Effects of cultivation and reforestation on suspended sediment concentrations: a case study in a mountainous catchment in China

摘要

Understanding how sediment concentrations vary with land use/cover is critical for evaluating the current and future impacts of human activities on river systems. This paper presents suspended sediment concentration (SSC) dynamics and the relationship between SSC and discharge (Q) in the 8973 km(2) Du catchment and its sub-catchment (4635 km(2)). In the Du catchment and its sub-catchment, 4235 and 3980 paired SSC-Q samples, respectively, were collected over 30 years. Under the influence of the Household Contract Responsibility System and Grain-for-Green projects in China, three periods were designated, the original period (1980s), cultivation period (1990s) and reforestation period (2000s). The results of a Mann-Kendall test showed that rainfall slightly increased during the study years; however, the annual discharge and sediment load significantly decreased. The annual suspended sediment yield of the Du catchment varied between 1.3 x 10(8) and 1.0 x 10(10) kg, and that of the sub-catchment varied between 6.3 x 10(7) and 4.3 x 10(9) kg. The SSCs in the catchment and sucatchment fluctuated between 1 and 22400 gm(-3) and between 1 and 31800 gm(-3), respectively. The mean SSC of the Du catchment was relatively stable during the three periods (+/- 83 gm(-3)). ANOVA (analysis of variance) indicated that the SSC did not significantly change under cultivation for low and moderate flows, but was significantly different under high flow during reforestation of the Du catchment. The SSC in the sub-catchment was more variable, and the mean SSC in the sub-catchment varied from 1058 +/- 2217 gm(-3) in the 1980s to 1256 +/- 2496 gm(-3) in the 1990s and 891 +/- 1558 gm(-3) in the 2000s. Reforestation significantly decreased the SSCs during low and moderate flows, whereas cultivation increased the SSCs during high flow. The sediment rating curves showed a stable relationship between the SSC and Q in the Du catchment during the three periods. However, the SSC-Q of the sub-catchment exhibited scattered relationships during the original and cultivation periods and a more linear relationship during the reforestation period.