# 1:3.6 Wind Speed

Wind speed is required by SWAT+ when the Penman-Monteith equation is used to calculate potential evapotranspiration. Mean daily wind speed is generated in SWAT+ using a modified exponential equation:

$$\mu *{10m}=\mu wnd*{mon}\*(-ln(rnd\_1))^{0.3}$$                                                                                                               1.3.6.1

where $$\mu *{10m}$$ is the mean wind speed for the day ($$m/s$$), $$\mu wnd*{mon}$$ is the average wind speed for the month   ($$m/s$$), and $$rnd\_1$$ is a random number between 0.0 and 1.0.

Table 1:3-6: SWAT+ input variables that pertain to generation of wind speed.

| Definition                                         | Source Name | Input Name | Input File                                                                                           |
| -------------------------------------------------- | ----------- | ---------- | ---------------------------------------------------------------------------------------------------- |
| Wind speed input: 'sim' for simulated or gage name | wgage       | wnd        | [weather-sta.cli](https://swatplus.gitbook.io/io-docs/introduction-1/climate/weather-sta.cli)        |
| $$\mu wnd\_{mon}$$: Observed wind speed ($$m/s$$)  | wnd         | wnd        | [.wnd](https://swatplus.gitbook.io/io-docs/introduction-1/climate/wnd.cli-and-wind-speed-data-files) |