Characteristics of gully development in Northern Shaanxi Hilly Loess Region before Grain-for-Green Programme (1976-1997)

Abstract: Gully erosion is a serious environmental problem and the primary source of sediment loss on the Loess Plateau. However, due to the limited technologies and approaches for monitoring gully, most of the early studies focused on the qualitative description of gully morphology and the division of gully development stages, confining the advancement of quantitative research for gully erosion and leading to insufficient knowledge about gully erosion. The objective of this study was to quantitatively estimate the gully development rates as well as assess the effect of land use and vegetation coverage in upslope drainage area on gully development over 1976-1997 period, after which the Grain-for-Green Programme had been completely implemented since 1999. The study area was in Fangta watershed in Ansai County (109°19’E, 36°52’N), which is located in the northern Shaanxi hilly loess region. Seven large gullies, as well as the small gullies which occurred along the large gullies, were selected along a transect placed randomly and throughout Fangta watershed. ArcGIS and MATLAB software were used to obtain boundary lines of the selected large gullies based on the DEM (digital elevation model) derived from topographic maps at the scale of 1:10000 in 1976 and 1997. By applying LOG (Laplacian of Gaussian) edge detection approach in MATLAB, the gully boundary lines were initially extracted; furthermore, in order to improve the accuracy of the boundary lines, visual interpretation based on the morphology reflected by contour line was conducted in ArcGIS. According to the gully boundary lines, the area and perimeter of large gully and the head retreat distance of small gullies within each large gully in 1976 and 1997 were calculated, respectively. Details about land use and vegetation coverage were extracted from the historical maps of land use and vegetation cover, respectively. The results showed that the rates of area and perimeter development of the 7 large gullies from 1976 to 1997 ranged from 11.01% to 180.46% and from 8.07% to 86.75%, respectively, and the mean head retreat distance of the small gullies within a large gully varied from 0.26 to 0.84 m/a. Head retreat of small gully caused by headward erosion was the dominant erosion form presented within a large gully in study area. In addition, valley widening and forming new small gullies forming could significantly contribute to the area and perimeter development of a large gully, respectively. Forest and grass coverage were both helpful to control gully development, especially when vegetation coverage exceeded 65% in upslope drainage area, which was more effective for controlling multiple erosion processes that simultaneously took place in a large gully, in which valley widening was mainly involved, as well as headward erosion and sidewall expansion of small gully. However, vegetation coverage exceeding 45% in upslope drainage area could only control the headward erosion of small gully effectively. Terrace could also control gully development and its efficiency depended on scale and position of terrace on the slope. This study indicates that the forms and rates of gully erosion can be determined based on GIS technology and topographic map, accomplishing the monitoring of gully dynamic development before implementing the Grain-for-Green Programme.