Shallow water table effects on water, sediment and pesticide transport in vegetative filter strips: Part B. model coupling, application, factor importance and uncertainty

Abstract

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.

Ardières‐Morcille in the Beaujolais, France: A research catchment dedicated to study of the transport and impacts of diffuse agricultural pollution in rivers

“…Data available on the variations in water table level and pesticide concentrations in the shallow groundwater under the grassed strip during an artificial runoff event also highlights a rapid decrease in peak infiltrated pesticide concentrations. This data set was used to test slope‐scale models (Carluer et al, 2017; Gatel et al, 2020; Lauvernet & Munoz‐Carpena, 2018).…”

Riparian Vadose Zone Preferential Flow: Review of Concepts, Limitations, and Perspectives

“…Muñoz‐Carpena et al (2018) showed that good results can be obtained when the slope of the wetting front is taken as α = 0 across a wide range of soil types, and extended the approach to include a surface water budget to calculate the infiltration rate, excess rainfall, θ redistribution profiles, and the wetting front position at each time step for initially non‐ponded soil under unsteady r . Lauvernet and Muñoz‐Carpena (2018) coupled SWINGO with a widely used vegetative filter strip and riparian buffer design model, VFSMOD (Muñoz‐Carpena et al, 1999; Muñoz‐Carpena and Parsons, 2004), that contains overland flow and transport processes, and Fox et al (2018) experimentally tested the coupled model under controlled laboratory conditions.…”

“…It is also dependent on a balance between constructive and destructive processes and, consequently, potentially influenced by changes in the relationship between the soil and the flora and fauna community, as well as by changes in external conditions such as climatic variations (Beven and Germann, 1982). In addition, riparian areas, especially those located in lowlands, may be strongly influenced by a seasonal shallow water table (SWT) (Fox et al, 2018; Lauvernet and Muñoz‐Carpena, 2018; Muñoz‐Carpena et al, 2018). A system with a SWT will have higher soil moisture and a lower infiltration capacity, resulting in a higher overland runoff transport rate and greater MF activation that ultimately can reduce riparian buffering efficiency.…”

Defining context-specific scenarios to design vegetated buffer zones that limit pesticide transfer via surface runoff

“…Similarly, slopes can be determined through field measurements or by using a GIS. Saturated hydraulic conductivity is a key parameter of the model and is often the most influential, together with water table depth depending on the hydraulic loading (Muñoz-Carpena et al, 2010;Lauvernet and Muñoz-Carpena, submitted). It is therefore important to assign a value as representative as possible of the real conductivity value.…”