HGS RESEARCH HIGHLIGHT – Disentangling runoff generation mechanisms: Combining isotope tracing with integrated surface/subsurface simulation
Chen, X., Yu, Z., Yi, P., Aldahan, A., Hwang, H.-T., & Sudicky, E. A. (2023). Disentangling runoff generation mechanisms: Combining isotope tracing with integrated surface/subsurface simulation. In Journal of Hydrology (Vol. 617, p. 129149). Elsevier BV. https://doi.org/10.1016/j.jhydrol.2023.129149
“The integrated analysis combining [HydroGeoSphere] simulation with isotope tracing revealed useful details about soil water storage and release processes and the complex surface–subsurface exchange of water.”
A new study co-authored by researchers at Hohai University and Aquanty Staff introduces an effective way to track runoff generation in a headwater catchment by combining isotopic tracer analysis and integrated hydrologic modelling using HydroGeoSphere. Many methods have been used to infer runoff generation methods, and the use of two methods in conjunction in this study helped to address the limitations of past methodologies.
Chen, X., et al (2023)
This research took place in a small study area on the Xin’an River in Eastern China. The authors used rainfall, temperature, and potential evapotranspiration data from 2001 to 2015 to calibrate the model. The resulting model was used to disentangle runoff generation mechanisms for a short rainfall event from 17 to 22 July, 2014. By tracking the signature of stable isotopes (δ2H and δ18O) in the stream prior to and after the rainfall event the authors could perform an isotope mass balance to determine the sources of streamflow and runoff. Runoff was categorized into three categories, rain-induced overland flow, exfiltration-induced overland flow, and subsurface flow, which amounted to 9%, 59% and 23% of streamflow respectively. The numerical HGS model was used to validate the results of the isotopic mass balance and model the characteristics of flow during the rainfall event.
This process helps determine what the capacity of the shallow soil layer is and what happens when the soils saturation limit is reached. When there is intense rainfall, the soil’s saturation point is rapidly reached, and overland flow occurs. This can be influenced by soil texture, topography, meteorology, and hydrological conditions. HydroGeoSphere proves to be the ideal software for this situation as its fully integrated capabilities allow for all components of the terrestrial hydrological cycle to be accounted for. Combining this software with isotope tracing to evaluate the mixing process between hillslope surface and shallow soil allowed for complex runoff processes to be accurately simulated. This mixing process of the shallow soil and rainwater is important for hillslope overland flow and runoff generation in headwater catchments.
“To more clearly understand how and where the stream runoff was generated, the temporal and spatial origins of the stream runoff were deduced using both isotopic analysis and numerical modeling.”
Abstract:
The majority of runoff studies to date have reported a similar pattern of rapid rain-to-runoff conversion via overland flow and shallow subsurface water flow, as well as the ubiquity of thresholds and hysteresis in rainfall-runoff response. But each of those approaches has limitations in terms of inference and process description. Processes of water transport and mixing and how they influence runoff generation are still not well understood or quantified. In this study, HydroGeoSphere, a fully-integrated surface/subsurface flow model, was used together with stable isotopes for disentangling runoff generation mechanisms in a headwater catchment. The study area (0.21 km2) is the first-level tributary of the Xin’an Jiang River located in a humid climate region of eastern China. Water flow simulation elucidated the spatial water transport process, and isotope tracing quantified the complex water mixing process between the shallow soil and hillslope surface. This study provides insights into the rapid transformation of rainfall infiltration and mixing in soil and exfiltration to a hillslope. Results show that the stream runoff was separated into rainfall-induced overland flow (9 %), exfiltration-induced overland flow (including the 53 % of mixed water from rainfall infiltration and 6 % of the stored soil water), and subsurface flow (32 %). Exfiltration-induced overland flow was the dominant water source in the headwater catchment during a rainfall-runoff event. The mutual influence of rainfall infiltration and soil water exfiltration caused the shallow soil to rapidly become saturated, thus forming saturation excess overland flow. The consistence of estimated soil water velocity based on the integrated isotope and numerical modeling approach, at about 0.4 m/ d nearby the stream channel, improved the reliability of model visualizations. Further analysis indicates that water flow paths and velocities responding to the rainfall event were attributed to the soil moisture conditions and topography. This study demonstrates that identifying the rapid mixing processes between the rain and the soil water was crucial for understanding hillslope overland flow and runoff generation in headwater catchments.