Impacts of land cover change on runoff and sediment yields in the headwater areas of the Yangtze River and the Yellow River, China

The small Natong and Kuarewarma Rivers in the headwater areas of the Yellow River and the Fenhuoshan River in the headwater area of the Yangtze River,where vegetation is simple and dominated by the alpine cold meadows and the alpine grassland meadows,are selected as typical catchments to study contrastive the impacts of the alpine land cover change on the rainfal-runoff processes and the soil erosion. Based on the studies of the plots,the effects of runoff and sediment yield at the same gradient from July to August are tested in terms of three vegetation coverages of 30%,68% and over 92%.The results indicate that on the 30° slope in the alpine meadow zone and under the condition of the same precipitation,the runoff in the meadow zone that holds 30% of land cover is apparently larger than that in the meadow zone that holds 95% and 68% land cover;furthermore,the sediment yield is also larger than that of the later two,and the average sediment yield caused by one precipitation event is 2-4 times as larger as that of them.The results support the concept of vegetation as a major control upon runof and sediment production. Moreover,the simulat ive results indicate that for the moderately degraded alpine meadow land with a coverage less than 68%,the apparent ef ects of runoff and sediment yield happen when the secondary precipitation is 3.5 mm,and 1400mL runoff produce in the 5 m² field when the precipitation reaches 7 mm and lasts 15 minutes.The sediment content,runof and its formation frequency in the seriously degraded alpine meadow land with coverage below 30% are appar ently higher than those of the alpine meadow land holding a higher land cover,on average,2-7 times as larger as that of the later than those of the alpine meadow land holding a higher land cover on the about 2-4 times as large as that of the latter. In the headwaters area of the Yangtze and the Yellow Rivers,the alpine meadow coverage,topographic gradient and precipitation characteristics(time,intensity)all influence the runof and sediment yield apparently.