Description and application of a distributed hydrological model based on soil–gravel structure in the Qinghai–Tibet Plateau
Abstract. The Qinghai–Tibet Plateau, known as the “Asian Water Tower”, has a thin soil layer with a thick gravel layer underneath. Its unique geological structure, combined with widespread snow and frozen soil in this area, profoundly affect the water circulation processes of the entire region. To thoroughly study the water cycle mechanism of the Qinghai–Tibet Plateau, this study considered the geological and climatic characteristics of this area and selected the Niyang River Basin as the study area. The Water and Energy transfer Processes in the Qinghai–Tibet Plateau (WEP-QTP) model was constructed based on the original Water and Energy transfer Processes in Cold Regions (WEP-COR) model. This model divides the single soil structure into two types of media: the soil layer and gravel layer. In the non-freeze–thaw period, two infiltration models based on the dualistic soil–gravel structure were developed based on the Richards equation in non-heavy rain periods and the multi-layer Green–Ampt model in heavy rain periods. During the freeze–thaw period, a hydrothermal coupling model based on the continuum of the snow–soil–gravel layer was constructed. This distributed hydrological model can dynamically simulate the changes in frozen soil and flow processes in this area. The addition of the gravel layer corrected the original model’s overestimation of the moisture content of the soil layer 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 reduces the temperature fluctuation of the surface soil, but also works with the gravel layers to revise the original model’s overestimation of the freeze–thaw speed of the frozen soil. 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 change in the frozen soil depth of WEP-QTP lags behind that of WEP-COR by approximately one month. There was more time for the river groundwater recharge, which better shows 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 is suitable for hydrological simulation of the Qinghai-Tibet Plateau.
Pengxiang Wang et al.
Pengxiang Wang et al.
Pengxiang Wang et al.
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The manuscript modified the a distributed hydrological model based on soil–gravel structure, and applied it to a watershed in the QTP. The topic fell into the scope of this journal and there are some issues need to be addressed as shown below:
About the novelty of the study. Considering the soil layer and unconfined aquifer in the cold region and QTP have been test for some previous studies, the author need to compare the current study with some previous studies. For example:
Song L., Wang L., Li X., et al. Improving permafrost physics in a distributed cryosphere-hydrology model and its evaluations at the upper yellow river basin. Journal of Geophysical Research: Atmospheres, 2020, 125(18):1-22.
Sun, A., Yu, Z., Zhou, J., Kumud,A., Ju, Q., Xing, R, Huang, D, Wen, L., 2020. Quantified hydrological responses to permafrost degradation in the headwaters of the Yellow River (HWYR) in High Asia. Sci. Total Environ. 712, 135632.
Gao B., Yang D., Qin Y., et al. Change in frozen soils and its effect on regional hydrology, upper Heihe basin, northeastern Qinghai-Tibetan Plateau, Cryosphere, 2018, 12(8):657-673
For the study area, the author need to describe the distribution of frozen soil. Where is the permafrost and seasonally frozen ground? For the experiment site, is it in permafrost region or seasonally frozen ground?
For figure 3, how do you determine the thickness of each soil layers? What is the maximum frozen depth of the study area? Do you consider the freezing front when you divide the soil layer?
Eq 14, I suggest to give the equations about how to calculate LE and H.
Are Supra-permafrost water and Sub-permafrost water both exit in the study area?
The radiation transfer in the snow layer are ignored in this study, the author may discuss the uncertainty from this? Another question, how do you estimate the snow albedo to get the net radiation of snow surface?
How do you calibrate the parameters of the hydrological model? And what are the major parameters you calibrated?
There seems an underestimation of river discharge by the WEP-QTP in the freeze season, why?
It seems that at 20 cm, the variation of soil temperature of WEP-QTP is reduced and the variation is lower than observations and WEP-COR, why?
I suggest to show the comparison of long term changes in the simulated runoff in the winter and summer and spring by different models and observations.