# 2:1.4 Surface Runoff Lag

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.

Once surface runoff is calculated with the Curve Number or Green & Ampt method, the amount of surface runoff released to the main channel is calculated:

$$Q\_{surf}=(Q'*{surf}+Q*{stor,i-1})\*(1-exp\[\frac{-surlag}{t\_{conc}}])$$                                                                                   2:1.4.1

where $$Q\_{surf}$$ is the amount of surface runoff discharged to the main channel on a given day (mm H$$\_2$$O), is the amount of surface runoff generated in the subbasin on a given day (mm H$$*2$$O), $$Q*{stor,i-1}$$ is the surface runoff stored or lagged from the previous day (mm H$$*2$$O), $$surlag$$ is the surface runoff lag coefficient, and $$t*{conc}$$ is the time of concentration for the subbasin (hrs).

The expression$$(1-exp\[\frac{-surlag}{t\_{conc}}])$$ in equation 2:1.4.1 represents the fraction of the total available water that will be allowed to enter the reach on any one day. Figure 2:1-3 plots values for this expression at different values for $$surlag$$and $$t\_{conc}$$.

<figure><img src="/files/gooVnu1zLJ3u6F9XStMJ" alt=""><figcaption><p>Figure 2:1-3: Influence of <span class="math">surlag</span> and <span class="math">t_{conc}</span> on fraction of surface runoff released.</p></figcaption></figure>

Note that for a given time of concentration, as $$surlag$$ decreases in value more water is held in storage. The delay in release of surface runoff will smooth the streamflow hydrograph simulated in the reach.

Table 2:1-6: SWAT+ input variables that pertain to surface runoff lag calculations.

| Definition                                 | Source Code Variable | Input Name                                                              | Input File                                                          |
| ------------------------------------------ | -------------------- | ----------------------------------------------------------------------- | ------------------------------------------------------------------- |
| $$surlag$$: Surface runoff lag coefficient | surlag               | [surq\_lag](/io-docs/introduction-1/basin-1/parameters.bsn/surq_lag.md) | [parameters.bsn](/io-docs/introduction-1/basin-1/parameters.bsn.md) |


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