Assessment of Contributions of Climatic Variation and Human Activities to Streamflow Changes in the Lancang River,China

The construction and operation of hydropower reservoirs on the Lancang River have drawn worldwide concern, because streamflow changes have occurred in this river since the introduction of dams. To address these concerns, it is necessary to quantitatively assess relative contributions of climatic variations and human activities to these changes. In this research, Mann-Kendall method was used to assess the trends in hydro-climatic data. The streamflow data were divided into a reference period (1956–1985) and a change period (1986–2008) based on hydropower reservoir construction timeline. The Back-Propagation Artificial Neural Network (BP-ANN) model was used to reconstruct natural streamflow. The contributions of climatic variations and human activities were investigated at the yearly, seasonal and monthly time scales. The results indicate that human activities exerted a slightly greater impact on flow changes than did climatic variations, at the yearly time scale (54.6 and 45.4 %, respectively). At the seasonal time scale, climatic variations made a greater contribution (65.8 %) during the wet season, while the contribution of human activities became the dominant factor during the dry season (85.3 %). At the monthly time scale, the contribution of climatic variations in January, June, August, and September was greater than that of human activities, while in the remaining eight months, human activities exerted a greater contribution than did climatic variation. The relative contributions of human activities and climatic variations (RCs) were also determined during the single- and cascade-dam periods; these did not always increase at the three time scales when dam system shifted from single-dam to cascade-dam.