QUANTIFYING LAND USE AND EROSION IN SOUTHWEST CHINA

Within the past sixty years, shifts in modern Chinese land use have changed the landscape of rural southwestern China. These policy changes were characterized by vast deforestation, often for agricultural purposes, and occurred in three periods. The “Three Great Cuttings” include the Great Leap Forward from 1958-1961, the Cultural Revolution from 1966-1976, and Deng’s Reform Era from 1978-1988. The “Great Cuttings” period corresponds with the time of cesium deposition related to nuclear weapons testing, with peak deposition in 1963. Following catastrophic flooding on the Yangtze in 1998, mandatory national reforestation efforts took place which are among the world’s largest Payments for Ecosystem Services programs.

Our study focuses on the Apiladda river valley in the Yangjuan Yi village, Sichuan Province, China (30 km²). Since the first Great Cutting, flooding and bank erosion have greatly impacted agricultural livelihoods and the environment. Thus, we are interested in examining how modern land use policies have influenced erosion and sediment sourcing. We combine GIS, repeat photography, and sediment fingerprinting techniques to answer this question. These techniques allowed us to further calculate and interpret geomorphological change.

Our GIS analysis reveals the channel widening considerably since 2008, partly due to the largest flood in 60 years (August 2015). Repeat photography (photos from 2008, 2010 and 2015) shows that the local restoration effort has been somewhat successful (sediment around fences, Lincoln logs, and poplar trees), but large floods can still wreak havoc.

We used an HPGe detector to measure activity of Cs-137 and Pb-210 in detrital sediment samples collected from the active channel. We find Cs-137 was undetectable, implying heavy erosion at the time of deposition (1950s-1960s), likely due to deforestation-induced erosion at that time. Yet, we find unsupported Pb-210 in three nested watersheds in the Yangjuan region all sourced from the same less-disturbed landscape upstream. These higher quantities of lead imply some modern accumulation, possibly due to reforestation.

Thus, we can conclude a correlation exists between the changes in land use policies and the increase in surface erosion. Moreover, reforestation has played a crucial role in mediating the problem to some degree.