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. When the particle size distribution of the transported sediment is compared to that of the soil surface layer, the sediment load to the main channel has a greater proportion of clay sized particles. In other words, the sediment load is enriched in clay particles. The sorbed phase of pesticide in the soil is attached primarily to colloidal (clay) particles, so the sediment load will also contain a greater proportion or concentration of pesticide than that found in the soil surface layer.

The enrichment ratio is defined as the ratio of the concentration of sorbed pesticide transported with the sediment to the concentration in the soil surface layer. SWAT+ will calculate an enrichment ratio for each storm event, or allow the user to define a particular enrichment ratio for sorbed pesticide that is used for all storms during the simulation. To calculate the enrichment ratio, SWAT+ uses a relationship described by Menzel (1980) in which the enrichment ratio is logarithmically related to sediment concentration. The equation used to calculate the pesticide enrichment ratio, $\varepsilon_{pst:sed}$, for each storm event is:

$\varepsilon_{pst:sed}=0.78*(conc_{sed,surq})^{-0.2468}$ 4:3.3.5

where $conc_{sed,surq}$ is the concentration of sediment in surface runoff (Mg sed/m$^3$ H$_2$O). The concentration of sediment in surface runoff is calculated:

$conc_{sed,surq}=\frac{sed}{10*area_{hru}*Q_{surf}}$ 4:3.3.6

where $sed$ is the sediment yield on a given day (metric tons), $area_{hru}$ is the HRU area (ha), and $Q_{surf}$ is the amount of surface runoff on a given day (mm H$_2$O).

Table 4:3-3: SWAT+ input variables that pertain to sorbed pesticide loading.

Pesticide attached to soil particles may be transported by surface runoff to the main channel. This phase of pesticide is associated with the sediment loading from the HRU and changes in sediment loading will impact the loading of sorbed pesticide. The amount of pesticide transported with sediment to the stream is calculated with a loading function developed by McElroy et al. (1976) and modified by Williams and Hann (1978).

$pst_{sed}=0.001*C_{solidphase}*\frac{sed}{area_{hru}}*\varepsilon_{pst:sed}$ 4:3.3.1

where $pst_{sed}$ is the amount of sorbed pesticide transported to the main channel in surface runoff (kg $pst$/ha), $C_{solidphase}$ is the concentration of pesticide on sediment in the top 10 mm (g $pst$/ metric ton soil), sed is the sediment yield on a given day (metric tons), $area_{hru}$ is the HRU area (ha), and $\varepsilon_{pst:sed}$ is the pesticide enrichment ratio.

The total amount of pesticide in the soil layer is the sum of the adsorbed and dissolved phases:

$pst_{s,ly}=0.01*(C_{solution}*SAT_{ly}+C_{solidphase}*\rho_b*depth_{ly})$ 4:3.3.2

where $pst_{s,ly}$ is the amount of pesticide in the soil layer (kg $pst$/ha), $C_{solution}$ is the pesticide concentration in solution (mg/L or g/ton), $SAT_{ly}$ is the amount of water in the soil layer at saturation (mm H$_2$O), $C_{solidphase}$ is the concentration of the pesticide sorbed to the solid phase (mg/kg or g/ton), $\rho_b$ is the bulk density of the soil layer (Mg/m$^3$), and $depth_{ly}$ is the depth of the soil layer (mm). Rearranging equation 4:3.1.1 to solve for $C_{solution}$ and substituting into equation 4:3.3.2 yields:

$pst_{s,ly}=0.01*(\frac{C_{solidphase}}{K_p}*SAT_{ly}+C_{solidphase}*\rho_b*depth_{ly})$ 4:3.3.3

which rearranges to

$C_{solidphase}=\frac{100*K_p*pst_{s,ly}}{(SAT_{ly}+K_p*\rho_b*depth_{ly})}$ 4:3.3.4

where $C_{solidphase}$ is the concentration of the pesticide sorbed to the solid phase (mg/kg or g/ton), $K_p$ is the soil adsorption coefficient ((mg/kg)/(mg/L) or $m^3$/ton) $pst_{s,ly}$ is the amount of pesticide in the soil layer (kg $pst$/ha), $SAT_{ly}$ is the amount of water in the soil layer at saturation (mm H$_2$O), $\rho_b$ is the bulk density of the soil layer (Mg/m$^3$), and $depth_{ly}$ is the depth of the soil layer (mm).