For comparative purposes, SWAT+ prints out sediment loadings calculated with USLE. These values are not used by the model, they are for comparison only. The universal soil loss equation (Williams, 1995) is:

$sed=1.292*EI_{USLE}*K_{USLE}*C_{USLE}*P_{USLE}*LS_{USLE}*CFRG$ 4:1.2.1

where $sed$ is the sediment yield on a given day (metric tons/ha), $EI_{USLE}$ is the rainfall erosion index (0.017 m-metric ton cm/(m$^2$ hr)), $K_{USLE}$ is the USLE soil erodibility factor (0.013 metric ton m$^2$ hr/(m$^3$-metric ton cm)), $C_{USLE}$ is the USLE cover and management factor, $P_{USLE}$ is the USLE support practice factor, $L_{USLE}$ is the USLE topographic factor and $CFRG$ is the coarse fragment factor. The factors other than $EI_{USLE}$ are discussed in the preceding sections.

The value of $EI_{USLE}$ for a given rainstorm is the product, total storm energy times the maximum 30 minute intensity. The storm energy indicates the volume of rainfall and runoff while the 30 minute intensity indicates the prolonged peak rates of detachment and runoff.

$EI_{USLE}=E_{storm}*I_{30}$ 4:1.2.2

where $EI_{USLE}$ is the rainfall erosion index (0.017 m-metric ton cm/(m$^2$ hr)), $E_{storm}$ is the total storm energy (0.0017 m-metric ton/m$^2$), and $I_{30}$ is the maximum 30-minute intensity (mm/hr).

The energy of a rainstorm is a function of the amount of rain and of all the storm’s component intensities. Because rainfall is provided to the model in daily totals, an assumption must be made about variation in rainfall intensity. The rainfall intensity variation with time is assumed to be exponentially distributed:

4:1.2.3

where is the rainfall intensity at time (mm/hr), is the maximum rainfall intensity (mm/hr), is the time (hr), and is the decay constant for rainfall intensity (hr).

The USLE energy equation is

4:1.2.4

where is the amount of rainfall during the time interval (mm HO), and is the time interval (hr). This equation may be expressed analytically as:

4:1.2.5

Combining equation 4:1.2.5 and 4:1.2.3 and integrating gives the equation for estimating daily rainfall energy:

4:1.2.6

where is the amount of precipitation falling on a given day (mm HO), and is the maximum rainfall intensity (mm/hr). To compute the maximum rainfall intensity, , equation 4:1.2.3 is integrated to give

4:1.2.7

and

4:1.2.8

where is the amount of precipitation falling on a given day (mm HO), is the maximum rainfall intensity (mm/hr), is the decay constant for rainfall intensity (hr), is the amount of rain falling during a time interval (mm HO), and is the time interval (hr). The maximum half-hour rainfall for the precipitation event is known:

4:1.2.9

where is the maximum half-hour rainfall (mm HO), is the maximum half-hour rainfall expressed as a fraction of daily rainfall, and is the amount of precipitation falling on a given day (mm HO). Calculation of is reviewed in Chapter 1:2 and Chapter 1:3. Substituting equation 4:1.2.9 and 4:1.2.7 into 4:1.2.8 and solving for the maximum intensity gives:

4:1.2.10

where is the maximum rainfall intensity (mm/hr), is the amount of precipitation falling on a given day (mm HO), and is the maximum half-hour rainfall expressed as a fraction of daily rainfall.

The maximum 30 minute intensity is calculated:

4:1.2.11

where is the maximum 30-minute intensity (mm/hr), is the maximum half-hour rainfall expressed as a fraction of daily rainfall, and is the amount of precipitation falling on a given day (mm HO).

Table 4:1-6: SWAT+ input variables that pertain to USLE sediment yield.