> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/~/changes/GiVD5ID0WVyRIAumaMy2/weather-generator/precipitation/maximum-half-hour-rainfall/solar-radiation.md).

# 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.

| Variable Name | Definition                                                                         | File Name |
| ------------- | ---------------------------------------------------------------------------------- | --------- |
| TMPSIM        | Temperature input code: 1-measured, 2-generated                                    | file.cio  |
| SLRSIM        | Solar radiation input code: 1-measured, 2-generated                                | file.cio  |
| TMPMX(mon)    | $$\mu mx\_{mon}$$: average maximum air temperature for month (°C)                  | .wgn      |
| TMPSTDMX(mon) | $$\sigma mx\_{mon}$$: standard deviation for maximum air temperature in month (°C) | .wgn      |
| TMPMN(mon)    | $$\mu mn\_{mon}$$: average minimum air temperature for month (°C)                  | .wgn      |
| TMPSTDMN(mon) | $$\sigma mn\_{mon}$$: standard deviation for minimum air temperature in month (°C) | .wgn      |
| SOLARAV(mon)  | $$\mu rad\_{mon}$$: average daily solar radiation for month (MJ m$$^{-2}$$)        | .wgn      |
| PCPD(mon)     | $$days\_{wet}$$: average number of days of precipitation in month                  | .wgn      |