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在对石羊河流域广泛实地调研的基础上,基于水资源约束条件,对石羊河流域水资源现状和流域农业发展的可持续性进行了分析.结果:若按照现有农业生产方式和产业结构进行农业生产,石羊河流域农业发展呈弱可持续性.提出了可持续发展对策. 相似文献
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Atlantic salmon, Salmo salar L., stocks worldwide have dwindled, and stricter recreational fishing regulations have been implemented. This study estimates the recreational value of salmon fishing under different regulatory schemes and identifies factors explaining changes in net economic value of sport fishing. A Contingent Valuation mail survey in the Verdal River, Norway, asked anglers their willingness-to-pay for increased fishing expenditures or willingness-to-accept compensation in reduced fishing expenditures, to estimate recreational value. The average annual recreational value per angler decreased under stricter regulations from about €420 (in 2007–2009) to €280 (2010–2012) to €180 for the future scenario (2013–2015). Although an overall net decline in recreational value was found with stricter regulations, anglers have heterogeneous preferences. Anglers with non-catch motives or high place attachment have higher recreational value with stricter regulations than others. If stricter regulations remain over time, younger, less consumptive-oriented anglers would probably dominate the fishery. 相似文献
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From the theory that four connecting rods mechanism has flexibility and any closed graphics of plane mechanism has three restrictions, the calculating formula of degree of freedom of mechanism is researched. The result of research indicate that the calculating formula of degree of freedom of space mechanism will be set up if the calculating formula of degree of freedom of plane and spherical surface mechanism are changed a little. The new formula could substitute for 29 traditional calculating formulas of degree of freedom of space mechanism but also the plane mechanism, the multiple rings space mechanism, the open chain space mechanism and mixed chain space mechanism and so on. Because that the new formula is easier and rapider in use than traditional formulas, it is a reliable theoretical calculating formula for calculating degree of freedom of space mechanism. 相似文献
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Chowell G Rivas AL Hengartner NW Hyman JM Castillo-Chavez C 《Preventive veterinary medicine》2006,73(4):297-314
A model of epidemic dispersal (based on the assumption that susceptible cattle were homogeneously mixed over space, or non-spatial model) was compared to a partially spatially explicit and discrete model (the spatial model), which was composed of differential equations and used geo-coded data (Euclidean distances between county centroids). While the spatial model accounted for intra- and inter-county epidemic spread, the non-spatial model did not assess regional differences. A geo-coded dataset that resembled conditions favouring homogeneous mixing assumptions (based on the 2001 Uruguayan foot-and-mouth disease epidemic), was used for testing. Significant differences between models were observed in the average transmission rate between farms, both before and after a control policy (animal movement ban) was imposed. They also differed in terms of daily number of infected farms: the non-spatial model revealed a single epidemic peak (at, approximately, 25 epidemic days); while the spatial model revealed two epidemic peaks (at, approximately, 12 and 28 days, respectively). While the spatial model fitted well with the observed cumulative number of infected farms, the non-spatial model did not (P<0.01). In addition, the spatial model: (a) indicated an early intra-county reproductive number R of approximately 87 (falling to <1 within 25 days), and an inter-county R<1; (b) predicted that, if animal movement restrictions had begun 3 days before/after the estimated initiation of such policy, cases would have decreased/increased by 23 or 26%, respectively. Spatial factors (such as inter-farm distance and coverage of vaccination campaigns, absent in non-spatial models) may explain why partially explicit spatial models describe epidemic spread more accurately than non-spatial models even at early epidemic phases. Integration of geo-coded data into mathematical models is recommended. 相似文献
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介绍了高层建筑结构优化设计软件ＨＢＯＰＴＣＡＤ的开发研究和系统结构，该程序在Ｗｉｎｄｏｗｓ平台上开发，具有良好的用户界面，软件在结构模型输入，计算约束条件输入和计算结果检查方面都十分方便，并将结构优化设计方法引入到ＣＡＤ中，使计算结果更经济合理，并能较好地符合工程习惯。 相似文献
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提取抗赤星病烤烟ＣＶ８７ 的总ＤＮＡ,采用浸种法导入感赤星病烤烟ＮＣ８９。Ｄ１ 代筛选出的优良株收种并种植后得到Ｄ２ 代,对其活体接种赤星病菌,检测其抗病性,并于接菌前后测定叶片过氧化物酶活性。结果表明,接菌的８ 个株系有３ 个株系的抗病性与供体亲本ＣＶ８７ 相似、并显著高于受体亲本ＮＣ８９;抗病性强的个体接菌后的过氧化物酶活性较接菌前有明显的升高,与ＣＶ８７ 没有明显差异。但ＣＶ８７ 的过氧化物酶活性在接菌前后没有明显变化,即接菌前就表现高的活性 相似文献
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Susan A. Schroeder Louis Cornicelli David C. Fulton Marrett D. Grund 《Human Dimensions of Wildlife》2014,19(4):301-318
Although considerable research has focused on satisfaction with recreation experiences, limited work has examined factors related to regulatory support. In 2005, an antler point restriction (APR) regulation was introduced for hunting white-tailed deer (Odocoileus virginianus) in Minnesota’s Itasca State Park. Hunter surveys were conducted following the 2005, 2006, 2007, and 2009 seasons. We modeled how (a) satisfaction with the deer seen, (b) type of deer pursued, (c) agency trust, and (d) years of deer-hunting experience, influenced support for the APR, and compared the model over the four survey years. Type of deer pursued and agency trust were the strongest predictors of APR support, followed by satisfaction with deer seen, and years of hunting experience. Hunters who targeted big bucks, had more trust in the agency, and expressed more satisfaction with deer seen in the field, were more supportive of the APR. Hunters who had more deer-hunting experience were less supportive. 相似文献