Abstract:A GIS-based RUSLE model is employed to study the global soil erosion potential for viewing the present situation, analysing changes over the past century, and projecting future trends with reference to global changes in land use and climate. Scenarios considered in the study include historical, present and future conditions of cropland and climate. This research gives the first overview of the global situation of soil erosion potential considering the previous century as well as the present and future. Present soil erosion potential is estimated to be about 0Ð38 mm year 1 for the globe, with Southeast Asia found to be the most seriously affected region in the world. It is estimated that nearly 60% of present soil erosions are induced by human activity. With development of cropland in the last century, soil erosion potential is estimated to have increased by about 17%. Global warming might significantly increase the potential for soil erosion, and the regions with the same increasing trend of precipitation and population might face much more serious problems related to soil erosion in the future.

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[1] The river discharges have decreased continuously during the last half century in the Yellow River, the second-largest river basin in China. In particular, a drying up of the main river along the lower reach has occurred since 1972, and the situation has become more and more serious during the 1990s. Using 50 years of meteorological data from 108 stations together with a collection of irrigation data, the long-term changes in the river discharge have been investigated with a view to identifying the reason for the drying up of the Yellow River. It was found that the annual precipitation generally decreased (À45.3 mm/50 yr) while the air temperature generally increased (+1.28°C/50 yr). From the 1960s to the 1970s the precipitation decreased by 29.6 mm/10 yr, the evaporation increased by 7 mm/10 yr (for pan evaporation), and the irrigation water usage increased by 10.5 mm/10 yr. As a consequence the drying up of the Yellow River has occurred since 1972. Irrigation was developed continuously in the 1980s, but the drying-up situation maintained at the same level as during the 1970s. The reason for this was the increase in precipitation (by 10.3 mm/10 yr) and the sharp decrease in the evaporation (by 133 mm/10 yr for pan evaporation). During the 1990s the irrigation was maintained at a level similar to that during the 1980s, but the drying-up situation was greatly aggravated. The reason for this was found to be a result of the decrease in precipitation (by 38.2 mm/10 yr) and the increase in evaporation (by 52 mm/10 yr for pan evaporation).

The anticipated water scarcity in the first half of this century is one of the international issues of most concern, which needs to be adequately addressed. However, even though the issue has an international impact and worldwide monitoring is critical, there are limited global estimates at present. In this study, annual water availability has been derived from annual runoff estimated by land surface models using total runoff integrating pathways (TRIP) with 0.5° by 0.5° longitude/latitude resolution globally. The global distribution of abstraction was estimated for each sector at the same spatial resolution based on country-based statistics of municipal water use, industrial water use, and agricultural intakes, using a global geographical information system with global distribution of population and irrigated crop land area. The total population under water stress estimated for 1995 corresponded very well with earlier estimates. However, the number is highly dependent on how one assumes the volume of water from upstream of a region, which can be considered as "available" water resources within the region. Therefore it is important, even for global scale analysis, to evaluate the regional water quality deterioration and the real consumption of water resources in the upper part of the stream, as well as the accessibility of water. Further studies should be promoted by an integrated approach to improve the accuracy of future projections on both the natural and social aspects of water resources.