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141.
142.
Environmental degradation, competition for resources, increasing food demands, and the integration of agriculture into the
international economy threaten the sustainability of many food production systems. Despite these concerns, the concept of
sustainable food production systems remains unclear, and recent attempts to appraise sustainability have been hampered by
conceptual inconsistencies and the absence of workable definitions. Six perspectives are shown to underpin the concept. Environmental
accounting identifies biophysical limits for agriculture. Sustained yield refers to output levels that can be maintained continuously.
Carrying capacity defines maximum population levels that can be supported in perpetuity. Production unit viability refers
to the capacity of primary producers to remain in agriculture. Product supply and security focuses on the adequacy of food
supplies. Equity is concerned with the spatial and temporal distribution of products dervied from resource use. Many studies
into sustainable agriculture cover more than one of these perspectives, indicating the concept is complex and embraces issues
relating to the biophysical, social, and economic environments. Clarification of the concept would facilitate the development
of frameworks and analytical systems for appraising the sustainability of food production systems.
LRRC Contribution No. 90–46. 相似文献
143.
G. N. Rao J. N. Beck H. E. Murray D. J. Nyman 《Journal of the American Water Resources Association》1991,27(1):47-58
ABSTRACT: Specific capacity data obtained from Well Construction reports which are available from USGS offices, can provide useful estimates of tranamissivity (T), and hydraulic conductivity (K), of an aquifer. The Chicot Aquifer in Louisiana is one of the largest sources of fresh ground water in North America. Hydrologic data collected for the Chicot Aquifer indicate that specific capacity tests can be used in estimating local and regional values for T and K, if the Cooper-Jacob equation for transient flow is used with proper corrections for well loss and partial penetration. Where full scale pumping test data are scarce, specific capacity test data that are adequately distributed spatially can be used to map changes in T and K values and can be summarized statistically to indicate applicable regional values. A computer program called “TGUESS” which is available from International Ground Water Modeling Center, Holcomb Research Institute, was used in this study. The contour maps for T and K values are prepared for different well depth intervals to avoid wide variation of values. 相似文献
144.
Under the United States Clean Air Act Amendments of 1977, a class I designation safeguards wilderness areas from the negative
effects of new sources of air pollution. We monitored streamwater chemistry in the class I Lye Brook Wilderness in southwestern
Vermont from May 1994 through August 1995. Stream samples were collected biweekly at nine sampling locations throughout the
wilderness and were analyzed for major cations and anions, dissolved organic carbon, pH, and acid-neutralizing capacity. Eight
of nine sites sampled had mean annual acid neutralizing capacity values below zero. During the study period, decreases in
streamwater acid neutralizing capacity values were caused primarily by SO4
2−. At some sites, however, NO3
− and naturally occurring, weak organic acids were seasonally important. During high discharge, the low pH and high concentrations
of inorganic monomeric Al were at levels that are toxic to acid-sensitive aquatic species. Watershed mass balances were calculated
to determine annual gains or losses for measured ions. These budgets indicate that S inputs and outputs were nearly equal,
there was a net loss of base cations, and a net gain in N. How long these watersheds can continue to assimilate additional
N inputs is unknown. 相似文献
145.
146.
黄河断流与黄河的水资源承载力 总被引:2,自引:0,他引:2
分析了黄河断流的时空特征、根源和危害,提出了水资源承载力的概念。黄河的水资源承载力即指在生态系统良性循环的条件下,黄河对人类取用水资源的最大承受能力。只有建立起水资源承载力的概念,人类才会谨慎地取用水资源,小心地保护水资源。 相似文献
147.
可持续发展能力建设的理论分析与重构 总被引:8,自引:1,他引:7
本文从《21 世纪议程》所述的可持续发展能力的解释出发,首次定义了可持续发展能力建设的概念,全面阐术述了能力建设的基本概念和实质内涵;同时,从理论上论述了能力建设的现实意义、能力建设的途径以及能力建设在实施可持续发展战略中基础作用。 相似文献
148.
关于中国土地资源人口承载力问题的思考 总被引:4,自引:0,他引:4
朱国宏 《中国人口.资源与环境》1996,6(1):18-22
土地资源的人口承载力问题也就是生存空间问题,它关系到人类未来的生存与发展问题,历来受学术界的注目。20世纪70年代联合国粮农组织(FAO)曾主持了一项大型的发展中地区土地资源人口承载力问题的研究,80年代后期,中国科学院—国家计委自然资源综合考察委员会也主持了一项大型的中国土地资源生产能力及人口承载量研究。相应地还有一系列有关中国未来的生存与发展和人口承载力问题的探讨。本文从中国人地关系变化的角度介入这一讨论,分析中国远期的土地资源是否超载的问题。 相似文献
149.
火灾后钢筋混凝土损伤程度的灰关联分析 总被引:5,自引:0,他引:5
由于火灾对钢筋混凝土结构的影响存在不确定性,使得人们不能准确地确定火灾后钢筋混凝土结构的损伤程度,给评估和加固修复带来很大困难。本文运用灰色系统理论中灰关联分析数学模型定量分析了高温下钢筋混凝土梁的3种影响因素,并确定了各因素间的主次关系,为高温后钢筋混凝土结构的安全评估及修复加固提供了理论依据。 相似文献
150.