DETERMINING SEDIMENT SOURCES AND SINKS IN A SMALL HEADWATER WATERSHED IN YUNNAN, CHINA USING SHORT-LIVED RADIONUCLIDES
We use radioisotope fingerprinting to better understand the relationship between fluvial sediment transport and upstream land use. Local agricultural techniques are typified by monoculture farming, often done on steep hillslopes; mean slope is 21.48°. Long-term, Chinese government hydrologic data makes Yunnan an ideal study site. Field observations, qualitative interpretation of Google Earth images, and Landsat 8 image analysis were used to understand land use trends.
A basin-wide absence of 137Cs in samples with 210Pbex indicates extensive surface erosion since the 1960’s that subsequently slowed down. The basin is divided into four regions: center upstream, western and eastern tributaries, and a main stem. The center upstream has the greatest percentage of farmed land (18.14%), including tobacco farming, which tends to increase erosion rates; and also has significantly lower 210Pbex activity than the rest of the watershed (p<0.002). The center upstream tributaries have an average 210Pbex activity of 0.0015 Bq/g (σ 0.0015 Bq/g), while the other areas have a mean activity of 0.0058 Bq/g (σ 0.0019 Bq/g). If contemporary erosion levels were higher, 210Pbex and 137Cs would not likely be found in the fluvial sediment of corresponding tributaries. However, relatively high 210Pbexactivity (0.0065 Bq/g) in the main stem suggests that the signature of intensive erosion is not present downstream, indicating that sediment from the central tributaries is being stored upstream.
These results suggest that the effects of land use on sediment supply to rivers could be quite localized in steep, agricultural landscapes, and may not translate into significant effects downstream in larger watersheds. This could partially explain why local and regional hydrologic records show no correlation between sediment yield and upstream land use: sediment yield may not accurately show the effects of land use changes on erosion.