延安北部丘陵沟壑区退耕还林(草)成效的遥感监测

将延安北部丘陵沟壑区的吴起、志丹、安塞、子长、延川、延长和宝塔等7县区做为一个整体,利用GIS综合解译分析SPOTVGT、NOAA/AVHRR、EOS/MODIS、TM和DEM数据,揭示该区域退耕还林(草)生态建设工程所取得的实效。1998-2009年NOAA/AVHRR和EOS/MODIS遥感监测表明,该区域在遥感影像图上凸现,在其北部和西部形成一条明显的与行政区边界相吻合的分界线,表明退耕还林后植被覆盖状况正在逐年改善,而且植被恢复情况明显好于其北部和西部。1999-2007年SPOTVGTNDVI演变表明,该区域NDVI正处于快速上升阶段,线性趋势值为0.0078,与其南部次生林区的NDVI差值在明显减小,与北部粮食种植区的NDVI差值在逐渐加大。退耕前后的TM影像解译结果对比分析说明,延安北部近年来退耕还林(草)的成绩是显著的,生态环境有了明显的改善。与1997年相比,到2007年延安北部有68.37%耕地不再耕种,而大于35°的坡耕地91.45%实现了退耕,25-35°的坡耕地有74.70%退耕,绝大部分高坡度耕地实现了退耕,而较低坡度的坡耕地退耕幅度也很大,达到了50%-70%。退出的耕地主要变为草地、林地和果园。TM影像解译结果表明,林草覆盖度增加非常显著,达到65.3%,增加24.3%,以草的面积增加最为明显。低植被覆盖度的土地面积在大幅度减少,植被覆盖度小于10%的面积减少了83.42%;中植被覆盖度面积在显著增加,到2007年,植被覆盖度在30%-50%的面积最大,占总面积的47.2%,将近一半;高植被覆盖度面积也明显增加,由1997年的6%增加到2007年22%。TM影像与DEM数据叠加分析表明,土壤侵蚀强度总体上降低,强度、极强度和剧烈等级土壤侵蚀的土地面积减少50%以上,但极强度和剧烈的高等级土壤侵蚀面积仍占2007年国土总面积的13.3%,土壤侵蚀状况仍然严重。植被以灌木和草为主,乔木面积较低,森林覆盖率只有22.4%,因此需要继续加强治理。

Remote sensing analysis of the effectiveness of converting farmland into forest or grass in the hilly gully region of Northern Yanan

A comprehensive analysis of the SPOT VGT, NOAA/AVHRR, EOS/MODIS, TM and DEM data by using the Geographic Information System (GIS) showed that the ecological construction project for changing cultivated land back into forest or grass land in seven counties of northern Yanan has achieved some effects. Examination of the NOAA/AVHRR and EOS/MODIS remote sensing monitoring data from 1998 to 2009 revealed that the area is highlighted in the remote sensing images, distinct from the adjacent areas to the north and west. All the results demonstrated that the vegetation coverage is improved and the vegetation recovery is better in the hilly gully region of northern Yanan. Evolution of the SPOT VGT NDVI from 1999 to 2007 illustrated that the difference between the NDVI in northern Yanan and the NDVI in secondary forest is significantly reduced. The difference of NDVI in Northern Yanan and that in grain-growing areas is obviously increasing. The NDVI is in a rapid increasing stage with the linear trend value being 0.0078, visibly higher than that of the surrounding area. Comparative interpretation of the TM images before and after the farmland reconstruction illustrated that the effect of converting farmland into forest is remarkable in recent years, with great improvement in the ecological environment. Compared with the situation in 1997, 68.37% of arable land in northern Yanan is no longer farmed in 2007. The cultivated land is changed mainly into grassland, woodland, or orchard. The forest/grass coverage has markedly increased, with an attainment of 65.3%, raised by 24.3%. The grass area has increased pronouncedly in particular. By the year 2007, the area with vegetation coverage of 30%50% is the largest, occupying nearly half (47.2%) of the total area, and the area with tall vegetation has also dilatated a lot, from 6% in 1997 to 22% in 2007. TM images and DEM data analyses showed that the soil erosion intensity decreases in general. Areas of violent soil erosion, extremely heavy soil erosion, and heavy soil erosion have diminished notably, by more than 50%. However, the areas of violent soil erosion and extremely heavy soil erosion still account for 13.3% of the total eroded soil land of China in 2007, so the situation is still formidable. Vegetation coverage is dominated by shrub and grass. The area covered by trees takes only a small percentage, and the forest coverage is only 22.4%. Therefore, strengthened governance is still imperative.