> 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/pesticide-transport/pesticide-lag-in-surface-runoff-and-lateral-flow.md).

# Pesticide Lag in  Surface Runoff and Lateral Flow

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

         Once the pesticide load in surface runoff and lateral flow is determined, the amount of pesticide released to the main channel is calculated:

$$pst\_{surf}=(pst'*{surf}+pst*{surstor,i-1})\*(1-exp\[\frac{-surlag}{t\_{conc}}])$$                               4:3.4.1

$$pst\_{lat}=(pst'*{lat}+pst*{latstor,i-1})\*(1-exp\[\frac{-1}{TT\_{lat}}])$$                                         4:3.4.2

$$pst\_{sed}=(pst'*{sed}+pst*{sedstor,i-1})\*(1-exp\[\frac{-surlag}{t\_{conc}}])$$                                  4:3.4.3

where $$pst\_{surf}$$ is the amount of soluble pesticide discharged to the main channel in surface runoff on a given day (kg $$pst$$/ha), $$pst'*{surf}$$ is the amount of surface runoff soluble pesticide generated in HRU on a given day (kg $$pst$$/ha), $$pst*{surstor,i-1}$$ is the surface runoff soluble pesticide stored or lagged from the previous day (kg $$pst$$/ha), $$pst\_{lat}$$ is the amount of soluble pesticide discharged to the main channel in lateral flow on a given day (kg $$pst$$/ha),$$pst'*{lat}$$ is the amount of lateral flow soluble pesticide generated in HRU on a given day (kg $$pst$$/ha), $$pst*{latstor,i-1}$$ is the lateral flow pesticide stored or lagged from the previous day (kg $$pst$$/ha), $$pst\_{sed}$$ is the amount of sorbed pesticide discharged to the main channel in surface runoff on a given day (kg $$pst$$/ha), $$pst'*{sed}$$ is the sorbed pesticide loading generated in HRU on a given day (kg $$pst$$/ha), $$pst*{sedstor,i-1}$$ is the sorbed pesticide stored or lagged from the previous day (kg $$pst$$/ha), $$surlag$$ is the surface runoff lag coefficient, $$t\_{conc}$$ is the time of concentration for the HRU (hrs) and $$TT\_{lag}$$ is the lateral flow travel time (days).

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

| Variable Name | Definition                                     | Input File |
| ------------- | ---------------------------------------------- | ---------- |
| SURLAG        | $$surlag$$: surface runoff lag coefficient     | .bsn       |
| LAT\_TTIME    | $$TT\_{lag}$$: Lateral flow travel time (days) | .hru       |