Filter Strips and Grassed Waterways

A widely used conservation practice to remove agricultural and urban pollutants before reaching nearby water bodies is the vegetative filter strip (VFS). A VFS is a strip of dense vegetation located to intercept runoff from upslope pollutant sources and filter it. The previous version of SWAT+ contained a VFS algorithm, but is had some limitations. It used the same filtering efficiency for sediment and all nutrient forms. Differing trapping efficiencies have been observed between soluble and particulate nutrients (Goel et al., 2004). In the previous version of SWAT+ the VFS model does not consider the effects of flow concentration apparent at the field and watershed scales. Due to widespread use of the SWAT+ to simulate VFSs (Chu et al., 2005; Arabi et al., 2008; Parajuli et al., 2008) improvements in these routines were needed.

A model to predict filter strip effectiveness under ideal uniform sheet flow conditions was developed from Vegetative Filter Strip MODel (VFSMOD) (Muñoz-Carpena, 1999) and measured data derived from 22 published studies. These studies were identified from a general search of the literature and other published summaries of VFS or riparian buffer effectiveness (Wenger, 1999; Helmers, 2003; Parkyn, 2004; Krutz et al., 2005; Mayer et al., 2005; Dorioz et al., 2006; Koelsch et al., 2006).

The filter strip model was adapted to operate at the field scale by considering the effects of flow concentration generally absent from plot scale experiments. Flow distribution through ten hypothetical filer strips was evaluated using high resolution (2m) topographical data and multipath flow accumulation (Quinn et al., 1991). Significant flow concentration was predicted at all sites, on average 10% of the filter strip received half of the field runoff. As implemented in SWAT+, the filter strip model contains two sections, a large section receiving relatively modest flow densities and a smaller section treating more concentrated flow. The combined model was incorporated into SWAT+ and verified for proper function. A full description of the filter strip model is presented below. A theoretical approach due to a lack of measured effectiveness data is used for grassed waterways. The model includes separate algorithms for the submerged and unsubmerged portions of the waterway. Particulate trapping in the submerged portion is based on the same sediment transport capacity algorithms employed in SWAT+’s channel reaches. The unsubmerged portion of the waterway is treated as a simplified filter strip. Runoff which enters laterally along the length of the waterway is subjected to this additional filtering effect. Larger events submerge a larger fraction of the waterway leaving less area to filter incoming runoff. Channel geometry for grassed waterways is defined as trapezoidal with 8:1 side slopes. Length, width, depth, and slope are required to simulate waterways in SWAT+.