· Human activities, such as dam construction, significantly altered the flow regimes in the Mekong River, particularly after the completion of two large dams, namely Xiaowan and Nuozhadu in 2010 and 2014, respectively. Stream flow data from 1960 to 2014 obtained from five stations located along the Mekong mainstream are divided into three periods, i.e., the pre-impact period (1960–1991), the transition period (1992–2009), and the post-impact period (2010–2014).
· The construction and operation of dams clearly have significant impacts on low pulse duration. It is observed that climate change dictated the changes in the annual stream flow during the transition period 1992–2009 (82.28%), whereas human activities contributed more in the post-impact period 2010–2014 (61.88%). The results of this study could provide a reference for reservoir operation in the upstream regions considering both ecological and economic benefits of such operations, as well as maximize the interests of stakeholders in this region.
* In river ecosystems, the flow regime of runoff plays a significant role in many fundamental ecological processes (Poff and Zimmerman, 2010). Changes in flow regimes within the context of climate change and human activities are significant to the hydrological community, receiving considerable global attention. Climate change and human activities have been considered as the two primary factors affecting flow regimes (Li et al., 2006 ; Ma et al., 2014). In some basins, human activities are the main factors that alter flow regimes, particularly during the construction and operation of large reservoirs (Poff et al., 1997 ; Fan et al., 2015). Climate change can also be the dominant factor that alters flow regimes (Li et al., 2006), which can change the pattern of precipitation and potential evaporation (Wang and Hejazi, 2011). Human activities, such as dam construction and water withdrawal activities (i.e., irrigation, industry, and municipal demands), directly change flow regimes (Ma et al., 2014), thereby changing river ecosystems (Gippel, 2001; Poff et al., 1997; Richter et al., 2003 ; Richter et al., 2006). The impacts of the two factors are often analyzed separately. However, the effects of climate change and human activities are always combined, and their effects on some river basins are difficult to identify. For example, climate change may cause changes in precipitation, increasing the impacts caused by dams as more water would be regulated by reservoirs during the long dry seasons (Lu et al., 2014).
Location of Mekong River Basin and the 6 completed dams and 19 planned dams
along the mainstream.
* Downstream of the LMB, several important ecological sites closely related to the flow regimes of the river exist; among them, Lake Tonle Sap and the Mekong Delta are the most famous. Tonle Sap, which is the largest lake in Southeast Asia (covering an area of 8800 km2 on average) and connected with the mainstream of the Mekong River at Phnom Penh, Cambodia, is regarded as the “natural reservoir” of the MRB. The water flowing into Tonle Sap from the Mekong River during the wet season (June to October) is nearly six times than that during the dry season (November to May) (Cochrane et al., 2014). Tonle Sap is rich in aquatic products and contains more than 300 species of freshwater fish. The downstream area of Phnom Penh is called the Mekong Delta, the area of which is ∼55,000 km2 (4000 km2 in Vietnam; 11,000 km2 in Cambodia). The Mekong Delta is subjected to frequent drought and flood events and influenced by tidal and seawater intrusion, particularly during dry seasons (Zhang et al., 2001).