A primary mechanism of disposal for manure generated by intensive animal operations such as confined animal feedlots is the land application of waste. In this type of a land management system, waste is applied every few days to the fields. Using the continuous fertilization operation allows a user to specify the frequency and quantity of manure applied to an HRU without the need to insert a fertilizer operation in the management file for every single application.

The continuous fertilizer operation requires the user to specify the beginning date of the continuous fertilization period, the total length of the fertilization period, and the number of days between individual fertilizer/manure applications. The amount of fertilizer/manure applied in each application is specified as well as the type of fertilizer/manure.

Nutrients and bacteria in the fertilizer/manure are applied to the soil surface. Unlike the fertilization operation or auto-fertilization operation, the continuous fertilization operation does not allow the nutrient and bacteria loadings to be partitioned between the surface 10 mm and the part of the 1st soil layer underlying the top 10 mm. Everything is added to the top 10 mm, making it available for transport by surface runoff.

Nutrient and bacteria loadings to the HRU are calculated using the equations reviewed in Section 6:1.7.

Table 6:1-9: SWAT+ input variables that pertain to continuous fertilization.

Urban areas differ from rural areas in the fraction of total area that is impervious. Construction of buildings, parking lots and paved roads increases the impervious cover in a watershed and reduces infiltration. With development, the spatial flow pattern of water is altered and the hydraulic efficiency of flow is increased through artificial channels, curbing, and storm drainage and collection systems. The net effect of these changes is an increase in the volume and velocity of runoff and larger peak flood discharges.

Impervious areas can be differentiated into two groups: the area that is hydraulically connected to the drainage system and the area that is not directly connected. As an example, assume there is a house surrounded by a yard where runoff from the roof flows into the yard and is able to infiltrate into the soil. The rooftop is impervious but it is not hydraulically connected to the drainage system. In contrast, a parking lot whose runoff enters a storm water drain is hydraulically connected. Table 6:3-1 lists typical values for impervious and directly connected impervious fractions in different urban land types.

During dry periods, dust, dirt and other pollutants build up on the impervious areas. When precipitation events occur and runoff from the impervious areas is generated, the runoff will carry the pollutants as it moves through the drainage system and enters the channel network of the watershed.