Simulating the role of gravel on the dynamics of permafrost on the Qinghai-Tibetan Plateau

Abstract. Gravel (particle size ≥ 2 mm) is common in soil profiles of the Qinghai-Tibetan Plateau (QTP). It has different thermal and hydrological properties than other fine mineral soils (particle size < 2 mm), which may have significant impacts on the thermal and hydrological processes of soil. However, few models have considered gravel. In this study, we implemented the thermal and hydraulic properties of gravel into the Dynamic Organic Soil-Terrestrial Ecosystem Model to develop new schemes to simulate the dynamics of permafrost on the QTP. Results showed that: (1) the widely used Farouki thermal scheme always simulated higher thermal conductivity of frozen soils than unfrozen soils with the same soil water content; therefore it tends to overestimate permafrost thickness strongly; (2) there exists a soil moisture threshold, below which the new set of schemes with gravel simulated smaller thermal conductivity of frozen soils than unfrozen soils; (3) soil with gravel has higher hydraulic conductivity and poorer water retention capability; and simulations with gravel were usually drier than those without gravel; and (4) the new schemes simulated faster upward degradation than downward degradation; and the simulated permafrost thicknesses were sensitive to the fraction of gravel, the gravel size, the thickness of soil with gravel, and the subsurface drainage. To reduce the uncertainties in the projection of permafrost degradation on the QTP, more effort should be made to: (1) developing robust relationships between soil thermal and hydraulic properties and gravel characteristics based on laboratory work; and (2) compiling spatial datasets of the vertical distribution of gravel content based on measurements during drilling or the digging of soil pits.