Understanding the soil loss at the permanent gully headcut area in the Mollisols region of Northeast China

Abstract. The development of permanent gullies can trigger both gravitational mass-wasting on over-steepen slopes and water erosion on the channel bed. This hydrogeomorphic process is typically driven by the hydrology process of the headcut area and the hydro-mechanical response within the soil mass. In this study, erosion intensities were observed at the headcut area of two permanent gullies in the Mollisols region of Northeast China during the rainy and snow-melting seasons. To understand water storage capacity and leakage process, as well as the suction stress level during the rainy and snow-melting seasons, critical parameters such as soil moisture, temperatures, and precipitation amounts were investigated. This analysis also incorporated the effects of pore water pressure rising and dissipation properties, and hydro-mechanical properties of Mollisols. The Mollisols at the interrupted headcut area of gully No. II exhibited a higher ratio and proxy of pore water rising and dissipation than those at the uninterrupted headcut of gully No. I. Moreover, the combination of soil and water characteristic curve along with the hydraulic conductivity function (HCF) indicate that the Mollisols of gully No. II has relatively higher air-entry pressure and saturated hydraulic conductivity during wetting and drying cycles than those of gully No. I. The headcut area of gully No. II exhibited rapid water infiltration and leakage responses during rain events, with high capacity in the water storage during torrential rain, rainstorm, and snow-melting season. Overall, the absolute suction stress within the Mollisols of gully No. II was lower than that of gully No. I, which could lead to high erosion intensity on the over-steepen slope. Importantly, we provided new evidence that the area erosion intensity of gravitational mass-wasting on the over-steepen slope was closely related to the soil suction stress level. Additionally, we observed a correlation between the erosion intensity of the gully bead near the headcut and the soil water storage. The findings of this study significantly deepen our understanding of the physical process of permanent gully development in the Mollisols region. We provide important insights that the accuracy of the Universal Soil Loss Equation could be improved by accounting for the effects of soil water storage pattern and soil suction stress status.