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61.
受玄武岩盖层的影响,石林地区的地下水在雨季和旱季对碳酸盐岩都具有侵蚀性,玄武岩盖层空气CO2呈现出低—高—低的双向变化梯度。溶蚀试验表明.地下0~0.6m,水平方向的溶蚀量大于垂直方向的溶蚀量,而随着深度的增加,垂直方向的溶蚀量大于水平方向。富含CO2的水通过具有最大渗透张量和较小主轴倾角的玄武岩裂隙下渗,对碳酸盐岩的溶蚀作用表现为一个脱钙、富铝铁、硅迁移的复杂过程,并在地下0~0.6m形成许多水平凹槽、穿洞等岩溶形态,而地表0.6m以下以垂向溶蚀为主,有利于石柱的形成与发育。 相似文献
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In accordance with Senegal’s decentralisation policy, important forest management tasks, including the right to allocate charcoal production rights, have been transferred to rural councils. This paper investigates the impact of these institutional reforms on charcoal production practices using the environmental entitlement framework developed by Leach et al. [Environmental entitlements: dynamics and institutions in community-based natural resource management. World Development 27 (2) (1999) 225]. The councils have not been able to turn their new endowments into entitlements because they lack sufficient strength and legitimacy. Informal institutions, notably the coalition between merchants, state agents and village chiefs, continue to run the charcoal business and are hardly affected by decentralisation efforts. Most rural people, especially those relying solely on agriculture for sustenance, do not benefit at all from the charcoal trade. They do suffer from the environmental costs it brings with it, however. Although tensions between pro-exploitation actors and pro-conservation actors are evident, the pro-exploitation actors’ firm grip on the informal institutions will probably lead to a prolonged subversion of the laws that seek to enhance local control and to sustain the forest. 相似文献
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中国西部退耕还林(草)和沙漠化土地绿化的区域性气候效应 总被引:10,自引:0,他引:10
中国正在实施的西部大开发必将导致较大规模的土地利用和地表覆盖(LULC)变化,其中最重要的两类LULC变化是退耕还林(草)和沙漠化土地绿化工程。利用最新的全球地表特征资料数据库(GLCCD)识别以上两项生态建设工程的实施规模与范围,并以此更新了目前比较流行的区域气候模式的下边界条件,利用修正了的区域气候模式研究上述大规模LULC变化可能对中国区域性气候造成的影响。使用的卫星资源数据库是由美国地质测绘局(USGS)和Nebraska-Lincoln大学牵头,根据NOAA卫星的AVHRR资料、USGS掌握的高分辨率数字化地图,以及生态区划和植被分布等资料组建的。 相似文献
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Zheng Feili Tang Keli Zhang Keli Cha Xuan Bai Hongying Northwest Institute of Soil Water Conservation CAS Ministry of Water Resources Yangling Shaanxi People''s Republic of China 《地理学报(英文版)》1997,(2)
Forlongtime,theproblemaboutnaturalerosionandartificiallyacceleratederosionisindispute.HistoricalgeographersconsiderthathumaninducedecoenvironmentaldestructionisaccountablefortheseveresoilerosionandecoenvironmentdeteriorationonLoessPlateau[7,8].Somegeolog… 相似文献
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Sergio Colombo Mario Chica-Olmo Francisco Abarca Hugh Eva 《ISPRS Journal of Photogrammetry and Remote Sensing》2004,58(5-6):330-341
Tropical forest mapping is one of the major environmental concerns at global and regional scales in which remote sensing techniques are firmly involved. This study examines the use of the variogram function to analyse forest cover fragmentation at different image scales. Two main aspects are considered here: (1) analysis of the spatial variability structure of the forest cover observed at three different scales using fine, medium and coarse spatial resolution images; and (2) the study of the relationship between rescaled images from the finest spatial resolution and those of the medium and coarse spatial resolutions. Both aspects are analysed using the variogram function as a basic tool to calculate and interpret the spatial variability of the forest cover. An example is presented for a Brazilian tropical forest zone using satellite images of different spatial resolutions acquired by Landsat TM (30 m), Resurs MSU (160 m) and ERS ATSR (1000 m). The results of this study contribute to establishing a suitable spatial resolution of remotely sensed data for tropical forest cover monitoring. 相似文献
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Pollen and charcoal records from two large, shallow lakes reveal that throughout most of the past 50,000 yr Noel Kempff Mercado National Park, in northeastern lowland Bolivia (southwestern Amazon Basin), was predominantly covered by savannas and seasonally dry semideciduous forests. Lowered atmospheric CO2 concentrations, in combination with a longer dry season, caused expansion of dry forests and savannas during the last glacial period, especially at the last glacial maximum. These ecosystems persisted until the mid-Holocene, although they underwent significant species reassortment. Forest communities containing a mixture of evergreen and semideciduous species began to expand between 6000 and 3000 14C yr B.P. Humid evergreen rain forests expanded to cover most of the area within the past 2000 14C yr B.P., coincident with a reduction in fire frequencies. Comparisons between modern pollen spectra and vegetation reveal that the Moraceae-dominated rain forest pollen spectra likely have a regional source area at least 2-3 km beyond the lake shore, whereas the grass- and sedge-dominated savanna pollen spectra likely have a predominantly local source area. The Holocene vegetation changes are consistent with independent paleoprecipitation records from the Bolivian Altiplano and paleovegetation records from other parts of southwestern Amazonia. The progressive expansion in rain forests through the Holocene can be largely attributed to enhanced convective activity over Amazonia, due to greater seasonality of insolation in the Southern Hemisphere tropics driven by the precession cycle according to the Milankovitch Astronomical Theory. 相似文献