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 SLRSIM in the master watershed (file.cio) file identifies the method used to obtain solar radiation data. To read in daily solar radiation data, the variable is set to 1 and the name of the solar radiation data file and the number of solar radiation records stored in the file are set. To generate daily solar radiation values, SLRSIM is set to 2. 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.

Variable Name	Definition	File Name
SUB_LAT	Latitude of the subbasin (degrees).	.sub
SLRSIM	Solar radiation input code: 1-measured, 2-generated	file.cio
NSTOT	Number of solar radiation records within the .slr file (required if SLRSIM = 1)	file.cio
SLRFILE	Name of measured solar radiation input file (.slr) (required if SLRSIM = 1)	file.cio
ISGAGE	Number of solar radiation record used within the subbasin (required if SLRSIM = 1)	.sub

see description of .slr file in the User’s Manual for input and format requirements if measured daily solar radiation data is being used.