Understanding soil loss in two permanent gully head cuts in the mollisol region of Northeast China

Abstract. Gravitational mass wasting on steep slopes plays an important role in permanent gully development. This is typically driven by hydrological processes in the head cut and the hydromechanical response within the soil mass. In this study, erosion intensities were observed in the head cuts of two permanent gullies in the mollisol region of Northeast China during the rainy and snow melting seasons. To understand the physical process, soil water storage and drainage capacity, and suction stress during the 111 d of the rainy season and 97 d of the snow melting season, critical parameters such as soil moisture, temperature, and precipitation were investigated. This analysis also examined the increase in pore water pressure, dissipation properties, and hydromechanical properties of the mollisols. Under the same confining stress, the mollisols in the interrupted head cut of Gully No. II increased more rapidly and dissipated pore water pressure more than at the uninterrupted head cut of Gully No I. The combination of the soil water characteristic curve and the hydraulic conductivity function indicates that the mollisols of Gully No. II had a higher air entry pressure and saturated hydraulic conductivity during the wetting and drying cycles than Gully No. I. The head cut area of Gully No. II exhibited rapid water infiltration and drainage responses during rain events, with a high soil water storage capacity during torrential rain, rainstorms, and snow melting seasons. The absolute suction stresses within the mollisols of Gully No. II was lower than that in Gully No. I, which could lead to high erosion per unit of steep slope area. Soil loss from gravitational mass wasting on steep slopes is closely related to soil suction stress and we observed a correlation between erosion per unit gully bed area and the soil water storage. These findings have deepened our understanding of the physical process of permanent gully development from the perspective of the hydrological and hydromechanical behavior of gully head cuts.