> 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/~/changes/GiVD5ID0WVyRIAumaMy2/surface-runoff/peak-runoff-rate.md).

# Peak Runoff Rate

The peak runoff rate is the maximum runoff flow rate that occurs with a given rainfall event. The peak runoff rate is an indicator of the erosive power of a storm and is used to predict sediment loss. SWAT+ calculates the peak runoff rate with a modified rational method.

The rational method is widely used in the design of ditches, channels and storm water control systems. The rational method is based on the assumption that if a rainfall of intensity $$i$$ begins at time $$t=0$$ and continues indefinitely, the rate of runoff will increase until the time of concentration, $$t=t\_{conc}$$, when the entire subbasin area\* is contributing to flow at the outlet. The rational formula is:

$$q\_{peak}=\frac{C*i*Area}{3.6}$$                                                                            2:1.3.1

where $$q\_{peak}$$ is the peak runoff rate ($$m^3 s^{-1}$$), $$C$$ is the runoff coefficient, $$i$$ is the rainfall intensity (mm/hr), Area is the subbasin area (km$$^2$$) and 3.6 is a unit conversion factor.