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981.
982.
Current human land use activities are altering many components of the river landscape, resulting in unstable channels. Instability may have serious negative consequences for water quality, aquatic and riparian habitat, and for river-related human infrastructure such as bridges and roads. Resource management agencies have developed rapid bioassessment surveys to help assess stability in a fast and cost-effective way. While this assessment can be done for a single site fairly rapidly, it is still time-consuming to apply over large watersheds and assessment activities must be prioritized. We constructed a system that employs commonly available map data as inputs to cost-sensitive variants of decision tree algorithms to predict the relative channel stability of different sites. In particular, we use bagged lazy option trees (LOTs) and bagged probability estimation trees (PETs) to identify all unstable channels while making the smallest number of errors of classifying stable channels as unstable, thereby minimizing cost and maximizing safety. We measured the performance of the classifiers using ROC curves and found that the PETs performed better than the LOTs in situations where the number of instances of the stable and unstable classes were relatively balanced, but the LOTs did better where unstable examples were relatively rare compared to stable, perhaps due to the LOTs’ ability to focus on individual examples. 相似文献
983.
S.?AlbonettiEmail author S.?Blasioli M.?Bugani C.?Lehaut-Burnouf S.?Augustine E.?Roncari F.?Trifirò 《Environmental Chemistry Letters》2003,1(3):197-200
The aim of the plesent investigation was to study the effect of SiO2 addition on the thermal deactivation of V2O5/WO3/TiO2 catalysts used for NOx pollution abatement. The results suggest that the degradation of the catalytic properties is strongly correlated to the structural ageing which is, in turn, mainly related to the anatase–rutile phase transformation and to the WO3 phase segregation. The addition of SiO2 strongly influences the temperature at which these phenomena occur. In fact, it was found that the introduction of this oxide stabilizes the material, retarding the collapse of surface area, and increases the temperature of the anatase to rutile phase transition. 相似文献
984.
The stability, mixing and effect of downstream control on axisymmetric turbulent buoyant jets discharging vertically into
shallow stagnant water is studied using 3D Reynolds-averaged Navier–Stokes equations (RANS) combined with a buoyancy-extended
k –ε model. The steady axisymmetric turbulent flow, temperature (or tracer concentration) and turbulence fields are computed using
the finite volume method on a high resolution grid. The numerical predictions demonstrate two generic flow patterns for different
turbulent heated jet discharges and environmental parameters (i) a stable buoyant discharge with the mixed fluid leaving the
vertical jet region in a surface warm water layer; and (ii) an unstable buoyant discharge with flow recirculation and re-entrainment
of heated water. A stratified counterflow region always appears in the far-field for both stable and unstable buoyant discharges.
Provided that the domain radius L exceeds about 6H, the near field interaction and hence discharge stability is governed chiefly by the jet momentum length scale to depth ratio
lM/H, regardless of downstream control. The near field jet stability criterion is determined to be lM/H = 3.5. A radial internal hydraulic jump always exists beyond the surface impingement region, with a 3- to 6-fold increase
in dilution across the jump compared with vertical buoyant jet mixing. The predicted stability category, velocity and temperature/concentration
fields are well-supported by experiments of all previous investigators. 相似文献
985.
Multiple mating by social insect queens is a common phenomenon despite likely imposing substantial costs on queens. Mating with several males could be adaptive if a more genetically diverse worker force is better able to always handle any task sufficiently well, leading to a higher colony homeostasis. If multiple-paternity colonies are more homeostatic, then I propose that they may constitute less stressful rearing environments for developing sexuals. The effective stress levels experienced by developing males and queens may, however, also depend on colony productivity and sex-ratio preferences. I tested these hypotheses in the ant Lasius niger by examining whether the fluctuating asymmetry, means and coefficients of variation of a set of phenotypic traits in males and new queens co-varied with the effective number of patrilines per colony, colony productivity or sex ratio.Little support was found that the level of intra-colonial genetic diversity affects the variation of phenotype in sexuals. In 1 out of 2 years, however, females from colonies with high effective patriline numbers were heavier relative to their head width than were females from colonies with few patrilines. Support was found for the hypothesis that colonies with more resources may invest more in individual sexuals, and tendencies suggested that sexuals may receive better treatment when they belong to the majority sex of their colony.Communicated by J. Heinze 相似文献
986.
草炭溶解态有机物质与Cu2+、Cd2+络合稳定性的研究 总被引:2,自引:0,他引:2
研究了由草炭提取的胡敏酸(HA)、富里酸(FA)、水溶解态有机物质(WSOM)的基团特征,分子量分布及其与Cu2+、Cd2+两种金属离子之间的络合稳定性。对HA、FA及WSOM的红外光谱研究显示三者的官能团组成相似,只是含量略有不同。固态13C-NMR对HA、FA及WSOM结构特征的描述以及水相凝胶色谱法对三种有机质样品的分子量及分子量分布的测定结果显示HA分子最大且含有大量的长链烷烃,而FA分子小得多,结构以碳水化合物及多糖为主,WSOM的分子量略低于FA,结构与FA相近。离子交换平衡法对三种有机质样品与Cu2+、Cd2+的络合配位数和稳定常数的测定结果显示两种介质条件下络合配位数(x)、络合稳定常数(logk)的次序均为:Cu2+>Cd2+;同一介质条件下,对不同有机质而言均满足:M-HA>M-FA>M-WSOM。 相似文献
987.
Assemblage stability in stream fishes: A review 总被引:6,自引:0,他引:6
We quantified the stability of nine stream fish assemblages by calculating coefficients of variation of population size for
assemblage members. Coefficients of variation were high and averaged over 96%; indicating that most assemblages were quite
variable. Coefficient of variation (CV) estimates were not significantly affected by: (1) years of study, (2) mean abundance,
(3) familial classification, or (4) mean interval between collections. We also detected minor regional differences in CVs.
The high variability exhibited by many stream fish assemblages suggests that it may be difficult to detect the effects of
anthropogenic disturbances using population data alone. Consequently, we urge managers to exercise caution in the evaluation
of the effects of these disturbances. More long-term studies of the ecological characteristics of undisturbed stream fish
assemblages are needed to provide a benchmark against which disturbed systems can be compared.
We suggest that CVs are a better estimator of population/assemblage stability, than either Kendall's W or the standard deviation
of the logarithms of numerical censuses. This conclusion is based on the following reasons. First, CVs scale population variation
by the mean and, hence, more accurately measure population variability. Second, this scaling permits the comparison of populations
with different mean abundances. Finally, the interpretation of CV values is less ambiguous than either of the aforementioned
metrics. 相似文献
988.
Thomas J. Myers Sherman Swanson 《Journal of the American Water Resources Association》1991,27(4):667-677
ABSTRACT: The quality of stream habitat varies for a variety of natural and anthropogenic reasons not identified by a condition index. However, many people use condition indices to indicate management needs or even direction. To better sort natural from livestock influences, stream types and levels of ungulate bank damage were regulated to estimates of aquatic habitat condition index and stream width parameters in a large existing stream inventory data base. Pool/riffle ratio, pool structure, stream bottom materials, soil stability, and vegetation type varied significantly with stream type. Pool/riffle ratio, soil and vegetation stability varied significantly with ungulate bank damage level. Soil and vegetation stability were highly cross-correlated. Riparian area width did not vary significantly with either stream type or ungulate bank damage. Variation among stream types indicates that riparian management and monitoring should be stream type and reach specific. 相似文献
989.
The choice of flood control techniques in Bangladesh is reviewed in the context of a case study on the impact of a small-scale
polder, the Dhaka-Narayanganj-Demra (DND) Project. The results of a questionnaire survey among the urban floodplain residents,
who have settled inside the polder and a control population from outside the polder, indicate that the project has achieved
significant flood alleviation but has also induced considerable environmental degradation due to stagnation of water within
the enclosed embankment. The project also experienced numerous problems of structural instability of embankments during major
flood events. An attempt is made to extrapolate some of these adverse environmental impacts of the polder to the compartmentalization
scheme, which has been proposed by a UNDP-sponsored Flood Policy Study as an innovative technique for floodplain management
in Bangladesh. The findings of the study, however, do not provide an adequate basis for assessing viability of the proposed
compartmentalization scheme. Nonetheless, the study illustrates the usefulness of questionnaire surveys among floodplain residents
for obtaining data for project evaluation. 相似文献
990.