Enrichment Ratio

The enrichment ratio is defined as the ratio of the concentration of phosphorus transported with the sediment to the concentration of phosphorus 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 phosphorus attached to sediment 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 phosphorus enrichment ratio, εP:sed\varepsilon_{P:sed}, for each storm event is:

εP:sed=0.78(concsed,surq)0.2468\varepsilon_{P:sed}=0.78*(conc_{sed,surq})^{-0.2468} 4:2.4.3

where concsed,surqconc_{sed,surq} is the concentration of sediment in surface runoff (Mg sedsed/m3^3 H2_2O). The concentration of sediment in surface runoff is calculated:

concsed,surq=sed10areahruQsurfconc_{sed,surq}=\frac{sed}{10*area_{hru}*Q_{surf}} 4:2.4.4

where sedsed is the sediment yield on a given day (metric tons), areahruarea_{hru} is the HRU area (ha), and QsurfQ_{surf} is the amount of surface runoff on a given day (mm H2_2O).

Table 4:2-4: SWAT+ input variables that pertain to loading of P attached to sediment.

Variable NameDefinitionInput File

SOL_BD

ρb\rho_b: Bulk density(Mg/m3^3)

.sol

ERORGP

εP:sed\varepsilon_{P:sed}: Phosphorus enrichment ratio

.hru

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