Development and testing of a simplified mechanistic algorithm to calculate the influence of a shallow water table on flow dynamics through vegetative filter strips
Développement et test d'un algorithme mécaniste pour calculer l'influence d'une nappe phréatique peu profonde sur la dynamique des flux au sein de bandes enherbées
Résumé
Vegetative filter strips (VFS) are a widely used best management practice to control surface runoff pollution from disturbed areas. In the USA, VFS are typically an upland practice, as the upper component of a 3-stage buffer protecting the streams and water bodies. In Europe, VFS are often placed on the bottomland next to the water body. But indeed, the existence of a seasonal water table in these areas can greatly influence the filter hydrodynamics and impact the intended efficiency of these buffer zones to control transport of contaminants like sediment and pesticides. The efficiency of the filters is controlled by a multifaceted interaction between hydrology, vegetation, transport and pollutant properties. Physically-based models can help to analyze the complex dynamics of these systems and in particular to quantify their potential loss of efficiency caused by the presence of a shallow water table. Salvucci and Enthekabi (1995) proposed an approximate, time-implicit integral solution to the ponded infiltration case for soils bounded by a water table (i.e. under a hydrostatic equilibrium bottom boundary condition). In this work, we further developed this solution to make it numerically explicit in time and to account for unsteady rainfall conditions. The extended solution was coupled with a numerical overland flow and transport model (VFSMOD) to simulate VFS hydrodynamics (infiltration and runoff) under shallow water table conditions. The new infiltration component of the tool was validated against a numerical solution of Richards'equation, and then used to study the effect of varying shallow water table depth (0-4 m) and rainfall intensity (0.1-20 cm/h) on 5 distinct soils. For the soils tested, results show that the influence of the shallow water table on the VFS infiltration and runoff is significant for depths shallower than 1-1.5 m, but that this influence becomes negligible when the water table is deeper. As expected, the influence of rainfall intensity varies with the duration of the storm, soil characteristics and water table depth. In general, the presence of a shallow water table was found to be more important for fine soils than coarse soils. Soils that exhibit a marked air entry (bubbling pressure) on their soil water characteristic curve introduce also a distinct behavior for very shallow water table depths. The results also show that the soil hydraulic function (Brooks and Corey, van Genuchten or Gardner) has to be chosen carefully since seemingly equivalent functions introduce important differences in the calculated filter efficiency.