Runoff and Sediment load of the Yan River, China: changes over the last 60 yr

Runoff and sediment load changes are affected by climate change and human activities in an integrated way. Historical insight into these effects can not only improve the knowledge of river processes, but also promote more effective land and water management. In this study, we looked at runoff and sediment change in the Yan River basin, Loess Plateau, China, using data sets on land use and land cover (LUC), monthly data of precipitation and temperature, and observed data on runoff and sediment load from 1952 to 2010 at the Ganguyi Hydrologic Station. Available data on soil and water conservation structures and their effect were also studied. Five main findings emerged from the data analysis. (1) The annual runoff and sediment load varied greatly during the last 60 yr, and both had coefficients of variation that were much larger than those of precipitation and temperature. (2) Annual runoff and sediment load both showed a significant trend of linear decline over the period studied. The climate data showed a non-significant decline in precipitation over the same period, and a very significant increase in temperature; both can help explain the observed declines in runoff and soil loss. (3) Based on a mass curve analysis with anomalies of normalized runoff and sediment load, 4 stages in the change of runoff and soil loss were identified: 1951 to 1971 (Stage I), 1972 to 1986 (Stage II), 1987 to 1996 (Stage III) and 1997 to 2010 (Stage IV). (4)When years were paired based on similar precipitation and temperature condition (SPTC) and used to assess the impacts of human activities, it was found that 6 sets of paired years out of 12 (50 %) showed a decline in runoff, 8 (67 %) a decline in sediment load, and 9 (75 %) a decline in sediment concentration. The other sets show an increasing change with time. It showed the complexity of human impacts. (5) Human impacts relating to LUC change and soil and water measures in this basin were significant because of both the transfer of sloping cropland into non-food vegetation or terraces, and the siltation in the reservoirs and behind check dams. Data indicated that about 56 Mt of sediment was deposited annually from 1960– 1999 as a result of the soil and water conservation structures, which is significantly more than the average 42 Mt, leaving the Yan River basin as sediment load each year. Although the effects of climate change and human action could not be separated, analysis of the data indicated that both had a significant impact on runoff and sediment load in the area.