Distributed hydrological model of the Qinghai Tibet Plateau based on the hydrothermal coupling: Ⅰ: hydrothermal coupling simulation of "snow-soil-sand gravel layer" continuum

The Qinghai-Tibet Plateau, known as the "Asian Water Tower", is a typical alpine mountain area. Snow-covered permafrost and seasonally frozen soil widely exist on this plateau, profoundly affecting the water cycle process of the entire region. In addition, the Qinghai-Tibet Plateau has the characteristics of a thinner soil layer and a thicker underlying sand and gravel layer, forming a special "snow-soil-sand gravel layer" hydrothermal medium structure. To further study the water cycle mechanism of the Qinghai-Tibet Plateau, this study selected the Niyang River basin as a typical area to perform field-heat-coupling experiments in the field. Combined with the geological and climatic characteristics of the Qinghai-Tibet Plateau, a 12-layer "snow-soil-sand gravel layer" continuum was constructed to describe the hydro-thermal coupling model of the Qinghai-Tibet Plateau. A complete hydro-thermal coupling simulation equation and parameter calculation method are described. The model was validated using the measured results of the temperature, liquid water content, and freezing depth of the soil and gravel layer in the freezing and thawing period of 0-160 cm from 2016 to 2017. The mean values of the simulated R² of the temperature of each layer and the moisture content during freezing and thawing were 0.91 and 0.52, respectively. The R² of the soil freezing depth is 0.76. The results show that the model can better reflect the hydrothermal variation of stratified soil and sand gravel during freezing and thawing processes. The results show that the model has adequate applicability in the Qinghai-Tibet Plateau and can reflect the special change process of moisture and temperature of the soil and gravel layer during the freezing and melting processes in this area.