|
E. Linacre |
11/'97 |
The surface energy balance of an oasis is quite different from that of the surrounding desert (1). The point has been studied in an oasis called Zhangye in China during 1991-'92. The oasis has an area of 15 km x 25 km (2). The albedo at Zhangye was about 25% in winter (Dec-Feb, when there was some snow) and 18% during the rest of the year. For the desert there was an annual swing from about 40% in winter to about 27% in summer. In winter, there was no significant difference in energy balance: the net radiation in winter was only 23 W/m2 or less in both areas.
During the summer months however, the daytime maximum net radiation was 174 W/m2 for the oasis and 104 W/m2 for the desert, on average. Monthly mean evaporation rates within the oasis were 0.2 mm/d in December, 2.8 in May, 3.9 in June, 3.4 in August, with the annual total reckoned to be 535 mm, equivalent to an average latent heat flux of 41 W/m2. This is only a quarter of the evaporation from a pan evaporimeter in the oasis, indicating the irrelevance of pan evaporimeter measurements. Much less evaporation occurred from the desert.
References
(1) Oke, T.R. 1987. Boundary-layer climates (Methuen) Second Edition, 435pp.
(2) Tsukamoto, O., K. Sahashi and J. Wang 1995. Heat budget and evapotranspiration at an oasis surface surrounded by desert. J. Meteor. Soc. Japan 73, 925-35.