Influence of topographic shadows on the thermal and hydrological processes in a cold region mountainous watershed in Northwest China

Authors: ZHANG, YANLIN - Hunan University Of Science And Technology; LI, XIN - Chinese Academy Of Sciences; CHENG, GUODONG - Chinese Academy Of Sciences; JON, HUIJUN - Chinese Academy Of Sciences; YANG, DAWEN - Chinese Academy Of Sciences; Flerchinger, Gerald; CHANG, XIAOLI - Hunan University Of Science And Technology; WANG, XIN - Hunan University Of Science And Technology

Submitted to: Journal of Advances in Modeling Earth Systems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/7/2018
Publication Date: 7/5/2018
Citation: Zhang, Y.L., Li, X., Cheng, G.D., Jin, H.J., Yang, D.W., Flerchinger, G.N., Chang, X.L., Wang, X., Liang, J. 2018. Influence of topographic shadows on the thermal and hydrological processes in a cold region mountainous watershed in Northwest China. Journal of Advances in Modeling Earth Systems. 10:1439-1457. https://doi.org/10.1029/2017MS001264.
DOI: https://doi.org/10.1029/2017MS001264

Interpretive Summary: The solar radiation incident in a mountainous area with complex terrain has a strong spatial heterogeneity due to the variation in slope orientation (self-shading) and shadows cast by the surrounding topography agents (topographic shading). Although slope self-shading has been well studied and considered in most land surface and hydrological models, topographic shading is usually ignored, and its influence on the thermal and hydrological processes in a mountainous area remains unclear. In this study, a topographic solar radiation algorithm with consideration for both slope self-shading and topographic shadows has been implemented and simply validated. Then the algorithm was incorporated into a distributed hydrological model with physically based descriptions for the energy balance, resulting in a base model with full topographical corrections for the solar irradiance in mountainous areas. Results show that the algorithm obtained a reasonable spatial-temporal estimation for the direct solar radiation occurrence with a timing error less than 20 minutes for the local sunset and sunrise at selected stations. The base model has a promising performance with most root mean square error smaller than 2 °C for the simulated ground temperature, and smaller than 0.1 for the liquid soil water content at various depths at the automatic weather stations. The simulated advancement of freezing and thawing fronts also agreed fairly well with fronts inferred from the observed ground temperature. A Nash-Sutcliffe coefficient of about 0.64 was achieved for the simulated daily discharge hydrograph at the basin outlet. After model evaluation, a control model was set up identical to the base model except without considering the topographic shading effect. Theoretical numerical analysis indicates that the simulated solar radiation incident in the study area in the control model was about 14.3 W/m2 higher than that in the base model on average, which in turn led to a higher simulated annual mean ground temperature at 4 m (by 0.41 °C) and evapotranspiration (by 16.1 mm/year), and a smaller permafrost extent (reduced by about 8%), as well as smaller maximal snow depth and shorter snow duration. Although the simulation was not improved for the discharge hydrograph in the base model, higher river runoff peaks and an increased runoff depth were obtained. In areas with rugged terrain and deep valleys, the influence of topographic shadows would even be stronger in reality than the presented results, which cannot be ignored when modelling the thermal and hydrological processes, especially in a refined model.

Technical Abstract: Solar radiation incident in a mountainous area with complex terrain has a strong spatial heterogeneity due to the variation in slope orientation (self-shading) and shadows cast by the surrounding topography (topographic shading). Although slope self-shading has been well studied and considered in most land surface and hydrological models, topographic shading is usually ignored, and its influence on the thermal and hydrological processes in a mountainous area remains unclear. In this study, a topographic solar radiation algorithm with consideration for both slope self-shading and topographic shadows has been implemented and tested. The model showed promising performance when compared to observations. In areas with rugged terrain and deep valleys, the influence of topographic shadows would even be stronger in reality than the presented results, which cannot be ignored when modelling the thermal and hydrological processes, especially in a refined model. These modifications to the hydrologic model will result in better and more reliable predictions of snowmelt, soil freezing, runoff, and streamflow from rugged mountainous terrain.