1:3.5.3 Adjustment for Clear/Overcast Conditions

To incorporate the effect of clear and overcast weather on generated values of relative humidity, monthly average relative humidity values can be adjusted for wet or dry conditions.

The continuity equation relates average relative humidity adjusted for wet or dry conditions to the average relative humidity for the month:

$R_{hmon}*days_{tot}=R_{hWmon}*days_{wet}+R_{hDmon}*days_{dry}$ 1:3.5.8

where $R_{hmon}$ is the average relative humidity for the month, $days_{tot}$ are the total number of days in the month, $R_{hWmon}$ is the average relative humidity for the month on wet days, $days_{wet}$ are the number of wet days in the month, $R_{hDmon}$ is the average relative humidity of the month on dry days, and $days_{dry}$ are the number of dry days in the month.

The wet day average relative humidity is assumed to be greater than the dry day average relative humidity by some fraction:

$R_{hWmon}=R_{hDmon}+b_H*(1-R_{hDmon})$ 1:3.5.9

where $R_{hWmon}$ is the average relative humidity of the month on wet days, $R_{hDmon}$ is the average relative humidity of the month on dry days, and $b_H$ is a scaling factor that controls the degree of deviation in relative humidity caused by the presence or absence of precipitation. The scaling factor, $b_H$, is set to 0.9 in SWAT+.

To calculate the dry day relative humidity, equations 1:3.5.8 and 1:3.5.9 are combined and solved for $R_{hDmon}$:

$R_{hDmon}=(R_{hmon}-b_H*\frac{days_{wet}}{days_{tot}})*(1.0-b_H*\frac{days_{wet}}{days_{tot}})^{-1}$ 1:3.5.10

To reflect the impact of wet or dry conditions, SWAT+ will replace $R_{hmon}$ with $R_{hWmon}$ on wet days or $R_{hDmon}$ on dry days in equations 1:3.5.4 through 1:3.5.7.

Table 1:3-5: SWAT+ input variables that pertain to generation of relative humidity.

Definition	Source Name	Input Name	Input File
Relative humidity input: 'sim' for simulated or gage name	hgage	hmd	weather-sta.cli
$\mu mn_{mon}$: average minimum air temperature for month (°C)	tmpmn	tmp_min_ave	weather-wgn.cli
$\mu mx_{mon}$: average maximum air temperature for month (°C)	tmpmx	tmp_max_ave	weather-wgn.cli
$\mu dew_{mon}$: average dew point temperature for month (°C)	dewpt	dew_ave	weather-wgn.cli
$days_{wet}$: average number of days of precipitation in month	pcpd	pcp_days	weather-wgn.cli