Study On The Differentiation Characteristics Of Vegetation Coverage In Shiyang River Basin Based On Dimidiate Pixel Model

Vegetation coverage can reflect information of the vegetation coverage, which can compensate for deficiencies of the normalized difference vegetation index(NDVI) in distinguishing low-coverage vegetation and easiness of being saturated when used for valuing high-coverage vegetation. And it can be used for the evaluation of the land degradation and desertification. Besides, vegetation coverage can reflect spatial distribution characteristics of vegetation coverage, so it will facilitate the analysis and monitoring of dynamic changes of vegetation, showing great significance in evaluating the merits of the ecological environment.In this study, MODIS product data, vegetation maps and soil type maps are adopted to assess the vegetation coverage of the Shiyang River Basin with the use of dimidiate pixel model and probabilistic method. And the spatial analysis method, spatial model of dynamic changes of vegetation coverage and the comprehensive indexes model of vegetation coverage evaluation analysis are chosen to reveal diversity law of vegetation coverage of the Shiyang River Basin. And here are research conclusions mainly achieved:In terms of the interannual scale, areas in the upper and middle reaches of Shiyang River Basin whose vegetation coverage is greater than 60% shows a steady rising tendency from 2000 to 2012, while vegetation area of other levels fluctuates. And downstream areas whose vegetation coverage is greater than 60% tend to decrease, with areas whose vegetation coverage is greater than 30%in Minqin Basin of an apparent decrease tendency. Generally speaking, areas whose vegetation coverage is greater than 60% throughout the Shiyang River Basin maintain a flat state, and areas whose vegetation coverage varies from 15% to 30% are generally of a slight decrease, while vegetation area of other levels increases slightly.In terms of the inter-monthly changes, area whose vegetation coverage is greater than 60% in the Shiyang River Basin in 2010 presents two kinds of change tendencies. From January to the March, the area shows a gradually increasing trend, while the trend changes into a decreasing fluctuation from April to November.The analysis made from the perspective of spatial characteristics shows that there is a generally steady change tendency in Shiyang River Basin, accounting for 47.89% of the total drainage area, and the positive and negative change proportion of other levels is relatively of a uniform distribution. The degradation and severe degradation areas are mainly in the edge of the downstream desert belt, while areas whose vegetation is restored or completely restored are mainly located in the upstream and midstream, with a small amount located in the downstream drainage network, showing a generally steady tendency of the whole Basin.The restored vegetation area in the upper reaches of the Shiyang River Basin is 8.04 times larger than the area of degeneration, while it is 1.53 times in the middle reaches, 2.50 times in the downstream, 3.20 times in Minqin Basin and 1.00 times in the whole area. Vegetation in the upper and middle reaches of the Shiyang River Basin has been well restored while vegetation in the downstream shows server degeneration. And the vegetation degradation in Minqin Basin is even severer than that in the downstream of the Shiyang River Basin. Therefore it is a necessity to strengthen the governance of the lower reaches and Minqin Basin in the whole management. Analyzing from the whole Shiyang River Basin, restoration and degradation areas are essentially flat, and the ecological environment of the whole area develops towards a state of stability.The annual rate of increase of the level-I vegetation is basically the same with the annual rate of decrease of the level-III vegetation, taking the whole area of the Shiyang River Basin into account. So an emphasis should be put on the vegetation of the level-I and level-III during the basin vegetation management. And the Level-IV vegetation in the basin shows a large rate of transferring, newly increasing and changing, forming a rapid variation during the twelve-year period. The Level-II vegetation in the basin shows a low rate of transferring, newly increasing and changing, forming a stable variation during the twelve-year period.If the vegetation coverage is evaluated, the comprehensive IIVFE of the vegetation coverage in the whole area in the Shiyang River Basin can be divided into 3 kinds: the IIVFE of the upper reaches is always within [4,5], while the IIVFE is within [3,4] in the middle reaches and [2,3] in the downstream. And if the IIVFE of those three areas are ranked, it is apparent that the upper reaches hold a highest IIVFE, followed by the middle reaches and the downstream, indicating that water resource is an important factor affecting vegetation coverage.