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301.
302.
Control of landscape diversity by catastrophic disturbance: A theory and a case study of fire in a Canadian boreal forest 总被引:3,自引:0,他引:3
A landscape may be envisioned as a space partitioned by a number of ecosystem types, and so it conforms to a neo-Clementsian model of succession. A corollary is that intermediate disturbance rates should maximize landscape (beta) diversity. This was confirmed using eight boreal forest landscapes in northwestern Ontario, Canada, where intermediate rates of forest fire were associated with highest landscape diversity. Because current measures of evenness subsume a richness measure, it is not, as yet, feasible to assess the relative contributions of evenness and richness to biological diversity, and thus it was not possible to determine the roles of numbers of habitat types and relative amounts of habitat types in the above situation. Both theory and observations suggest that forest fire control in fire-prone landscapes increases landscape diversity, but that it is lowered by fire control in landscapes of intermediate to low diversity. 相似文献
303.
Catherine A. Jamieson John C. Clausen 《Journal of the American Water Resources Association》1988,24(6):1219-1226
ABSTRACT: The computer model, CREAMS, has been developed for field-sized agricultural areas to aid in best management practices evaluation and planning. A test of CREAMS was performed by comparing monthly observed and simulated values for runoff, sediment, and phosphorus exports from two agricultural fields in Vermont to determine the applicability of the model in cold climates. Water quality samples were collected from field runoff and analyzed for both total suspended solids and total phosphorus. Generally, exports were overestimated during low flow months and underestimated during high flow months. Significant r2values (p <0.05), ranging from 0.78 to 0.90, between simulated and observed data were found for all comparisons except for sediment export from one field. Comparisons of the slopes of the regressions between observed and simulated values and the ideal slope of one using t-tests revealed significant differences between simulated and observed monthly runoff, sediment, and phosphorus exports. It is postulated that this lack of adequate prediction could be attributed to the use of average monthly, instead of daily, temperature and solar radiation in calculations of evapotranspiration and snowmelt, and the use of static parameter values for parameters that vary seasonally. 相似文献
304.
Correlational studies have identified worksite coping resources such as social support and perceived control, and have suggested a positive role for such resources in employee stress processes. However, little experimental evidence has demonstrated the causal role of worksite coping resources in improving mental health, nor how worksite coping resources can be enhanced. This paper reports the results of a field experiment intended to provide such evidence. Human service workers participated in a theory-driven training program designed to increase individual and group psychosocial coping resources and individuals' abilities to use those resources when coping with job demands. The inclusion of selection variables in models used to estimate the impact of the intervention controlled for selection biases and also allowed for the assessment of the impact of the training on those workers identified as most prone to turnover and on those most likely to participate in such an intervention. Results indicated that the program enhanced the work team climate and reduced depresssive symptoms and somatization in those most at risk for leaving their jobs. The program was also effective in increasing the amount of supervisor support received on the job and strengthening perceptions of coping abilities in those workers most likely to participate in the program. 相似文献
305.
306.
Natural attenuation of contaminated soils 总被引:5,自引:0,他引:5
Natural attenuation is increasing in use as a low cost means of remediating contaminated soil and groundwater. Modelling of contaminant migration plays a key role in evaluating natural attenuation as a remediation option and in ensuring that there will be no adverse impact on humans and the environment. During natural attenuation, the contamination must be characterized thoroughly and monitored through the process. In this paper, attenuation mechanisms for both organic and inorganic contaminants, use of models and protocols, role of monitoring and field case studies will be reviewed. 相似文献
307.
We assessed the relationship between the quality (i.e., job characteristics, role stressors, and interpersonal relationships) and quantity of part-time employment, and work-related attitudes and aspirations among 349 students. Neither the linear nor the quadratic components of employment quantity predicted any of the outcome variables. Motivation to do good work was predicted both by role stressors and by family achievement orientation. Career maturity was predicted by role stressors at work, and cynicism by role stressors and satisfaction with interpersonal relationships. In contrast, job involvement was significantly related to family achievement orientation but not to any perceived job factors. These findings question the assumption that only intensive work during high school exerts negative effects for teenagers, and support the importance of employment quality for teenagers as well as adults. © 1998 John Wiley & Sons, Ltd. 相似文献
308.
Catherine Chamberlin Marilyn ten Brink Kate Munson Alyssa Le Naomi Detenbeck 《Journal of the American Water Resources Association》2023,59(1):178-196
Excess loading of nitrogen and phosphorus to river networks causes environmental harm, but reducing loads from large river basins is difficult and expensive. We developed a new tool, the River Basin Export Reduction Optimization Support Tool (RBEROST) to identify the least-cost combinations of management practices that will reduce nutrient loading to target levels in downstream and mid-network waterbodies. We demonstrate the utility of the tool in a case study in the Upper Connecticut River Basin in New England, USA. The total project cost of optimized lowest-cost plans ranged from $18.0 million to $41.0 million per year over 15 years depending on user specifications. Plans include both point source and non-point source management practices, and most costs are associated with urban stormwater practices. Adding a 2% margin of safety to loading targets improved the estimated probability of success from 37.5% to 99%. The large spatial scale of RBEROST, and the consideration of both point and non-point source contributions of nutrients, make it well suited as an initial screening tool in watershed planning. 相似文献
309.
Thirty-two brownfield sites from the city of Wolverhampton were selected from those with a former industrial use, wasteland
or areas adjacent to industrial processes. Samples (<2 mm powdered soil fraction) were analysed, using inductively coupled
plasma–atomic emission spectrometry (ICP–AES) for 20 elements. Loss on ignition and pH were also determined. A five-step chemical
sequential extraction technique was carried out. Single leach extraction with 0.12 M hydrochloric acid of Pb, Cu and Zn in
soil was determined as a first approximation of the bioavailability in the human stomach. Some of the sites were found to
have high concentrations of the potentially toxic elements Pb, Zn, Cu and Ni. The partitioning of metals showed a high variability,
however a number of trends were determined. The majority of Zn was partitioned into the least chemically stable phases (steps
1, 2 and 3). The majority of Cu was associated with the organic phase (step 4) and the majority of Ni was fractionated into
the residue phase (step 5). The majority of Pb was associated with the residue fraction (step 5) followed by Fe–Mn oxide fraction
(step 3). The variability reflects the heterogeneous and complex nature of metal speciation in urban soils with varied historic
histories. There was a strong inverse linear relationship between the metals Ni, Zn and Pb in the readily exchangeable phase
(step 1) and soil pH, significant at P < 0.01 level. There was a significant increase (P < 0.05) in the partitioning of Cu, Ni and Zn into step 4 (the organic phase) in soils with a higher organic carbon content
(estimated by loss on ignition). Copper was highly partitioned into step 4 as it has a strong association with organics in
soil but this phase was not important for the partitioning of Ni or Zn. The fractionation of Ni, Cu and Zn increased significantly
in step 3 when the total metal concentration increases (P < 0.01). The Fe–Mn oxide fraction becomes more important in soils elevated in these metals, possibly due to the scavenging
of metals by oxides. Cu and Pb extracted by HCl was statistically similar to the sum of the metals in steps 1 to 4 (P < 0.01) and HCl available Zn was statistically similar to the sum of Zn in steps 1 to 3 (P < 0.01). Step 4 (the organic phase) was not an important phase for Zn, so it was concluded that any Cu, Zn and Pb present
in soil in a nonresidue phase would be potentially available for uptake into the human system once soil has been ingested. 相似文献
310.
Recency preference of odour memory retrieval in honeybees 总被引:1,自引:0,他引:1
Free-flying honeybees (Apis mellifera) were trained on tasks in which they had to choose one of three odours for a reward of sugar water. In acquisition, the bees
learned this task in about five trials of training. Unrewarded retention tests showed that the odour memory was retained after
24-h delay. These findings are unsurprising. Integration experiments were then performed in which the bees had to learn two
successive tasks of odour discrimination with conflicting demands. In task 1 (20 trials), one of three odours provided sugar
water while the other two provided tap water. In task 2 (ten trials), which followed task 1 immediately, a different odour
provided the reward. The bees were given unrewarded tests immediately after training on task 2 and then tested again after
10 min, 22 h or 24 h. The 22-h delay coincided with the circadian time for the start of task 1 training, while the 24 h coincided
with the circadian time for the end of the task 2 training. Bees strongly preferred the rewarded odour for task 2 on immediate
testing and after a 10-min delay. After delays of 22 and 24 h, they still preferred the rewarded odour for task 2. We conclude
that the most recently acquired odour memory dominates behaviour in honeybees. The close association between floral odour
and reward availability under natural circumstances may predispose honeybees to rely more on the most recently rewarded odour
cue rather than on circadian time. 相似文献