Zeng Qunzhu, Xie Yingqin
Land evaporability, dry-wet balance and their distribution on the Plateau have been calculated and discussed by using H. L. Penman’s model. Based on observation data of Plateau and of more than 100. meterological stations in its close vicinity the paper gives the result of the study.1. The amount of annual land evaporability on the Plateau is 600-1400mm. It gradually increases from southeast to northwest. The high value regions, more than 1200 mm/year, are observed in the arid Qaidam, Tarim, Junggar basins and the Hexi Corridor; the second high, more than 1000mm/year, on the northern slope of the Himalaya and Yaluzangbujiang valley, i.e. Dingri, Xigaze, Lasa; and the low regions, less than 800mm/year, on the eastern periphery of the Plateau and the Sichuan basin.2. During the transitional period between the dry and wet seasons, i.e. in May, in the south of Tangula the weather is clear, the air dry, the sum of solar radiation strong, and the evaporation is more than 120mm/month, the maximum value in a year. In July the warm high pressure in the upper atmosphere and the ground hot low pressure form strong convection, and the warm and air flowing from the surounding low atmosphere layer converges upon the Plateau. The south-west monsoon flows from the lower reaches to the upstreams of Bulamputela river -Yaluzangbu river, the Nujiang river, the Lancang river etc., bringing abundant rainfall to the southern and southeastern parts of the Plateau, and lowering the evaporation month by month south of the Tanggula Mts. However, on the north slope of the Tanggula Mts. where the influence of southwest monsoon is difficult to reach, the evaporation is more than 230mm/month,the maximum in a year.The land evaporation in winter on whole Plateau surface is negligible, and it decreases from 30mm/month to 1 mm/month from south to north.3. The state of the dry-wet balance on the Plateau is influenced by the factors of radiation balance, precipitation, evaporation, etc.. The zero line of the dry-wet annual balance starts in the east on the southern slope of the Qingling and Bashan Mountains, running towards the west, along the southern border of the Gansu and Qinghai, crossing the Hengduan Mountains, entering into Qinghai-Xizang Plateau, through Suoxian, Jiali, Linzhi and further westward and finally along the southern slope of the Himalaya Mts. leaving the territory of China. In the western section of this line the evaporation is higher than the precipitation, and the annual deficit is over 500 mm. The most arid is located in the centre of the arid region, i.e. in the Qaidam, Tarim and Junggar basins, where the shortage of water is over 1000mm/year.. 1. In the south Tanggula Mts., the evaporation is greatest in May, and the shortage of water is 5(1-150mm/month. On the northern slope of Tanggula Mts. and in the arid regions of Gansu, Xinjiang and Qinghai the most shortage of water is in July, more tha 150mm month. On the eastern side of the Plateau the remaining water of abundant precipitation is highest in July in a year, reaching 100-150mm, month in the central region (the west Sichuan basin).
