There are five structural practices that can be applied in SWAT+:

• Tile drainage: tiledrain.str

• Filterstrip: filterstrip.str

• Grassed waterway: grassedww.str

• User BMP: bmpuser.str

• Septic system: septic.str

The name of the tile drain record is a primary key referenced by in . All names in the tiledrain.str file must be unique.

The lateral saturated hydraulic conductivity is determined based on the vertical saturated hydraulic conductivity (soil_k).

Tile drains remove excess water from an area to optimize plant growth.

The name of the filter strip record is a primary key referenced by in . All names in the filterstrip.str file must be unique.

This parameter defines the fraction of the flow within the most concentrated 10% of the filter strip that is fully channelized. Flow that is fully channelized is not subject to filtering or infiltration effects.

Values from 30-60 are most common.

Runoff generated upslope a filter strip is not uniformly distributed across the entire length of the strip. 10% of the filter strip can receive between 25 and 75% of the runoff from the field.

This user-defined coefficient is used to compute the sediment transport capacity in the grassed waterway.

The name of the grassed waterway record is a primary key referenced by in . All names in the grassedww.str file must be unique.

The name of the user BMP record is a primary key referenced by in . All names in the bmpuser.str file must be unique.

The name of the septic system record is a primary key referenced by sep in landuse.lum. All names in the septic.str file must be unique.

The number of permanent residents for a typical US residence is 2.5.

The typically recommended drainfield area per person is about 40 to 70 square meters. This varies from state to state in the United States. For a household with 2.5 people, a drainfield area of 100 square meters is generally recommended. Users can modify the area based on the number of people in a household. The parameters area and cap may be modified appropriately to study the effects of a larger population size using septic systems.

An active system automatically becomes failing as the biozone layer gets clogged over time. A failing system turns to an active system after the user specified number of days for rehabilitation defined by t_fail.

Grassed waterways are vegetated channels that transport runoff from a field. Vegetation within the waterways reduces flow velocities and protects the waterway from the scouring potential of concentrated flow. Grassed waterways are generally broad and shallow channels; the channel simulated in SWAT+ has a side slope of 8:1. Grassed waterways trap sediment and other contaminants by reducing flow velocities, which increases deposition of particulate contaminates.

Data contained in the septic.str data file are: type of septic system, geometry of biozone, characteristics of biomass, and bio-physical reaction coefficients occurring in the biozone (adapted from Siegrist et al. (2005)).

References

Siegrist, R.L., J. McCray, L. Weintraub, C. Chen, J. Bagdol, P. Lemonds, S. Van Cuyk, K. Lowe, R. Goldstein, and J. Rada (2005): Quantifying Site-Scale Processes and Watershed-Scale Cumulative Effects of Decentralized Wastewater Systems. Project No. WU-HT-00-27. Prepared for the National Decentralized Water Resources Capacity Development Project, Washington University, St. Louis, MO, by the Colorado School of Mines.

The thickness includes the top soil layer and septic tank effluent (STE) distribution chamber including perforated pipe.

The biozone layer is thin soil layer underneath the septic tank effluent (STE) distribution chamber where pollutants are degraded by naturally existing live biomass bacteria.

An active system becomes failing as the biozone gets clogged and hydraulic failure occurs. A failing system automatically turns active during the simulation and septic parameters are re-initialized to default values after the specified number of days for rehabilitation. For testing long term failure, t_fail can be increased as per the failing duration. The parameter operation should be set to 2 for simulating failing conditions.

The septic system type is a foreign key referencing name in septic.sep.

Biozone fecal coliform coefficient is normalized by the volume of biomass in the formula.

The biozone nitrification rate coefficient is normalized by the volume of biomass in the formula.

The biozone denitrification rate coefficient is normalized by the volume of biomass in the formula.

Biozone BOD coefficient is normalized by the volume of biomass in the formula.

A filter strip is a strip of dense vegetation located to intercept runoff from upslope pollutant sources and filter it. Filter strips remove contaminants by reducing overland flow velocity which results in the deposition of particulates. The filter strip area also acts as an area of increased infiltration, reducing both the runoff volume and non-particulate contaminants. The filter strip algorithm used in SWAT+ was derived from White and Arnold (2009). Filter strips reduce sediment, nutrients, bacteria, and pesticides, but do not affect surface runoff in SWAT+.

References:

White, Michael J. and Arnold, Jeff G. (2009): Development of a simplistic vegetative filter strip model for sediment and nutrient retention at the field scale. Hydrological Processes 23, 1602– 1616. DOI: 10.1002/hyp.7291

There are many conservation practices that are not implemented in SWAT+, but for which approximate removal efficiencies have been established. To allow these practices to be included, this generic Best Management Practice (BMP) operation allows fixed removal efficiencies to be specified by constituent.