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:

1:3.4.20

where is the average daily solar radiation of the month on wet days (MJ m), is the average daily solar radiation of the month on dry days (MJ m), and is a scaling factor that controls the degree of deviation in solar radiation caused by the presence or absence of precipitation. The scaling factor, , 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 :

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:

1:3.4.22

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

1:3.4.23

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