Features and mechanisms of low flow runoff in the upper reaches of the Weihe River

Based on the monthly runoff data from Qin'an, Shetang, Tianshui, Wushan and Beidao Hydrology Stations of the main stream or main branches (Huluhe River, Niutouhe River and Xihe River) of the Weihe River for 50 years from 1956 to 2005, features of annual minimum low flow runoff, monthly minimum low flow runoff and duration of low flow are analyzed by using traditional probability statistics and time series analysis methods. Furthermore, formation mechanisms of low flow runoff and its extreme value are analyzed from two aspects as meteorology and human activity. The research indicates that annual mean runoff is many times larger than annual mean minimum runoff. The more complicated the underlying surface and the more frequent the human activities, the more percussive the minimum runoff. The minimum monthly mean runoff is more stable than the instant minimum runoff. Agricultural area shows a negative correlation with low flow runoff.