# 1:1.2.3 Daily Solar Radiation

The solar radiation reaching the earth's surface on a given day, $$H\_{day}$$, may be less than $$H\_{mx}$$ due to the presence of cloud cover. The daily solar radiation data required by SWAT+ may be read from an input file or generated by the model.

The variable slr in the master weather station ([weather-sta.cli](/io-docs/introduction-1/climate/weather-sta.cli.md)) file identifies the method used to obtain solar radiation data. To read in daily solar radiation data, the slr variable is set to the name of the solar radiation data file. To generate daily solar radiation values, set the name of the solar radiation input file (slr) to sim. The equations used to generate solar radiation data in SWAT+ are reviewed in Chapter 1:3. SWAT+ input variables that pertain to solar radiation are summarized in Table 1:1-2.

Table 1:1-2: SWAT+ input variables used in solar radiation calculations.

| Definition                                                                  | Source Name | Input Name | Input File                                                                           |
| --------------------------------------------------------------------------- | ----------- | ---------- | ------------------------------------------------------------------------------------ |
| Latitude of the solar radition station (degrees).                           | lat         | lat        | [slr.cli](/io-docs/introduction-1/climate/slr.cli-and-solar-radiation-data-files.md) |
| Name of measured solar radiation input file (.slr) to simulate set to "sim" | sgage       | slr        | [weather-sta.cli](/io-docs/introduction-1/climate/weather-sta.cli.md)                |

See the description for the .slr files on the [slr.cli](/io-docs/introduction-1/climate/slr.cli-and-solar-radiation-data-files.md) page for input and format requirements if measured daily solar radiation data is being used.


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