Temporal change of runoff and sediment load and their differential response to human activities: A case study for a well-vegetated mountain watershed of southern China

Abstract

This study examined the temporal trends of runoff and sediment load and their differential response to human activities in the Lishui river, a tributary of the Yangtze river in southern China. The long-term observation data at four gauging stations, generally involving two periods from 1954 to 1985 and from 2007 to 2011, were used. We detected no significant temporal trend for both the annual runoff volume (Q) and the annual suspended Sediment Load (SL) over more than 30 years before 1985. The flow duration curves and the Suspended Sediment Concentration (SSC) also hold constant before 1985. Compared with the period before 1985, SL has decreased by about 80% though Q remains unchanged for the period after 2007. Detailed examination shows that the flow duration curves after 2007 have changed with a significant decrease in the high-flow component, which acts as a major cause for the decreasing SL. In addition, SSC has decreased by several times, which also contributes to the decrease in SL after 2007. Both decreases in high-flow discharges and in SSC can be linked with recent human activities, mainly including vegetation establishment and dam constructions. The constant Q and the decreasing SL are also reported for the main stream of the Yangtze River and other major rivers in southern China, although they are orders of magnitude larger than our study area in drainage area size. The present study highlights the importance of high-flow discharges on SL and suggests that the use of SL is more appropriate to reflect environmental change than Q.