Base flow is one of the most important river hydrological characteristics in dry season. In recent years, with the global water issues becoming more prominent, base flow has become research focus in eco-hydrology. For the stability and hysteresis, base flow is significant to maintain river's ecological functions, determines river's ecological water requirement, and provides food and service for the locals in arid and semi-arid regions. The study area, upper reaches of Heihe River Basin, is located in the Qilian Mountains, northern part of Tibetan Plateau, China, with elevations of 2300-4000 m. The watershed area is 1.0009×104 km2.
The recursive digital filter technique was used to separate annual base flow of three main hydrological stations in the upper reaches of Heihe River. The methods of moving average, Mann-Kendall abrupt test and wavelet variance were employed to explain the characteristics of the annual change of base flow. Precipitation and temperature data of six adjacent meteorological observational stations and human activities were used to analyze the influencing factor of base flow. The results indicated that the Base flow Index (BFI) was greater than 0.4 in Qilian, Zhamashike and Yingluoxia stations. The notable increase of base flow was observed in 1979, when there was an abrupt point at Qilian and Yingluoxia stations. The base flow fluctuated at Zhamashike station. The periods of base flow were 11, 6 and 15 years respectively at Qilian, Zhamashike and Yingluoxia stations. The change of base flow was mainly affected by precipitation. The influence of human activities on base flow was another factor. The forest areas were decreased by human activities, which made the capacity of recharging and adjusting base flow in decline. In turn, the base flow went down and was greatly influenced by climatic change. The base flow change showed the consistency with the response of Northwest China to the global climate change and also reflects the differences in response to global climate change.
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