Streamflow changes of the Changjiang (Yangtze) River in the recent 60 years: Impacts of the East Asian summer monsoon, ENSO, and human activities

Streamflow in the Changjiang River has experienced significant changes in recent decades due to the coupling of environmental factors and intensive anthropogenic activities in associated catchments. Based on a long-term data set, including water discharge, precipitation, temperature, East Asian Summer Monsoon (EASM), El Niño – Southern Oscillation (ENSO), and reservoir volumes in the most recent 60 years, the modes of streamflow changes along the Changjiang and associated factors was discussed. Analysis of streamflow observations by empirical mode decomposition show that streamflow along the Changjiang consist of a trend and four intrinsic components. Trend component in streamflow had obvious downward changes, which could be mainly attributed to dam construction. In addition, increased snowmelt caused by a warming climate led to more water being discharged into the upper reaches. The resultant intrinsic component of streamflow changes can be characterized by two modes using empirical orthogonal function analysis. The main mode represents periodic oscillations in baseflow due to the coupling of a weak EASM and weak ENSO. The secondary mode reflects differences in streamflow changes between the upper and lower reaches of the Changjiang River, which is anti-phased relative to changes in streamflow between the upper and lower reaches. These differences may be caused by a weak EASM and intensive ENSO. Moreover, the combination of a weak EASM and ENSO can lead to extreme flood. Extreme drought years may be significantly impacted by intensive EASM and ENSO.