基于Landsat-TM和MODIS的非洲水体分布格局研究

卫星遥感是进行洲际或者全球尺度内陆水体信息提取的最有效、最为切实可行的技术手段。作为全球水资源较为匮乏的大洲，以遥感技术进行非洲内陆水体信息提取研究具有重要的应用价值。文章以2010年Landsat TM/ETM+卫星遥感影像数据为基础，辅以MODIS遥感影像，基于面向对象的分类方法，选用归一化差值水体指数（Normalized Difference Water Index，NDWI）及波段组合（Band 4/Band 2）进行水体信息提取。并根据不同气候区，选取4个典型区，监测年内水体的变化情况。结果表明：1）非洲内陆水体面积为311 479.2 km2，其中湖泊总面积为209 018.5 km2，占水体总面积的67.1%；河渠总面积64 550.1 km2，占20.7%；水库坑塘总面积37 910.7 km2，占12.2%。非洲拥有水库211个，总面积13 438.7 km2。2）撒哈拉以南的非洲是水体主要的分布区，统计分析表明约97%的水体分布在该地区。3）热带沙漠气候区湖泊面积呈减少趋势，热带草原气候区湖泊受干湿季节影响上下波动较大，热带雨林气候区和地中海气候区湖泊面积变化较小，总体上4个典型区水体面积变差系数＜10%。

Distribution Pattern of African Surface Water Based on Landsat-TM and MODIS Imagery

An object-oriented classification approach was applied in this study to extract surface water area across the African continent based on Landsat TM/ETM+ imagery acquired in 2010, combined with MODIS images to examine the water surface areal fluctuation caused by seasonality. Normalized Difference Water Index (NDWI) and band ratio (band 4/band 2) were used as the major input for eCognition software package to extract surface water area. Further, surface water area was extracted over four typical regions determined from various climatic zones to investigate the surface water areal fluctuation due to precipitation and runoff variability. Our results showed that the surface waters area totaled 311 479.2 km2 across the African continent. The lake area was 209 018.5 km2, accounting for 67.1% of the total inland waters area, while river and channels area 64 550.1 km2, accounting for 20.7%, and reservoirs and ponds area 37 910.7 km2, accounting for 12.2% of the total inland waters across the African continent. Altogether, there were 211 reservoirs distributed all over the continent, and the area amounted to 13 438.7 km2. Inland surface waters for Africa were mainly distributed across regions sitting down to the south of Sahara. Our investigation also showed that inland waters decreased about 100 km2 in the target area from tropical desert climatic zone. A large areal fluctuation was observed for the target region from tropical steppe climatic zone, and the surface waters area changed in a range of 1 000 km2. Comparatively, surface waters showed small variation due to seasonality, and the areas only changed in a range of tens of square kilometers. As indicated by the coefficient of variation (less than 4.85%) in these four target regions, the surface water areal fluctuation across the African continent might be less than 10%.