XI'AN, Nov. 28 -- Hongjiannao Lake, China's largest desert freshwater lake, has been shrinking sharply over the past 13 years, warned local meteorological authorities on Thursday.

The lake, sandwiched between the Muus Desert in Shaanxi Province and the Erdos Plateau in Inner Mongolia Autonomous Region, has shrunk by about one-third since the year 2000, according to latest statistics from the remote sensing information center for agriculture under the Shaanxi Provincial Meteorological Bureau.

The water area of the lake now stands at 32.16 square km. At its largest, the lake covered 67 square km in 1969.

The bank of the lake has recessed over 800 meters in the worst case from the beginning of the 1990s. The water quality and ecological functions of the lake, which also serves as the world's largest habitat for the rare Ichthyaetus relictus species of bird, have also kept deteriorating, said the center.

The lake's water volume saw a significant shrinkage of 7.4 percent in 2005 and dwindled again sharply by 5.55 percent in 2011, though the shrinkage slowed to 2.89 percent in 2012, the center added.

Experts said human activities including reservoir construction, mining and agricultural irrigation are the main causes for the sad phenomenon.