INTEGRATED ASSESSMENT AND ANALYSIS OF PHYSICAL LANDSCAPE PROCESSES THAT IMPACT THE QUALITY AND MANAGEMENT OF AGRICULTURAL WATERSHEDS
Location: Watershed Physical Processes Research Unit
Title: Macropore Flow and Mass Wasting of Gullies in the Loess Plateau, China
Submitted to: International Journal of Sediment Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 10, 2005
Publication Date: September 1, 2005
Citation: Wilson, G.V., Xu, M., Chen, Y., Liu, G., Romkens, M.J. 2005. Macropore Flow and Mass Wasting of Gullies in the Loess Plateau, China. International Journal of Sediment Research. 20(3): 249-258.
Interpretive Summary: Soils in the north central area of China were developed from wind blown sediment called loess. Much of the loess region is being converted from row-crop agriculture to grass vegetation. The effects of this conversion on infiltration and gullies has not been established. Gullies can form by collapsing of large pores due to erosion of the pore walls and gullies can enlarge by slumping off of gully walls due to flow through large pores. This study assessed changes in infiltration following conversion to grass vegetation and the amount of soil loss by slumping of gully walls. Areas that had been vegetated with grass for different lengths of time were used to count the number of large pores in a fixed area and measure their size and the infiltration rate into these large pores. The number and size of large pores was also measured on adjacent gully walls and the amount of soil loss by slumping was determined. The total number of large pores and the volume of large pores per volume of soil increased with time following conversion of land to grass vegetation if the land was tilled before the conversion but not if the land was contour-ditched. The infiltration rate through individual large pores was 100 times higher than the infiltration rate through the soil surfaces outside these pores. If the land was recently (<1 year) converted to grass, the pore walls eroded and enlarged by over 200% during the infiltration event. The number of large pores on the gully walls was three times higher and the porosity of the large pores four times higher than on the soil surfaces. Therefore, the large pores are likely to cause rapid flow from the ridge areas to the gullies and result in gullies expanding due to slumping of gully walls. The total soil loss due to slumping of gully walls was 43,374 lbs per acre which is many times higher than observed for runoff on the soil surface.
Due to the extensive gullying from historically excessive erosion in the loess plateau of China, much of this region is being removed from cropping and converted to native grass and shrub vegetation. The effects of this conversion on soil physical properties that result in preferential flow have not been established. Tunnel scour and mass wasting are important gully erosion processes resulting from preferential flow through macropores (pores > 1 mm diameter). The objective of this study was to assess the changes with time in macropore flow characteristics of soils on the loess plateau following conversion to grass vegetation and the associated degree of mass wasting of gully faces. Ridge areas that had been revegetated for 1 year, 6 year, and > 15 years, following tilling and for 6 years following contour-ditching and the adjacent gully faces were characterized for their macropore and soil matrix properties on a 50 cm by 50 cm area. The macropore and soil matrix infiltration rates were measured on the ridge areas and soil loss by mass wasting was measured on gully faces. The total number of macropores increased from 11.6 m-2 to 39.6 m-2 from 1 to 6 years and to 51.6 m-2 after 15 years of revegetation following tillage. The macroporosity increased from 0.0008 m3 m-3 to 0.0018 m3 m-3 from 1 to 6 years of revegetation following tillage but the lowest macroporosity (0.0005 m3 m-3) was 6 years of revegetation following contour-ditching. The contour-ditched area had the lowest infiltration rate (95 m d-1) through the soil matrix (areas without macropores) with the tilled areas having similar infiltration rates regardless of the years revegetated (averaged 146 m d-1). Due to macropore erosion in the 1 year revegetated area enlarging pore diameters by > 200% during the infiltration event, this condition had the highest macropore infiltration rates (7967 m d-1). Macropores in all other areas were stable, no macropore erosion. The 6 year, tilled areas had significantly higher macropore infiltration rates (4665 m d-1) than the 6 years since contour-ditching areas (91 m d-1). The number of macropores on the gully faces was triple (92.8 m-2) and the macroporosity quadruple (0.004 m3 m-3) that of the ridge surfaces. The upper gully faces exhibited 1.1 slumps m-1 for a total soil loss of 48622 kg ha-1.