Dynamic changes of land surface variables on grassland in Northern Tibet in recent 20 years

The vegetation cover, surface reflectance and surface temperature on grassland are important factors for assessment and monitoring of grass growing and grassland degradation. Using 10-day composite data of NOAA/AVHRR channels 1, 2, 4 and 5 in 1982-2000 and common methods of remote sensing retrieval and statistic, spatial-temporal variation of above three factors in Naqu of Northern Tibet in recent 20 years were analyzed. The main results showed that the vegetation cover in vegetation region changed between 0.10 and 0.95. Its annual variability changed between -0.012a-1 and 0.012a-1 and its variation coefficients changed between 0.05 and 0.60 and the grassland of 24% of vegetation region degraded. The more the vegetation cover was, the less variation coefficient was. The mean surface reflectance from June to September changed between 0.152 and 0.300. Its annual variability changed between -0.009a-1 and 0.007a-1 and its variation coefficients changed between 0.02 and 0.23 and the grassland of 51% of vegetation region degraded. The mean surface temperature from July to August changed between 20℃ and 27℃. The conclusions show that the better the vegetation grows, the more the vegetation cover is and the smaller the surface reflectance is and the lower the surface temperature is. Although grassland degradation can be assessed and monitored by only one in three factors, it is assessed and monitored by three or more factors is more objective and reasonable.