Understanding land use and cover change impacts on run-off and sediment load at flood events on the Loess Plateau, China

The Loess Plateau has been experiencing large-scale land use and cover changes (LUCCs) over the past 50 years. It is well known about the significant decreasing trend of annual streamflow and sediment load in the catchments in this area. However, how surface run-off and sediment load behaved in response to LUCC at flood events remained a research question. We investigated 371 flood events from 1963 to 2011 in a typical medium-sized catchment within the Plateau in order to understand how LUCC affected the surface run-off generation and sediment load and their behaviours based on the analysis of return periods. The results showed that the mean annual surface run-off and sediment load from flood events accounted for 49.6% and 91.8% of their mean annual totals. The reduction of surface run-off and associated sediment yield in floods explained about 85.0% and 89.2% of declines in the total annual streamflow and sediment load, respectively. The occurrences of flood events and peak sediment concentrations greater than 500 kg/m(3) showed a significantly downward trend, yet the counterclockwise loop events still dominated the flood event processes in the catchment. The results suggest that LUCC over the past 50 years resulted in significant changes in the water balance components and associated soil erosion and sediment transportation in the catchment. This was achieved mainly by reducing surface run-off and sediment yield during floods with return period of less than 5 years. Runoff-sediment load behaviour during the extreme events with greater than 10-year return periods has not changed. Outcomes from this study are useful in understanding the eco-hydrological processes and assisting the sustainable catchment management and land use planning on the Loess Plateau, and the methodologies are general and applicable to similar areas worldwide.