Wind Speed

Wind speed is required by SWAT+ if the Penman-Monteith equation is used to estimate potential evapotranspiration and transpiration. SWAT+ assumes wind speed information is collected from gages positioned 1.7 meters above the ground surface.

When using the Penman-Monteith equation to estimate transpiration, the wind measurement used in the equation must be above the canopy. In SWAT+, a minimum difference of 1 meter is specified for canopy height and wind speed measurements. When the canopy height exceeds 1 meter, the original wind measurements is adjusted to:

$z_w=h_c+100$ 1:1.4.1

where $z_w$ is the height of the wind speed measurement (cm), and $h_c$ is the canopy height (cm).

The variation of wind speed with elevation near the ground surface is estimated with the equation (Haltiner and Martin, 1957):

$u_{z2}=u_{z1}[\frac{z_2}{z_1}]^{aa}$ 1:1.4.2

where $u_{z1}$ is the wind speed (m s$^{-1}$) at height $z1$(cm), $u_{z2}$ is the wind speed (m s$^{-1}$) at height $z2$(cm), and $aa$ is an exponent between 0 and 1 that varies with atmospheric stability and surface roughness. Jensen (1974) recommended a value of 0.2 for $aa$ and this is the value used in SWAT+.

The daily wind speed data required by SWAT+ may be read from an input file or generated by the model. The variable WNDSIM in the master watershed (file.cio) file identifies the method used to obtain wind speed data. To read in daily wind speed data, the variable is set to 1 and the name of the wind speed data file and the number of different records stored in the file are set. To generate daily wind speed values, WNDSIM is set to 2. The equations used to generate wind speed data in SWAT+ are reviewed in Chapter 1:3.

Table 1:1-9: SWAT+ input variables used in wind speed calculations.

Variable Name	Definition	File Name
WNDSIM	Wind speed input code: 1-measured, 2-generated	file.cio
NWTOT	Number of wind speed records within the .wnd file (required if WNDSIM = 1)	file.cio
WNDFILE	Name of measured wind speed input file (.wnd) (required if WNDSIM = 1)	file.cio
IWGAGE	Number of wind speed record used within the subbasin (required if WNDSIM = 1)	.sub

see description of .wnd file in the User’s Manual for input and format requirements if measured daily wind speed data is being used.