Assessment of soil shear strength in gully systems: A case-study in the Wangjiagou (WJG) watershed on the Loess Plateau of China

摘要

Soil erodibility is one of the major parameters for soil erosion prediction and conservation planning. Soil shear strength is an effective indicator of soil erodibility. Although shear strength has been extensively measured on agricultural lands, very limited measurements have been made within gullies. In this study, shear strength was measured at various geomorphic positions of gullies, in different types of deposits and gullies, as well as in inter-gully areas in the Wangjiagou watershed on the Loess Plateau. In total, over 5,000 measurements were taken in the field prior to the rainy seasons in 2006 and 2008. The results demonstrated: (a) there is a drastic difference in soil shear strength among different types of geomorphic features and deposits in the gullies, with the highest on gully sidewalls and the lowest in new dry flow materials (a mean value of 110.7 vs. 1.3 kPa); (b) Among different types of gullies, the mean shear strength of headwater gully and hillside gully shows no statistically significant difference (14.8 vs. 14.3 kPa), but is much smaller than that of valleyside gully (26.4 kPa); (c) Soil shear strength has much greater variation in the gully area than in the inter-gully area, with a standard deviation of 28.3 versus 12.3 kPa. Different erosion/deposition processes play a major role in the spatial heterogeneity of soil shear strength in the gully area, whereas land use and tillage practices are the controlling factors in the inter-gully area; (d) All types of mass failures reduce soil shear strength dramatically, and therefore increase subsequent water erosion.