EXAMINING LAND USE AND EROSION IN SOUTHWEST CHINA

Although deforestation is associated with increased erosion, prior research in southwestern China finds no increase in sediment yield corresponding with extensive deforestation starting in the 1960s. In order to understand the timing and nature of landscape response to deforestation in southwest China, we measured the fallout radionuclides unsupported 210Pbex and 137Cs in 19 in-channel detrital sediment samples collected at or near Chinese hydrology stations with upstream areas ranging from 114.8 km2 to 145,184 km2. These isotopes are found in the upper 30 cm of soil; their presence in river sediment implies shallow, slow erosion, while absence suggests rapid and/or deep erosion. We also compared isotope activity to land use and topographic parameters for each upstream basin.

Overall isotope activities are low and we find no correlation between unsupported 210Pbex and percent cultivated land or any other landscape factor. Three samples had both unsupported 210Pbex and 137Cs, eight had no detectable unsupported 210Pbex or 137Cs, and eight had unsupported 210Pbex but not 137Cs. The presence of 137Cs in only three samples suggests that there has been significant shallow erosion in this region since the 1960s removing at least the top 20-30cm of soil. Samples with measurable levels of both isotopes suggest that the upstream landscape has been fairly stable for the past 100 years, while the absence of both isotopes suggests a landscape shaped by extensive, modern surface erosion, or one with sediment sourced from deep erosional processes such as gullying and landsliding. Samples with 210Pbex but no 137Cs indicate rapid erosion followed by slower erosion, which corresponds with deforestation starting in the 1960s and ending in the 1990s with widespread reforestation policies. Although we find no correlations between land use or topographic parameters and isotope activities, overall low activities of isotopes and relative activities of each isotope are consistent with Chinese policies encouraging deforestation from the 1960s-1990s and reforestation since then.