Study on the Zone of Maximum Precipitation in the North Slopes of the Central Qilian Mountains

During the period of June 2006 through September 2008,an investigation in spatial variations in precipitation in the upper catchment of the Heihe River in the north slopes of the central Qilian Mountains was conducted.Based on the data observed,it was found that: 1) summer precipitation displayed a decreasing trend from east to west in the intermediate altitude with a rate of approximately 80 mm·(100 km-1);2) the zone of maximum precipitation lay between 4 500 to 4 700 m a.s.l.(where annual precipitation is 485 mm),which coincided with the zone of maximum relative humidity(above 4 400 m a.s.l.,with a central height of 4 600 m a.s.l.) and the height of 0 ℃ in summer(about 4 680 m a.s.l.).By analysis of the difference between air temperature and dew point at different heights in the days that precipitation occurred,it is deduced that the average condensation height was 4 900 m a.s.l.in the summertime of 2008,and it might decrease to 4 460 m a.s.l.sporadically.In the studied mountainous area under the zone of maximum precipitation,mean annual precipitation increased with the height with a rate of 17.2 mm·(100 m-1),and summer precipitation increased with a rate of 11.5 mm·(100m)-1.