> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/theoretical-documentation/section-4-erosion/nutrient-transport/organic-and-mineral-p-attached-to-sediment-in-surface-runoff/enrichment-ratio.md).

# 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, $$\varepsilon\_{P:sed}$$, for each storm event is:

              $$\varepsilon\_{P:sed}=0.78\*(conc\_{sed,surq})^{-0.2468}$$                                              4:2.4.3

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:

                $$conc\_{sed,surq}=\frac{sed}{10\*area\_{hru}\*Q\_{surf}}$$                                                      4:2.4.4

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

| Variable Name | Definition                                            | Input File |
| ------------- | ----------------------------------------------------- | ---------- |
| SOL\_BD       | $$\rho\_b$$: Bulk density(Mg/m$$^3$$)                 | .sol       |
| ERORGP        | $$\varepsilon\_{P:sed}$$: Phosphorus enrichment ratio | .hru       |