# Assessment of Grassland Degradation on the Tibetan Plateau Based on Multi-Source Data
## 1 Introduction  
Grassland, covering about 40% of the earth's terrestrial area, is an important part of the global terrestrial ecosystem [1]. Grassland is not only vital for supporting ecosystem functions, services, and livelihoods of millions of humans but also plays a key role in the conservation of biodiversity, climate regulation and global biogeochemical cycles [2, 3]. However, grasslands have been severely degraded for decades due to frequent human activities and environmental changes [4, 5]. At present, 39.06% of grasslands in the world have been degraded, of which nearly 5% experienced extreme degradation [6]. Grassland degradation not only causes huge environmental problems such as soil erosion and land desertification but also poses a great threat to the food, fodder, fuel and medicine that human beings rely on [7, 8]. A series of problems brought about by grassland degradationwill seriously affect the sustainable development of the pastoral area. Thus, grassland degradation has gradually become one of the key issues to be solved urgently [9]. The Tibetan Plateau (TP) is known as the \"Rool of the World\" and the \"Third Pole of the Earth\". Due to its unique geographical location and altitude, the TP plays an important role in regulating climate and water resources in East Asia and even the Northern Hemisphere [10]. The grassland is the most widely distributed type of ecosystem on the TP, accounting for about 70% of the total area and providing a source of livelihood for more than 6 million herders [11]. The grassland of the TP is a treasured reservoir of biodiversity and a key carbon pool [12]. Due to the limitation of harsh environmental conditions, the TP grassland ecosystem is very sensitive to external disturbances [13]. Once the grassland on the TP is degraded, it will be difficult to recover [14; 15]. Under the combined influence of potential factors such as overgrazing, climate change, permafrost and soil disturbance by small mammals, the grasslands of the TP face serious threats of degradation [16]. According to net primary productivity (NPP, NPP is the product of photosynthesis) data, nearly 38.8% of grasslands on the TP have been degraded from 2001 to 2013 [17]. [PERSON] et al. found that 63% of the \"Three-River Headwaters\" region experienced grassland degradation from 2000 to 2016, and 2% of the region was at risk of desertification [18]. Grassland degradation is a process from quality degradation to area reduction, and the dynamic process of grassland degradation on the TP has not been well described and understood in previous studies. Therefore, a dynamic analysis of the TP grassland ecosystem from quality degradation to area reduction is necessary for grassland ecosystem restoration. Field surveys and location monitoring are capable of obtaining accurate information on grassland degradation, while they are time-consuming and costly [19]. In recent years, remote sensing monitoring has been widely used as a cost-effective method for large-scale grassland degradation monitoring [20; 21]. As satellite-based earth observation techniques improve and extraction algorithms develop, different land cover products and various vegetation indexes have emerged [22]. Meanwhile, the inconsistencies among land cover products increase the uncertainty of the study results [23; 24]. Although many researchers using vegetation indexes to reflect the grassland degradation on the TP have been published, most studies only considered the degradation status of grassland under a single remote sensing vegetation index. There is still a high degree of uncertainty in monitoring and assessing grassland degradation using a single vegetation index. Meanwhile, the differences in sensors, classification systems and resolutions of different remote sensing products cause great inconsistencies among them, making it difficult to obtain accurate grassland degradation information [25].