Abstract:

In this paper, the normalized difference vegetation index (NDVI) was calculated based on the TM satellite image data taken in June, 1995 and June, 2009, and was in turn used to estimate vegetation coverage in Xi'an city. The vegetation change was quantitatively assessed with the NDVI when the vegetation coverage was more than 0.1. The result showed that the vegetation situation was improved. In 1995, the average value of NDVI was 0.252 2, but in 2009, the value reached 0.388 2. The highest NDVI was in mountain area and the ecotone region between the mountain and the plain, while the lowest NDVI was in plain region because the crops were just harvested by then. From 1995 to 2009, the area with extreme low, low and high vegetation coverage reduced by 1.05, 22.25 and 1.81 percentage, respectively, and the area with the medium and extreme high coverage increased by 12.68 and 12.43 percentage. The difference value index of NDVI indicated that the moderate and extraordinary amelioration area were occupied 86.81% of total area. The degraded area only occupied 5.58%, and mainly distributed in the Xi’an districts, the highest peak in a Taibai mountain range and the northern part of ecotone area between Zhouzhi County and Huxian County. The amelioration area was mainly distributed in the ecotone area among Qinling Mountain and Guanzhong plain, Heihe river valley, Lishan mountain and the plain part of Zhouzhi County. The driving forces of the vegetation change would include the implementation of natural forest protection and grain for green in the mountain, orchard development, new village construction, as well as the greening in the city.

Key words: NDVI, vegetation coverage, NDVI difference value index, Xi'an