访问控制是应用系统中的重要问题之一。传统的基于角色的访问控制(RBAC)方案需要预先定义和同步用户-角色赋值关系,这会带来管理成本和同步开销,并且限制了应用系统的灵活性和动态性。文章提出一种基于策略的动态角色分配模型(Policy-ba...访问控制是应用系统中的重要问题之一。传统的基于角色的访问控制(RBAC)方案需要预先定义和同步用户-角色赋值关系,这会带来管理成本和同步开销,并且限制了应用系统的灵活性和动态性。文章提出一种基于策略的动态角色分配模型(Policy-based Dynamic Role Assignment Model——PDRA),它无需同步用户就可以自定义角色,并通过策略匹配的方式实现动态分配。模型完全兼容RBAC,可以成为RBAC良好的扩展机制。文章给出了模型的定义和算法,评估了模型的性能,并在华东师范大学的数据治理平台中进行了应用,验证了该方案的可行性和有效性。展开更多
Understanding the factors that drive variation in species distribution is a central theme of ecological research. Although several studies focused on alpine vegetation, few efforts have been made to identify the envir...Understanding the factors that drive variation in species distribution is a central theme of ecological research. Although several studies focused on alpine vegetation, few efforts have been made to identify the environmental factors that are responsible for the variations in species composition and richness of alpine shrublands using numericalmethods. In the present study, we investigated vegetation and associated environmental variables from 45 sample plots in the middle Qilian Mountains of the northwestern China to classify different community types and to elucidate the speciesenvironment relationships. We also estimated the relative contributions of topography and site conditions to spatial distribution patterns of the shrub communities using the variation partitioning. The results showed that four shrub community types were identified and striking differences in floristic composition were found among them. Species composition greatly depended on elevation, slope,shrub cover, soil p H and organic carbon. The important determinants of species richness were soil bulk density and slope. No significant differences in species richness were detected among the community types. Topography and site conditions had almost equal effects on compositional variation. Nonetheless,a large amount of the variation in species composition remained unexplained.展开更多
Hydrogen and oxygen isotopes in precipitation have been widely used as effective traces to investigate hydrological processes such as evaporation and atmospheric moisture source. This study analyzed δD and δ^(18)O o...Hydrogen and oxygen isotopes in precipitation have been widely used as effective traces to investigate hydrological processes such as evaporation and atmospheric moisture source. This study analyzed δD and δ^(18)O of precipitation in continuous event-based samples at three stations of Pailugou Catchment from November 2012 to December 2013. The δ^(18)O and δD values ranged from-32.32‰ to +3.23‰ and from-254.46‰ to +12.11‰, respectively. Results show that the δ^(18)O displayed a distinct seasonal variation, with enriched values occurring in summer and relatively depleted values in winter, respectively. There was a statistically significant positive correlation between the δ^(18)O and δD values and local surface air temperature at all the three stations. The nearest Global Network of Isotopes in Precipitation(GNIP) station(Zhangye), compared to the Meteoric Water Lines for this study, showed the obvious local evaporation effects with lower intercept and slope. Additionally, d-excess(δD- 8δ^(18)O) parameter in precipitation exhibited an anti-phase seasonal variability with the δ^(18)O. The 96-h back trajectories for each precipitation event using Hybrid Single Particle Lagrangian Integrated Trajectory(HYSPLIT) model indicated a dominant effect of westerly air masses in summer and the integrated influence of westerly and polar air masses in winter.展开更多
文摘访问控制是应用系统中的重要问题之一。传统的基于角色的访问控制(RBAC)方案需要预先定义和同步用户-角色赋值关系,这会带来管理成本和同步开销,并且限制了应用系统的灵活性和动态性。文章提出一种基于策略的动态角色分配模型(Policy-based Dynamic Role Assignment Model——PDRA),它无需同步用户就可以自定义角色,并通过策略匹配的方式实现动态分配。模型完全兼容RBAC,可以成为RBAC良好的扩展机制。文章给出了模型的定义和算法,评估了模型的性能,并在华东师范大学的数据治理平台中进行了应用,验证了该方案的可行性和有效性。
基金supported by the National Natural Science Foundation of China (Grant No. 91025002)
文摘Understanding the factors that drive variation in species distribution is a central theme of ecological research. Although several studies focused on alpine vegetation, few efforts have been made to identify the environmental factors that are responsible for the variations in species composition and richness of alpine shrublands using numericalmethods. In the present study, we investigated vegetation and associated environmental variables from 45 sample plots in the middle Qilian Mountains of the northwestern China to classify different community types and to elucidate the speciesenvironment relationships. We also estimated the relative contributions of topography and site conditions to spatial distribution patterns of the shrub communities using the variation partitioning. The results showed that four shrub community types were identified and striking differences in floristic composition were found among them. Species composition greatly depended on elevation, slope,shrub cover, soil p H and organic carbon. The important determinants of species richness were soil bulk density and slope. No significant differences in species richness were detected among the community types. Topography and site conditions had almost equal effects on compositional variation. Nonetheless,a large amount of the variation in species composition remained unexplained.
基金Under the auspices of National Natural Science Foundation of China(No.41501085,41461003)Postdoctoral Science Foundation of China(No.2013M532094)
文摘Hydrogen and oxygen isotopes in precipitation have been widely used as effective traces to investigate hydrological processes such as evaporation and atmospheric moisture source. This study analyzed δD and δ^(18)O of precipitation in continuous event-based samples at three stations of Pailugou Catchment from November 2012 to December 2013. The δ^(18)O and δD values ranged from-32.32‰ to +3.23‰ and from-254.46‰ to +12.11‰, respectively. Results show that the δ^(18)O displayed a distinct seasonal variation, with enriched values occurring in summer and relatively depleted values in winter, respectively. There was a statistically significant positive correlation between the δ^(18)O and δD values and local surface air temperature at all the three stations. The nearest Global Network of Isotopes in Precipitation(GNIP) station(Zhangye), compared to the Meteoric Water Lines for this study, showed the obvious local evaporation effects with lower intercept and slope. Additionally, d-excess(δD- 8δ^(18)O) parameter in precipitation exhibited an anti-phase seasonal variability with the δ^(18)O. The 96-h back trajectories for each precipitation event using Hybrid Single Particle Lagrangian Integrated Trajectory(HYSPLIT) model indicated a dominant effect of westerly air masses in summer and the integrated influence of westerly and polar air masses in winter.
