Permanent gully increases the heterogeneity of soil water retention capacity across a slope-gully system

The coexistence of permanent gully and slope farmland in the northern Loess Plateau catchment has experienced great changes since the implementation of the Grain for Green project in 1999. This program swaps agricultural production in the slope land for soil and water conservation measures. To date, the slope-gully system soil water retention capacity are poorly understood. In our study, undisturbed soil samples at 0-25 cm depth were taken from three transects along a gully bottom, bank and nearby slope, and the effects of gully terrain and slope position on the soil water retention curves (WRC) in two soil layers (surface layer: 0-12 cm; sub-surface layer: 12-25 cm) were evaluated. The results showed that: (1) with the increasing suction, soil water retention capacity within the vertical direction of the gully bottom was constant and increased on the nearby slope. For all the slope positions on the gully bank, the WRC shapes of surface soil differed from those of sub-surface soil; (2) The capillary rupture moisture in the surface soil at the lower (L-) slope of the gully bank occurred at low suction pressure. The available water content of the L-slope was lower than the upper (U-) slope in the gully bank; (3) The gully increased the complexity of soil pore distribution across the slope-gully system. The gully bank reduced the water retention capacity of surface soil. In conclusion, the permanent gully enhanced the heterogeneity of soil water retention capacity. These results provide specific information about hydrological functioning in gullied areas and improves our understanding of soil hydrology in the post-farmland environment in the northern Loess Plateau.