The soil moisture distribution, thawing-freezing processes and their effects on the seasonal transition on the Qinghai-Xizang (Tibetan) plateau

The soil energy-water distribution and freezing-thawing processes varied at different sites in the northern part of the Qinghai-Xizang (Tibetan) Plateau. The temporal and spatial variations of the soil moisture content were more complex than those of temperature. At the observation site, the soil moisture content increases with depth in certain layers but decreases in other layers. The freezing/thawing processes and the temperature distribution were strongly influenced by soil moisture content. During the summer monsoon period, the soil moisture contents at a depth of 10 cm for all sites were relatively high but differed spatially. In general, the shallow layers started to freeze in October and thaw in April at all sites, with a freezing period of about 6 months. However, the onset of freezing/thawing varied at different sites. The results suggest that the freezing process may prevent the soil moisture from evaporating. Although there is low precipitation in winter (freezing period), the soil moisture content is high when the soil begins to thaw. The abrupt increase/decrease of net atmospheric heating synchronized with the thawing/freezing of soil indicates that these processes may strongly influence the seasonal transition on the Qinghai-Xizang (Tibetan) Plateau. (C) 2002 Elsevier Science Ltd. All rights reserved.