> 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/bacteria-transport/bacteria-lag-in-surface-runoff.md).

# Bacteria Lag in Surface Runoff

In large subbasins with a time of concentration greater than 1 day, only a portion of the surface runoff will reach the main channel on the day it is generated. SWAT+ incorporates a storage feature to lag a portion of the surface runoff release to the main channel. Bacteria in the surface runoff are lagged as well. 

             Once the bacteria load in surface runoff is determined, the amount of bacteria released to the main channel is calculated:

$$bact\_{lp,surf}=(bact'*{lp,surf}+bact*{lp,surstor,i-1})\*(1-exp\[\frac{-surlag}{t\_{conc}}])$$                4:4.3.1

$$bact\_{p,surf}=(bact'*{p,surf}+bact*{p,surstor,i-1})\*(1-exp\[\frac{-surlag}{t\_{conc}}])$$                   4:4.3.2

$$bact\_{lp,sed}=(bact'*{lp,sed}+bact*{lp,sedstor,i-1})\*(1-exp\[\frac{-surlag}{t\_{conc}}])$$                    4:4.3.3

$$bact\_{p,sed}=(bact'*{p,sed}+bact*{p,sedstor,i-1})\*(1-exp\[\frac{-surlag}{t\_{conc}}])$$                       4:4.3.4

where $$bact\_{lp,surf}$$ is the amount of less persistent bacteria discharged to the main channel in surface runoff on a given day (#cfu/m$$^2$$),$$bact'*{lp,surf}$$ is the amount of surface runoff less persistent bacteria generated in the HRU on a given day (#cfu/m$$^2$$), $$bact*{lp,surstor,i-1}$$ is the surface runoff less persistent bacteria stored or lagged from the previous day (#cfu/m$$^2$$), $$bact\_{p,surf}$$ is the amount of persistent bacteria discharged to the main channel in surface runoff on a given day (#cfu/m2), $$bact'*{p,surf}$$ is the amount of surface runoff persistent bacteria generated in the HRU on a given day (#cfu/m$$^2$$), $$bact*{p,surstor,i-1}$$ is the surface runoff persistent bacteria stored or lagged from the previous day (#cfu/m$$^2$$), $$bact\_{lp,sed}$$ is the amount of sediment-attached less persistent bacteria discharged to the main channel in surface runoff on a given day (#cfu/m$$^2$$),$$bact'*{lp,sed}$$ is the amount of sediment-attached less persistent bacteria generated in the HRU on a given day (#cfu/m$$^2$$), $$bact*{lp,sedstor,i-1}$$ is the sediment-attached less persistent bacteria stored or lagged from the previous day (#cfu/m$$^2$$), $$bact\_{p,sed}$$ is the amount of sediment-attached persistent bacteria discharged to the main channel in surface runoff on a given day (#cfu/m$$^2$$), $$bact'*{p,sed}$$ is the amount of sediment-attached persistent bacteria generated in the HRU on a given day (#cfu/m$$^2$$), $$bact*{p,sedstor,i-1}$$ is the sediment-attached persistent bacteria stored or lagged from the previous day (#cfu/m$$^2$$), $$surlag$$ is the surface runoff lag coefficient, and $$t\_{conc}$$ is the time of concentration for the HRU (hrs).

Table 4:4-3: SWAT+ input variables that pertain to bacteria lag calculations.

| Variable Name | Definition                                 | Input File |
| ------------- | ------------------------------------------ | ---------- |
| SURLAG        | $$surlag$$: surface runoff lag coefficient | .bsn       |


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