Shallow water table effects on water, sediment and pesticide
transport in vegetative filter strips: Part B. model coupling,
application, factor importance and uncertainty
Abstract. Vegetative filter strips are often used for protecting surface waters from pollution transferred by surface runoff in agricultural watersheds. In Europe, they are often prescribed along the stream banks, where a seasonal shallow water table (WT) could decrease the buffer zone efficiency. In spite of this potentially important effect, there are no systematic experimental or theoretical studies on the effect of this soil boundary condition on the VFS efficiency. In the companion paper, we developed a physically-based numerical algorithm (SWINGO) that allows representing soil infiltration with a shallow water table. Here we present the dynamic coupling of SWINGO with VFSMOD, an overland flow and transport mathematical model to study the WT influence on VFS efficiency in terms of reductions of overland flow, sediment and pesticide transport. This new version of VFSMOD was evaluated with two contrasted benchmark field studies in France (sandy-loam soil under Mediterranean semi-continental climate, and silty-clay under temperate Oceanic climate), where testing of the model with field data showed promising results. The analysis showed that for the conditions of the studies, VFS efficiency decreases markedly when the water table is 0 to 1.5 m from the surface. In order to evaluate the relative importance of WT among other input factors controlling VFS efficiency, two global sensitivity and uncertainty analysis (GSA) methods, Morris and eFAST, were applied on the benchmark studies. The most important factors found for VFS overland flow reduction were saturated hydraulic conductivity and WT, added to sediment characteristics and VFS dimensions for sediment and pesticide reductions. The relative importance of WT varied as a function of soil type (most important at the silty-clay soil) and hydraulic loading (rainfall + incoming runoff) at each site. The presence of WT introduced more complex responses dominated by strong interactions in the modelled system response, reducing the predominance of saturated hydraulic conductivity on infiltration under typical deep water table conditions. This study demonstrates that when present, WT should be considered as a key hydrologic factor in buffer design and evaluation as a water quality mitigation practice.