Xi'an lies on the Guanzhong Plain in central China, on a flood plain created by the eight surrounding rivers and streams, most of which are too polluted to be used as sources of fresh water.The city has an average elevation of 400 metres (1,312 ft) above sea level and an annual precipitation of 550 millimetres (22 in). The urban area of Xi'an is located at 34°16′N 108°56′E / 34.267°N 108.933°E / 34.267; 108.933 The Wei River provides potable water to the city.The city borders the northern foot of the Qinling Mountains to the south, and the banks of the Wei River to the north. Hua Shan, one of the five sacred Taoist mountains, is located 100 km away to the east of the city.At the beginning of Han Dynasty, Prime Minister Zhang Liang advised the emperor Liu Bang to choose Guanzhong as the capital of the Han Dynasty: 'Guanzhong Plain, which is located behind Xiao Pass and Hangu Pass, connects Long Plain and Shu Plain. Land of thousands miles and rich in harvest can be found here, as if this place is belongs to the nation of the heaven.' ("关中左崤函, 右陇蜀, 沃野千里, 此所谓金城千里, 天府之国也" 《史记·留侯世家》) Since then, Guanzhong is also known as 'Nation of the Heaven'.