# Sediment 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 surface runoff storage feature to lag a portion of the surface runoff release to the main channel. Sediment in the surface runoff is lagged as well.

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

         $$sed=(sed'+sed\_{stor,i-1})\*(1-exp\[\frac{-surlag}{t\_{conc}}])$$                                 4:1.4.1

where $$sed$$ is the amount of sediment discharged to the main channel on a given day (metric tons), $$sed'$$  is the amount of sediment load generated in the HRU on a given day (metric tons), $$sed\_{stor,i-1}$$ is the sediment stored or lagged from the previous day (metric tons), $$surlag$$ is the surface runoff lag coefficient, and $$t\_{conc}$$ is the time of concentration for the HRU (hrs).

The expression $$(1-exp\[\frac{-surlag}{t\_{conc}}])$$ in equation 4:1.4.1 represents the fraction of the total available sediment that will be allowed to enter the reach on any one day. 

Figure 4:1-1 plots values for this expression at different values for $$surlag$$ and $$t\_{conc}$$.

![](/files/MFVovde3rTwiK3YgqqAs)

Note that for a given time of concentration, as $$surlag$$ decreases in value more sediment is held in storage.

Table 4:1-7: SWAT+ input variables that pertain to sediment lag calculations.

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


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