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 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 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 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.

The name of the hydrology record is a primary key referenced by in .

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

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.

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 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 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.