Abstract
This study aimed to improve runoff simulations and explore deep soil hydrological processes for a watershed in the center of the Loess Plateau (LP), China. This watershed, the Wuding River Basin (WRB), has very complex topography, with soil depths ranging from 0 to 197 m. The hydrological model used for our simulations was the Community Land Model (CLM) version 5 developed by the National Center for Atmospheric Research. Actual soil depths and river channels were incorporated into CLM to realistically represent the physical features of the WRB. Through sensitivity tests, CLM with 150 soil layers produced the most reasonable results and was adopted for this study. Our results showed that CLM with actual soil depths significantly suppressed unrealistic variations of the simulated sub-surface runoff when compared to the default simulations with a fixed soil depth of 8 m. In addition, CLM with higher-resolution soil layering slightly improved runoff simulations, but generated simulations with much smoother vertical water flows that were consistent with the uniform distribution of soil textures in our study watershed. The runoff simulations were further improved by the addition of river channels to CLM, where the seasonal variability of the simulated runoff was reasonably captured. Moreover, the magnitude of the simulated runoff remarkably decreased with increased soil evaporation by lowering the soil water content threshold, which triggers surface resistance. The lowered threshold was consistent with the loess soil, which has a high sand component. Such soils often generate stronger soil evaporation than soils dominated by clay. Finally, with the above changes in CLM, the simulated total runoff matched very closely with observations. When compared with those for the default runoff simulations, the correlation coefficient, root-mean-square error, and Nash Sutcliffe coefficient for the improved simulations changed dramatically from 0.02, 10.37 mm, and −12.34 to 0.62, 1.8 mm, and 0.61. The results in this study provide strong physical insight for further investigation of hydrological processes in complex terrain with deep soils.
Highlights
Understanding runoff processes in regions with very complex topography is important to managing and predicting water resources
The default soil depth in Community Land Model version 5 (CLM5) is set to a constant of 8.03 m and is discretized into 20 layers defined as hydrological active layers (HALs) to distinguish them from the five bedrock layers set in the model
We examined how the simulated runoff for the Wuding River Basin (WRB) was affected by the actual soil depths (40-197 m) that were inputted into CLM5 with a default soil layer numbers (SLNs) of 20
Summary
Understanding runoff processes in regions with very complex topography is important to managing and predicting water resources. Complex topography leads to intricate runoff processes (Jencso et al, 2011), causing uncertain estimation of water resources. It is essential to accurately estimate the spatiotemporal distribution of water resources in this region of complex terrain. Soil depth in the LP can reach 350 m (Zhu et al, 2018; Li et al, 2019), making it difficult to measure deep soil hydrological processes and understand runoff generation (Shao et al, 2018; Liu et al, 2012). Terrain in the LP includes loess tablelands, ridges, hills, gullies, and river channels (Fu, 1989), all of which have quite different runoff generation processes (Liu et al, 2012). In loess tablelands with deep water tables (Huang et al, 2013; Shao et al, 2018), the soils store most infiltrated water, generate insignificant surface runoff, and remarkably delay sub-surface runoff
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