DETERMINING THE SPATIAL AND TEMPORAL IMPACTS ON EROSION OF HILLSLOPE LAND USE IN SOUTHWESTERN CHINA

Anthropogenic changes such as deforestation can have widespread impacts on soil, and erosional measurements are useful to quantify these effects and better understand their relationship to the environment. From 1950 to 1980, extensive deforestation of hillslopes was commonly practiced throughout southwestern China. Consequently, the Chinese government established the Returning Farmland to Forest Program (RFFP) in 1998 to combat flooding and downstream erosion. To analyze the effects of the deforestation and subsequent reforestation, we utilized short-term erosion estimates and the concentration of fallout radionuclides (FRNs) in detrital river samples. We hypothesize that deforestation efforts from the 1950s-1980s led to increased surface erosion, whereas reforestation practices from the late 1990s onward have resulted in decreased erosion.

Cs-137 and Pb-210ex provide depth and temporal constraints on erosion within upstream watersheds of detrital sediments. Cs-137 is a tracer of surface material erosion from the 1950s-1960s due to atmospheric nuclear weapons testing. The presence of Cs-137 in samples suggests minimal or shallow erosion since the 1950s. In contrast, due to its continual atmospheric decay from U-238, Pb-210ex provides a tracer for current surface material and indicates slow surface erosion at the present time. Therefore, basins affected by deforestation will show minimal Cs-137 whereas basins with reduced erosion should have detectable amounts of Pb-210ex.

Using gamma spectroscopy, we analyzed the presence and absence of these FRNs and concluded that the implementation of RFFP lowered hillslope erosion, decreasing sediment yield to river channel basins. Cs-137 presence in fine-grained silt-clay fractions is minimal (4 out of 133), while in coarse-grained medium sand fractions the isotope is slightly more abundant (20 out of 117). Pb-210ex presence in coarse-grained fractions is moderate (20 out of 57), indicating possible slowing of erosion. Preliminary data of fine-grained fractions have more detectable Pb-210ex (18 out of 23), further supporting our hypothesis. Our ongoing research examines the seasonality and replicability of isotope concentrations between summer and winter sampling periods.