DUAN Shuiqiang, FAN Shixiong, CAO Guangchao, LIU Xisheng, SUN Yongshou
Based on the Landsat satellite imagines over the source regions of the Yellow River from 1976 to 2014, forty-two lakes, of which the area was larger than 1 km2,have been interpreted. The results showed that Gyaring Lake and other forty medium and small lakes, except for the Eling Lake, wholly had appeared four changing processes: a steady stage (1976 to 1994), a shrinking stage (1994 to 2004), an expanding stage (2004 to 2010) and again a steady stage (2010 to 2014), with a minimum in 2004 and a sharply expanding in 2005; and in 2007 the area had exceeded that in 1976.Both the Gyaring Lake and the Eling Lake had shrunk by 1.4% with a smaller rate from 1994 to 2004.In 2005 the area of the Gyaring Lake had restored to that before the shrinking. The water level of the Eling Lake has risen rapidly after 2005, up to more than 4 270 m in July 2007.The average water level of the lake had reached 4 270.58 m from 2008 to 2014, 2.33 m higher than the water level (4 268.25 m) averaged from 1986 to 1999, with an expanded area of 30.0~45.2 km2.The total area of the forty medium and small lakes had shrunk from 288.0 km2 to 193.0 km2, with a rate of 33.0%, from 1994 to 2004, and a fastest shrinking year in 2004, followed by a rapidly expanding year in 2005.The annual mean changing rates of the lake area in the two years reached-14.5%·a-1and 32.9%·a-1, which were the maximum rates among all the study years. Analyzing the meteorological and hydrological data of the study area from 1956 to 2014 shows that air temperature had risen significantly during the 58 years, with an increasing rate of 0.32℃·(10a)-1. Evaporation, precipitation and runoff had increased significantly since 2003, 2004 and 2005 to 2014, respectively, with the increments of 53.8 mm (6.9%), 57.4 mm(18.5%) and 3.523×108 m3 (52.7%). Analyzing the relations between lake area change and climate change, hydrology change and human activities indicates that there is relatively small effect of precipitation and runoff supply changes on the Gyaring Lake and the Eling Lake, two large external flow lakes. The expansion of the Eling Lake is caused by water level uplift due to the downstream reservoir operation. The area change of the forty medium and small lakes has close relation with precipitation and runoff; recent expansion of the area is caused by significant increase in precipitation and runoff. In watershed scale, the temperature rise and evaporation enhancement are not the directly reasons for the lake expansion after 2005.
