The average slope length describes the distance that sheet flow is the dominant surface runoff flow process. It should be measured to the point where flow begins to concentrate. It is easily observable after a heavy rain on a fallow field when the rills are well developed. In this situation, the slope length is the distance from the micro-watershed divide to the origin of the rill. The average slope length can also be determined from topographic maps.

The average slope length is used to compute soil erosion using the MUSLE.

There are four files containing the basic topographical and hydrological properties of HRUs:

• topography.hyd defines the topographic characteristics of the HRUs and routing units,

• hydrology.hyd defines the hydrological characteristics of the HRUs,

• field.fld specifies the properties of representative fields,

• snow.sno controls the simulation of snowfall and snowmelt processes.

The name of the topography record is a primary key referenced by topo in hru-data.hru and topo in rout_unit.rtu.

The average slope length for lateral flow is typically longer than the surface slope length.

The name of the hydrology record is a primary key referenced by hydro in hru-data.hru.

The maximum amount of water that can be trapped in the canopy when the canopy is fully developed.

The plant canopy can significantly affect infiltration, surface runoff and evapotranspiration. As rain falls, canopy interception reduces the erosive energy of droplets and traps a portion of the rainfall within the canopy. The influence the canopy exerts on these processes is a function of the density of plant cover and the morphology of the plant species.

When calculating surface runoff, the SCS curve number method lumps canopy interception in the term for initial abstractions. This variable also includes surface storage and infiltration prior to runoff and is estimated as 20% of the retention parameter value for a given day. When the Green & Ampt infiltration equation is used to calculate infiltration, the interception of rainfall by the canopy must be calculated separately.

SWAT+ allows the maximum amount of water that can be held in canopy storage to vary from day to day as a function of the leaf area index.

This coefficient has been incorporated to allow the user to modify the depth distribution used to meet the soil evaporative demand to account for the effect of capillary action, crusting and cracks.

The amount of water uptake that occurs on a given day is a function of the amount of water required by the plant for transpiration and the amount of water available in the soil. If upper layers in the soil profile do not contain enough water to meet the potential water uptake, users may allow lower layers to compensate.

The average slope steepness of the HRUs is calculated based on the DEM.

Biological mixing is the redistribution of soil constituents as a result of the activity of biota in the soil (e.g. earthworms, etc.). Studies have shown that biological mixing can be significant in systems where the soil is only infrequently disturbed. In general, as a management system shifts from conventional tillage to conservation tillage to no-till there will be an increase in biological mixing.

SWAT+ allows biological mixing to occur to a depth of 300 mm (or the bottom of the soil profile if it is shallower than 300 mm). The efficiency of biological mixing is defined by the user and is conceptually the same as the mixing efficiency of a tillage implement. The redistribution of nutrients by biological mixing is calculated using the same methodology as that used for a tillage operation. Biological mixing is performed at the end of every calendar year.

The organic nitrogen enrichment ratio is defined as the ratio of the concentration of organic nitrogen transported with the sediment to the concentration in the soil surface layer.

As surface runoff flows over the soil surface, part of the water’s energy is used to pick up and transport soil particles. The smaller particles weigh less and are more easily transported than coarser particles. Therefore, the sediment load transported to the main channel has a greater proportion of clay sized particles than the soil surface layer. In other words, the sediment load is enriched in clay particles. Organic nitrogen in the soil is attached primarily to colloidal (clay) particles, so the sediment load will also contain a greater proportion or concentration of organic nitrogen than that found in the soil surface layer.

This parameter gives the user control over the level of saturation of the soil that has to be reached before the model switches from using the Curve Number for moisture condition II to moisture condition III. Thus, it can be used to delay the onset of surface runoff after dry periods.

The percolation coefficient is input to limit percolation from the bottom soil layer due to an impermeable layer or high water table. Since percolation can be a slow process, that can occur over days or weeks, the percolation coefficient is non-linear and difficult to parameterize and calibrate.

This parameter can be used to decrease or increase PET calculated using any of the three PET equations implemented in SWAT+.

The phosphorus enrichment ratio is defined as the ratio of the concentration of phosphorus transported with the sediment to the concentration in the soil surface layer.

As surface runoff flows over the soil surface, part of the water’s energy is used to pick up and transport soil particles. The smaller particles weigh less and are more easily transported than coarser particles. Therefore, the sediment load transported to the main channel has a greater proportion of clay sized particles than the soil surface layer. In other words, the sediment load is enriched in clay particles. Phosphorus in the soil is attached primarily to colloidal (clay) particles, so the sediment load will also contain a greater proportion or concentration of phosphorus than that found in the soil surface layer.

This parameter is used to compute the sediment transport capacity of surface runoff across an HRU. A large coefficient will result in a high transport capacity and thus a larger amount of sediment leaving the HRU.

The snow pack will not melt until the snow pack temperature exceeds melt_tmp.

Mean air temperature at which precipitation is equally likely to be rain as snow/freezing rain.

The name of the snow record is a primary key referenced by snow in hru-data.hru.

The variables melt_max and melt_min allow the rate of snow melt to vary through the year and account for the impact of snowpack density on snow melt.

If the watershed is in the Northern Hemisphere, melt_max will be the maximum melt factor. If the watershed is in the Southern Hemisphere, melt_max will be the minimum melt factor.

Soil lateral flow is computed for each soil layer using a hillslope storage method. The equation is a function of excess water above field capacity, total soil water capacity, hydraulic conductivity, slope, and flow length. The lateral flow coefficient is a direct linear coefficient applied to the hillslope storage equation.

SWAT+ assumes a non-linear relationship between snow water and snow cover. This parameter defines the fraction of the snow volume represented by snow_h2o that corresponds to 50% snow cover.

The variables melt_max and melt_min allow the rate of snow melt to vary through the year and account for the impact of snowpack density on snow melt.

If the watershed is in the Northern Hemisphere, melt_min will be the maximum melt factor. If the watershed is in the Southern Hemisphere, melt_min will be the minimum melt factor.

Due to factors such as drifting, shading, and topography, the snow pack in a HRU will rarely be uniformly distributed over the total area. A fraction of the HRU area will be bare of snow. This fraction must be quantified to accurately compute snow melt in the HRU.

The factors that contribute to variable snow coverage are usually similar from year to year, making it possible to correlate the areal coverage of snow with the amount of snow present in the HRU at a given time. This correlation is expressed as an areal depletion curve, which is used to describe the seasonal growth and recession of the snow pack as a function of the amount of snow present in the HRU.

The areal depletion curve requires a threshold depth of snow to be defined, above which there will always be 100% cover. The threshold depth will depend on factors such as vegetation distribution, wind loading of snow, wind scouring of snow, interception, and aspect and will be unique to the watershed of interest.

The width of the field is used to compute the sediment transport capacity of surface runoff across an HRU.

The name of the field is a primary key referenced by field in hru-data.hru and field in rout_unit.rtu.

The influence of the previous day’s snowpack temperature on the current day’s snow pack temperature is controlled by a lagging factor, which inherently accounts for snow pack density, snowpack depth, exposure and other factors affecting snowpack temperature.

The length of the field is used to compute time of concentration.