HGS RESEARCH HIGHLIGHT – Fractal Behaviors of Hydraulic Head and Surface Runoff of the Nested Groundwater Flow Systems in Response to Rainfall Fluctuations
Zhang, X., Li, H., Jiao, J. J., Luo, X., Kuang, X., Mao, R., & Hu, W. (2022). Fractal Behaviors of Hydraulic Head and Surface Runoff of the Nested Groundwater Flow Systems in Response to Rainfall Fluctuations. In Geophysical Research Letters (Vol. 49, Issue 2). American Geophysical Union (AGU). https://doi.org/10.1029/2021gl093784
“Numerical modeling is conducted using HydroGeoSphere. HGS solves the Richards equation for subsurface flow and wave approximations of the Saint Venant equation for surface flow simultaneously, in one fully integrated globally implicit matrix.”
Fig. 1. (a and b) A conceptual model for the problem. The green dashed lines in Figure 1b indicate 4 monitoring profiles. (c) Rainfall fluctuation and its spectral; (d) Fluctuations of hydraulic head in different depth (at x/L = 0.1) and their spectral of the Case 1.
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Researchers Xiaolang Zhang, Hailong Li, Jiu Jimmy Jiao, Xin Luo, Xingxing Kuang, Rong Mao and Wenli Hu have undertaken a comprehensive investigation into the behavior of nested groundwater flow systems (NGFS) in response to rainfall fluctuations and their influence on surface runoff. Through the utilization of a fully coupled variably saturated groundwater-surface water model alongside spectral analysis, the team delves into the fractal characteristics of hydraulic head and surface runoff across different scenarios. Their findings showcase how rainfall can induce changes in the inlets, outlets, and flow patterns of NGFS sub-systems, uncovering the concealed pathways through which water and chemicals traverse. Moreover, the study introduces the concept of scaling-exponent, offering insights into the variable behaviors within specific zones and the consistent trends within broader systems. Additionally, the proportion of baseflow in surface runoff emerges as a pivotal factor shaping the relationship between fractal behavior and watershed size.
At the heart of this investigation lies the integration of HydroGeoSphere (HGS), a sophisticated modeling platform known for its ability to simulate coupled surface water-groundwater interactions with unparalleled accuracy. By leveraging HGS, the researchers were able to simulate the complex dynamics of NGFS under transient conditions, providing valuable insights into how these systems respond to rainfall fluctuations. The versatility of HGS allowed for the exploration of various scenarios, shedding light on the relationship between rainfall, groundwater flow, and surface runoff.
Plain Language Summary:
This study investigates the responses of the unsaturated zone, groundwater flow systems, and surface runoff to rainfall fluctuations. The results reveal that the inlets and outlets of the sub-systems can change with rainfall fluctuations. This has strong implications for understanding the sources and destinations of solutes in an aquifer because these sub-systems are hidden channels of water flow and also conveyors for chemicals or pollutants. The method to separate the sub-systems under steady-state conditions is not applicable under transient conditions. Our study finds that the vertical distributions of the fractal behaviors of the hydraulic heads can be used to separate sub-systems of the groundwater flow systems under transient conditions. The proportion of the baseflow in the surface runoff can determine whether the fractal behaviors of the runoff are linearly related to the watershed size. This can explain the controversy in previous studies that the fractal behavior of river runoff can have either positive linear relation or no relation with its watershed size. The spectral analysis is proven to be a useful way of analyzing the hydraulic features of groundwater and surface runoff under transient conditions.
Fig 2. (a–d) Seasonal changes of the flow patterns of Case 1 in response to rainfall fluctuations. Panels (e and f) are the insets of (a) and (d), respectively. Darcy velocity magnitude Log(V) (|V|, m/day) contours (in color bands), streamlines (black solid lines), velocity direction (black arrows with uniform length), and transient SP (in yellow circle).
Abstract:
The responses of the nested groundwater flow systems (NGFS) to rainfall fluctuations and the impacts of the NGFS on surface runoff are explored using a fully coupled variably saturated groundwater-surface water model in conjunction with spectral analysis. Numerical experiments are designed to investigate the fractal behaviors of hydraulic head and surface runoff in different scenarios. The results show that the inlets, outlets, and flow patterns of the sub-systems of NGFS can change with rainfall. The scaling-exponent, defined as the slope of the power spectra of the fluctuations of hydraulic head or runoff, is used to depict the fractal behaviors. The scaling-exponent of the hydraulic head is highly variable within the unsaturated zone and local flow systems but remains constant within the intermediate and regional systems. Whether the scaling-exponent of surface runoff exhibits a linear relation with the watershed size depends on the proportion of baseflow in the runoff.