Study area

The Akesu alluvial plain is bounded on the west by Keping Mountain, on the north by interjunction plain of Toushigan River and Kumalake River, on the east by Tailan River, and on the south by the Taklimakan Desert. The plain has an area of 4842 km². There are Xidaqiao hydrometric station at the upper boundary and Yimapaxia hydrometric station at the lower boundary of the study area. A wetland, locally known as Aiximan, locates in the west of the plain.

In the study area, annual evaporation exceeds precipitation much. It is a typical continental dry arid climate. Mean monthly temperature varies between -28and 27, and annual mean temperature is about 10(Tang, 2003). Annual precipitation is only 57 mm, and annual water surface evaporation is about 1900 mm (evaporating dish with a 20 cm diameter) (Tang et al., 2004). The annual reference evapotranspiration calculated from FAO Penman-Monteith formula is 1015 mm, which is about eighteen times of the precipitation (Tang, 2003; Allen et al., 1998). The river water from upstream is the main water source of the study area because of little precipitation. The stream flow at upper boundary control station Xidaqiao concentrates in high water period from May to September due to precipitation and glacier melting in upstream mountainous area in summer season. On the contrary, there is a minuscule amount of stream flow in low water period from October to April when the alluvial plain is dry.