Impacts of land cover change on runoff and sediment yields in the headwater areas of the Yangtze River and the Yellow River, China
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摘要: 在长江和黄河源区的左冒西孔曲和纳通河、垮热洼尔玛河流域的不同植被覆盖下建立了天然径流观测场,利用观测天然降水和人工模拟降水,初步研究了江河源区不同植被覆盖下降水的产流产沙效应。结果表明,长江黄河源区的3个小流域内,在典型高寒草甸草地30°坡面上,退化较为严重的30%覆盖度以下的场地内,地表径流产出量明显大于覆盖度较好的95%、92%和68%场地,同时产沙量显著高于这3个场地,其平均单次降水形成的泥沙量是这三种盖度的2~4倍,由此造成地表侵蚀量平均为这3种盖度的3~10倍。通过对几次典型的降水形态的分析,在长江黄河源区高寒草甸草地的坡面上,不但降水量影响着产流产沙量,降水形态也影响着产流产沙量,降雨仍是引起水土流失的主要降水形态,在降水量相同的条件下,降雪可比降雨和雨加雪增加产流量2.1~3.5倍,可比降雨减少泥沙侵蚀45.4%~80.3%。人工模拟结果表明:对于覆盖度为5%和30%的强度退化草地,次降水量在3.5 mm时,就形成了较为明显的径流和产沙效应,当次降雨量达到7 mm,降雨持续时间15 min,5 m2场地内就会形成1 400 mL以上的径流量;在地表土壤含水量(FDR测0~5 cm平均含水量为36.7%)较高的情况下,次降雨量达4mm,降雨强度超过0.4 mm/min,在5 m2场地内历时5 min就能形成1 060 mL的地表径流,每100 mL径流中含泥沙高达1.6 g。这一试验结果在长江黄河源区3个不同的河源小流域是一致的。Abstract: 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.
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