篇
  2007年   37篇
  2006年   26篇
  2005年   32篇
  2004年   45篇
  2003年   44篇
  2002年   34篇
  2001年   31篇
  2000年   32篇
  1999年   37篇
  1998年   36篇
  1997年   25篇
  1996年   43篇
  1995年   20篇
  1994年   19篇
  1993年   15篇
  1992年   14篇
  1991年   18篇
  1990年   9篇
  1989年   6篇
  1988年   7篇
  1987年   14篇
  1985年   8篇
  1983年   4篇
  1982年   12篇
  1981年   6篇
  1980年   15篇
  1979年   6篇
  1978年   7篇
  1977年   2篇
  1976年   4篇
  1975年   8篇
  1974年   2篇
  1973年   5篇
  1972年   6篇
  1971年   8篇
排序方式: 共有955条查询结果,搜索用时 156 毫秒
911.
ABSTRACT: This paper uses the varying perspectives of researchers, funders, users or purchasers, and the wider community of stakeholders who have an interest in the outcome of research, to describe the socio-economic and organizational context in which forest hydrology research, as a particular form of environmental science, is carried out. The kinds of products from research, and the outcomes from research desired by these different parties are discussed. Implications for research organizations, their research strategies, and for the researchers within those organizations are discussed using the Porter-Harvard model of the context for the competitive strategies of enterprises. Some possible implications for research funding, research management, career paths for researchers, and the initial and lifetime education/learning needs of researchers and their managers are suggested. These considerations and implications are then related to the kinds of knowledge, skills, experience, and behaviors which are likely to be needed for a 21st century career in environmental research.  相似文献   
912.
ABSTRACT: The U.S. Department of Agriculture Curve Number (CN) method is one of the most common and widely used techniques for estimating surface runoff and has been incorporated into a number of popular hydrologic models. The CN method has traditionally been applied using compositing techniques in which the area weighted average of all curve numbers is calculated for a watershed or a small number of sub-watersheds. CN compositing was originally developed as a time saving procedure, reducing the number of runoff calculations required. However, with the proliferation of high speed computers and geographic information systems, it is now feasible to use distributed CNs when applying the CN method. To determine the effect of using composited versus distributed CNs on runoff estimates, two simulations of idealized watersheds were developed to compare runoff depths using composite and distributed CNs. The results of these simulations were compared to the results of similar analyses performed on an urbanizing watershed located in central Indiana and show that runoff depth estimates using distributed CNs are as much as 100 percent higher than when composited CNs are used. Underestimation of runoff due to CN compositing is a result of the curvilinear relationship between CN and runoff depth and is most severe for wide CN ranges, low CN values, and low precipitation depths. For larger design storms, however, the difference in runoff computed using composite and distributed CNs is minimal.  相似文献   
913.
ABSTRACT: While much is known about the hydrology of forested mountain catchments in the Pacific Northwest, important research questions remain. For example, the dynamics of storm precipitation amounts and the modeling of catchment outflows represent a continuing research need. Without an improved understanding of the spatial and temporal aspects of storm precipitation patterns, our ability to evaluate and improve physically-based hydrologic models is limited. From a practical perspective, tens of thousands of kilometers of access roads have been constructed across forested catchments of the Pacific Northwest. Yet, few forestry research programs focus on road drainage (e.g., ditches, culverts, fords). The few studies that address this issue indicate road drainage systems need to function effectively over a wide range of flow events and terrain conditions. In addition, historical forest practices associated with hillslopes and riparian systems have altered the character of many Pacific Northwest aquatic ecosystems. If restoration of these systems is to be effective, research efforts are needed to better understand the linkages between riparian forests, geomorphic processes, and hydrologic disturbance regimes.  相似文献   
914.
荒漠植物红砂研究进展   总被引:2,自引:0,他引:2  
主要综述了荒漠植物红砂的分布区及其生境、解剖学特征、种子萌发及幼苗生长影响因素、组织培养、生理生态特征、繁殖及育苗、分子生物学特性等方面的研究。简要阐述了修复和重建红砂植被的必要性,并着重研究了红砂种子萌发的影响因素及育苗技术的发展现状,在此基础上指出红砂研究中的不足之处,并对今后的研究方向提出了建议。  相似文献   
915.
Hao, Yonghong, Jiaojuan Zhao, Huamin Li, Bibo Cao, Zhongtang Li, and Tian‐Chyi J. Yeh, 2012. Karst Hydrological Processes and Grey System Model. Journal of the American Water Resources Association (JAWRA) 48(4): 656‐666. DOI: 10.1111/j.1752‐1688.2012.00640.x Abstract: The karst hydrological processes are the response of karst groundwater system to precipitation. This study provided a concept model of karst hydrological processes. The hydraulic response time of spring discharge to precipitation includes the time that precipitation penetrates through the vadose zone, and the subsequent groundwater pressure wave propagates to a spring outlet. Due to heterogeneities in karst aquifers, the hydraulic response time is different in different areas. By using grey system theory, we proposed a karst hydrological model that offers a calculation of hydraulic response time, and a response model of spring discharge to precipitation. Then, we applied the models to the Liulin Springs Basin, China. In the south part of the Liulin Springs Basin, where large fields of carbonate rocks outcrop with intensive karstification, the hydraulic response time is one year. In the north, where the Ordovician karst aquifer is covered by Quaternary loess sediments, the response time is seven years. The grey system GM(1,3) response model of spring discharge to precipitation was applied in consideration of the hydraulic response time. The model calibration showed that the average error was 6.55%, and validation showed that the average error was 12.19%.  相似文献   
916.
Abstract: Snowmelt largely affects runoff in watersheds in Nordic countries. Neural networks (NN) are particularly attractive for streamflow forecasting whereas they rely at least on daily streamflow and precipitation observations. The selection of pertinent model inputs is a major concern in NNs implementation. This study investigates performance of auxiliary NN inputs that allow short‐term streamflow forecasting without resorting to a deterministic snowmelt routine. A case study is presented for the Rivière des Anglais watershed (700 km2) located in Southern Québec, Canada. Streamflow (Q), precipitations (rain R and snow S, or total P), temperature (T) and snow lying (A) observations, combined with climatic and snowmelt proxy data, including snowmelt flow (QSM) obtained from a deterministic model, were tested. NN implemented with antecedent Q and R produced the largest gains in performance. Introducing increments of A and T to the NNs further improved the performance. Long‐term averages, seasonal data, and QSM failed to improve the networks.  相似文献   
917.
ABSTRACT: This study evaluated the use of a modified pebble count procedure (Bevenger and King, 1995) to detect cumulative watershed effects on headwater streams in the Routt National Forest in northcentral Colorado. The 42 sample reaches were stratified by disturbance (reference or disturbed) and geologic terrene (granitic or mixed sedimentary-volcanic). Water surface slope was a significant control on the number of fine particles in the reference reaches in both terranes, and the data from the disturbed reaches were adjusted accordingly. The disturbed reaches in the granitic terrene generally had a higher percentage of fine particles, and the adjusted number of fine particles was significantly correlated with the number of road crossings. Disturbed reaches in the sedimentary-volcanic (s-v) terrane generally did not have significantly more fine particles, nor were the adjusted numbers of fine particles significantly correlated with any management index. The lack of significant trends in the s-v streams is probably due to differences in weathering between the two rock types, and the location of the sample reach relative to sedimentary outcrops. Two other procedures were also used to assess cumulative watershed effects, with the Pfankuch channel stability rating yielding stronger and more consistent differences between the reference and the disturbed streams than the Tarzwell substrate ratio. We conclude that it may be difficult to define a standard reference condition, and that the number of road crossings is more strongly correlated with the number of fine particles than equivalent clearcut area.  相似文献   
918.
ABSTRACT: Few water budgets exist for specific types of wetlands such as peatlands, even though such information provides the basis from which to investigate linkages between wetlands and upland ecosystems. In this study, we first determined the water budget and then estimated nutrient loading from an upland farm field into a 1.5 ha, kettle-block peatland. The wetland contains highly anisotropic peat and has no distinct, active layer of groundwater flow. We estimated the depth of the active layer using Fick's law of diffusion and quantified groundwater flow using a chemical mass balance model. Evapotranspiration was determined using MORECS, a semi-physical model based on the Penman-Monteith approach. Precipitation and surface outflow were measured using physical means. Groundwater provided the major inflow, 84 percent (44,418 m3) in 1993 and 88 percent (68,311 m3) in 1994. Surface outflow represented 54 percent (28,763 m3) of total outflows in 1993 and 48 percent (37,078 m3) in 1994. A comparison of several published water budgets for wetlands and lakes showed that error estimates for hydrologic components in this study are well within the range of error estimates calculated in other studies. Groundwater inflow estimates and nutrient concentrations of three springs were used to estimate agricultural nutrient loading to the site. During the study period, nutrient loading into the peatland via groundwater discharge averaged 24.74 kg K ha-1, 1.83 kg total inorganic P had, and 21.81 kg NO3-N ha-1.  相似文献   
919.
A ‘forest–hydrology–poverty nexus’ hypothesis asserts that deforestation in poor upland areas simultaneously threatens biodiversity and increases the incidence of flooding, sedimentation and other damaging hydrological processes. This paper uses rough heuristics to assess the applicability of this hypothesis to two montane forested countries in Central America: Guatemala and Honduras. We do so by using simple rules of thumb to identify watersheds at greater risk of hydrologically significant land use change, using information about land cover, slope, and watershed size. The location of these watersheds is compared to spatial maps of poverty and forests. We find plausible evidence for a forest–biodiversity–poverty connection in Guatemala, and to a lesser extent in Honduras.  相似文献   
920.
ABSTRACT: Usability assessments were used to obtain feedback on the development of a flood forecasting decision support system. The feedback was used to guide design of system functionality, interface, training, implementation, and operations. The usability process was user focused and was dependent upon implementation of a prototype system in an operational setting. This paper describes concepts and methods applied to collect reflective and objective data on DSS components and information outputs. The general structure of the usability assessments is discussed and results of assessments are summarized.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号