1:3.4.3.2 Solar Radiation

The continuity equation relates average daily solar radiation adjusted for wet or dry conditions to the average daily solar radiation for the month:

$\mu rad_{mon}*days_{tot}=\mu Wrad_{mon}*days_{wet}+\mu Drad_{mon}*days_{dry}$ 1:3.4.19

where $\mu rad_{mon}$ is the average daily solar radiation for the month (MJ m$^{-2}$), $days_{tot}$ are the total number of days in the month, $\mu Wrad_{mon}$ is the average daily solar radiation of the month on wet days (MJ m$^{-2}$), $days_{wet}$ are the number of wet days in the month, $\mu Drad_{mon}$ is the average daily solar radiation of the month on dry days (MJ m$^{-2}$), and $days_{dry}$ are the number of dry days in the month.

The wet day average solar radiation is assumed to be less than the dry day average solar radiation by some fraction:

$\mu Wrad_{mon}=b_R*\mu Drad_{mon}$ 1:3.4.20

where $\mu Wrad_{mon}$ is the average daily solar radiation of the month on wet days (MJ m$^{-2}$), $\mu Drad_{mon}$ is the average daily solar radiation of the month on dry days (MJ m$^{-2}$), and $b_R$ is a scaling factor that controls the degree of deviation in solar radiation caused by the presence or absence of precipitation. The scaling factor, $b_R$, is set to 0.5 in SWAT+.

To calculate the dry day average solar radiation, equations 1:3.4.19 and 1:3.4.20 are combined and solved for $\mu Drad_{mon}$:

$\mu Drad_{mon}=\frac{\mu rad_{mon}*days_{tot}}{b_R*days_{wet}+days_{dry}}$ 1:3.4.21

Incorporating the modified values into equation 1:3.4.12, SWAT+ calculated the solar radiation on a wet day using the equation:

$H_{day}=\mu Wrad_{mon}+\chi_i(3)*\sigma rad_{mon}$ 1:3.4.22

and the solar radiation on a dry day using the equation:

$H_{day}=\mu Drad_{mon}+\chi_i(3)*\sigma rad_{mon}$ 1:3.4.23

Table 1:3-4: SWAT+ input variables that pertain to generation of temperature and solar radiation.

Definition	Source Name	Input Name	Input File
Temperature input: 'sim' for simulated or gage name	tgage	tmp	weather-sta.cli
Solar radiation input: 'sim' for simulated or gage name	sgage	slr	weather-sta.cli
$\mu mx_{mon}$: average maximum air temperature for month ($\degree C$)	tmpmx	tmp_max_ave	weather-wgn.cli
$\sigma mx_{mon}$: standard deviation for maximum air temperature in month ($\degree C$)	tmpstdmx	tmp_max_sd	weather-wgn.cli
$\mu mn_{mon}$: average minimum air temperature for month ($\degree C$)	tmpmn	tmp_min_ave	weather-wgn.cli
$\sigma mn_{mon}$: standard deviation for minimum air temperature in month ($\degree C$)	tmpstdmn	tmp_min_sd	weather-wgn.cli
$\mu rad_{mon}$: average daily solar radiation for month (MJ m$^{-2}$)	solarav	slr_ave	weather-wgn.cli
$days_{wet}$: average number of days of precipitation in month	pcpd	pcp_days	weather-wgn.cli