陕北长城沿线地区土地退化态势分析

以陕北长城沿线农牧交错区为例，运用遥感与GIS技术，定性与定量相结合的方法，对研究区1985－1998年土地利用类型转换及土地退化时空规律，趋势和机制等进行了评价与分析。研究表明：该区土地退化的主导类型及土地沙漠化。从成因分析，土地沙漠化主要是长期以来的人口超载，不合理的土地利用方式和高强度的土地开发行为激发了地表自然过程伯退化性演替，致使潜在的自然环境脆弱怀转化为现实的破坏。过去13年土地退化程度部体上在不断剧，而且退化类型正向多样化发展。最后，依据评价与分析结果，进一步对该区土地退化防治和沙漠化土地生态恢复的措施与对策进行了讨论。

Nitrogen export by runoff and sediment under different types of land use in West Tiaoxi catchment

Through the analysis of historical natural settings in the farming and grazing interlocked zone along the Great Wall in northern Shaanxi Province of China, this study has ascertained that its land degradation is both natural and anthropogenic. The overlay of desertification severity layers interpreted from multi-temporal remotely sensed materials in a GIS, in conjunction with field investigation, has revealed that the spatial extent of desertified land in the area has drastically expanded during the 13-year period in certain localities. Both natural and anthropogenic factors in this study are realistically considered to assess land degradation severity. It is found that desertified area of most of the counties in the study area has exceeded 50% of each county's total area. The percentage is even higher in a few counties. The overall severity of land degradation has worsened during the last 13 years with extremely serious and serious degraded areas accounting for 88.5% of the total study area in 1998. The spatial distribution of the desertified land is uneven in terms of degrees. A comparison of the recent satellite image with historical aerial photographs reveals that the extent of degraded land in the study area has expanded while the overall severity of land degradation has worsened. Confirmed by the field investigation, the desert front in the vicinity of Xincheng, Jingbian County has encroached by over 10 km. In the worst affected region between Yulin and Hengshan, the encroachment is as far as over 40 km. Because of the expansion of the Mu Us Desert and the impact of sandstorms, the Great Wall is no longer the divide between sandy land and loessial area. The worsened desertification is attributed to the intensified conflicts among mounting population pressure, limited land resources, and the fragile ecosystem. Inappropriate human activities such as excessive exploitation of natural resources and malmanagement of land, to a certain extent, have inevitably resulted in the destruction of the environment. Mining activities have been identified as the sole cause of rapid spread of desertification in the vicinities of coalfields. The findings in this study have profound implications on how to reduce the severity of desertification hazard in the study area. As the cause of this problem is both natural and anthropogenic in origin, any measure must deal with problems of rural economic development, especially development of agriculture and animal husbandry for increasing farmer's income. A mechanism should be established to compensate for farmers whose income has dropped as a consequence of diminished land productivity caused by mining-induced degradation in the vicinity. Secondly, environmental laws aimed at controlling desertification and protecting environment should be formulated to mitigate the detrimental influence of human economic activities on land. For instance, in severely degraded, poverty-stricken regions the scattered small villages must be shifted to areas with relatively richer water resources so as to enable natural vegetation to recover, thus reversing the trend of desertification.