Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> The Qinghai-Tibet Plateau has a thin soil layer on top of a thick gravel layer, while its unique geological structure, combined with the snow and frozen soil in this area, significantly affect water circulation in the entire region. To investigate the mechanism of the underlying surface structure on the hydrothermal migration and water circulation process in the Qinghai-Tibet Plateau, we performed comprehensive study combining field experiments of the water and heat transfer processes and development of a Water and Energy transfer Processes model in the Qinghai–Tibet Plateau (WEP-QTP), based on the original Water and Energy transfer Processes model in Cold Regions (WEP-COR). The Niyang River Basin located on the Qinghai-Tibet Plateau was selected as the study area to evaluate the agreement between theoretical hypothesis, observation, and modeling results. This model divides the single soil into a dualistic structure of soil and gravel. In the non-freeze–thaw period, two infiltration models based on the dualistic soil-gravel structure were developed using the Richards equation in a non-heavy rain scenario and the multi-layer Green-Ampt model in a heavy rain scenario. During the freeze–thaw period, a hydrothermal coupling model based on the continuum of the snow-soil-gravel layer was constructed. The addition of the gravel layer corrected the original model’s overestimation of the moisture content below the surface soil and reduced the moisture content relative error (RE) from 33.74 to -12.11 %. The addition of the snow layer not only reduced the temperature fluctuation of the surface soil, but also, with the help of the gravel layers, revised the original model’s overestimation of the freeze-thaw speed. The temperature RE was reduced from -3.60 to 0.08 %. In the non-freeze-thaw period, the dualistic soil-gravel structure improved the regulation effect of groundwater on flow, stabilizing the flow process. The maximum RE at the flow peak and valley decreased by 88.2 and 21.3 %, respectively. In the freeze-thaw period, by considering the effect of the snow-soil-gravel layer continuum, the freezing and thawing process of WEP-QTP lagged that of WEP-COR by approximately one month. The groundwater simulated by WEP-QTP had more time to recharge the river, which better showed the "tailing" process after October. The flow simulated by the WEP-QTP model was more accurate and closer to the actual measurements, with Nash > 0.75 and |RE| < 10 %. The improved model reflects the effects of the Qinghai-Tibet Plateau special environment on the hydrothermal transport and water cycle process and can be used for hydrological simulation of the Qinghai-Tibet Plateau.
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