Hillslope Hydrographs Analyzed Using 1D and 2D Numerical Models

Abstract:

Stable isotopes of water naturally occurring in rainwater have the potential to reveal principal transport mechanisms at multiple scales – from soil profile to hillslope and catchment scale. In this contribution, we study transport processes at the hillslope scale by combining field observations of hillslope discharge and the associated oxygen-18 contents with detailed process-based numerical modeling. In the one-dimensional (1D) approach, 1D dual-continuum vertical flow and transport model (based on Richards and advection-dispersion equations) is coupled with 1D single-continuum lateral flow and transport model (based on diffusion wave equation for saturated subsurface flow and advection-dispersion equation for isotope transport). In the two-dimensional (2D) approach, the movement of water in a variably saturated hillslope segment is modeled as vertical planar flow (i.e., the vertical and lateral flow components are fully integrated into one flow system). Both modeling approaches, taking into account flow and transport through the soil matrix and the system of preferential pathways, were used to simulate the subsurface processes during observed rainfall–runoff episodes. The observed subsurface runoff and its oxygen-18 composition were compared with the model predictions. In addition, contributions of pre-event and event water to hillslope runoff during major rainfall–runoff episodes were evaluated by means of numerical experiments involving synthetic oxygen-18 rainfall signatures. The simulated hillslope responses showed a reasonable agreement with the experimental data in terms of subsurface runoff and oxygen-18 transport dynamics. Pre-event water was found to be the significant runoff component in most major rainfall–runoff events, though preferential flow played an important role in the hillslope runoff formation.