SWAT+ requires daily data for precipitation, maximum and minimum air temperature, solar radiation, relative humidity, and wind speed. The model can read in observed weather data or generate values using the weather generator. Climate data will be generated in two instances: when the user specifies that simulated weather data will be used or when there are missing values in the observed weather data. A Global Weather Generator Database containing weather generator datasets in SWAT+ format for almost 180,000 stations across the globe can be downloaded from the SWAT website: https://swat.tamu.edu/data/.

If observed data is used, one data file has to be provided for each station and variable. The data files for precipitation, temperature, solar radiation, relative humidity, and wind speed should have the file extensions .pcp, .tem, .slr, .hmd, and .wnd, respectively. The names of all available data files for precipitation, temperature, solar radiation, relative humidity, and wind speed will be listed in pcp.cli, tmp.cli, slr.cli, hmd.cli, and wnd.cli, respectively.

Each spatial object in a SWAT+ setup will be assigned the weather stations that are closest to its centroid. Because the precipitation, temperature, solar radiation, relative humidity, and wind speed stations might be at different locations, several combinations of weather stations might be needed for a SWAT+ setup. These combinations will be listed as a record in weather-sta.cli. Each of them will be given a unique name, which is referenced by the connect files for the different spatial objects. In addition, the name of the closest weather generator station will be specified, which points to weather-wgn.cli. Finally, the user has the option to specify the name of an atmospheric deposition record, which points to atmo.cli.

The flowchart below illustrates the relationships between the different SWAT+ climate files.

The name of the weather station is a primary key referenced by in .

The name of the weather generator station is a foreign key referencing the primary key in .

The name of the precipitation station is a foreign key referencing the filenames listed in .

The name of the relative humidity station is a foreign key referencing the filenames listed in .

The name of the solar radiation station is a foreign key referencing the filenames listed in .

The name of the temperature station is a foreign key referencing the filenames listed in .

The name of the wind speed station is a foreign key referencing the filenames listed in .

This variable is used to calculate values for .

The name of the atmospheric deposition station is a foreign key referencing the station names in .

This value is calculated by summing the maximum air temperature for every day in the month for all years of record and dividing the sum by the number of days:

where $μmx_{mon}$is the mean daily maximum temperature for the month $mon$ (ºC), $T_{mx,mon}$ is the daily maximum temperature on day $d$ in month $mon$ (ºC), and $N$is the total number of daily maximum temperature records for month $mon$.

The name of the weather generator station is a primary key referenced by wgn in weather-sta.cli.

This value is calculated by summing the minimum air temperature for every day in the month for all years of record and dividing the sum by the number of days:

where $μmn_{mon}$is the mean daily minimum temperature for the month $mon$ (ºC), $T_{mn,mon}$ is the daily minimum temperature on day $d$ in month $mon$ (ºC), and $N$is the total number of daily minimum temperature records for month $mon$.

This parameter quantifies the variability in minimum temperature for each month. The standard deviation is calculated as

where $σmn_{mon}$is the standard deviation for daily minimum temperature in month $mon$ (ºC), $T_{mn,mon}$ is the daily minimum temperature on day $d$ in month $mon$ (ºC), $μmn_{mon}$ is the average daily minimum temperature for the month $mon$ (ºC), and $N$ is the total number of daily minimum temperature records for month $mon$.

This parameter quantifies the variability in maximum temperature for each month. The standard deviation is calculated as

where $σmx_{mon}$is the standard deviation for daily maximum temperature in month $mon$ (ºC), $T_{mx,mon}$ is the daily maximum temperature on day $d$ in month $mon$ (ºC), $μmx_{mon}$ is the average daily maximum temperature for the month $mon$ (ºC), and $N$ is the total number of daily maximum temperature records for month $mon$.

The weather generator file contains weather generator data for any number of stations. For each weather generator station, there will be one line specifying the name of the station, its latitude, longitude, and elevation, and the number of years of maximum monthly 0.5 h rainfall data used to define values for pcp_hhr. There are no headers for this line. These variables are listed in the first table below. The second line for each weather generator station contains the headers for the following 12 lines, which list the weather generator data for each month of the year. An overview of the weather generator data variables is given in the second table below.

The average or mean total monthly precipitation is calculated as

where $R_{mon}$ is the mean monthly precipitation (mm H2O), $R_{day,mon}$ is the daily precipitation for day $d$ in month $mon$ (mm H2O), $N$ is the total number of records in month $mon$ used to calculate the average, and $yrs$ is the number of years of daily precipitation records used in calculation.

This parameter quantifies the variability in precipitation for each month. The standard deviation is calculated as

where $σ_{mon}$ is the standard deviation for daily precipitation in month $mon$ (mm H2O), $R_{day,mon}$ is the amount of precipitation for day $d$ in month $mon$ (mm H2O), $R_{mon}$ is the average precipitation for the month (mm H2O), and $N$ is the total number of daily precipitation records for month $mon$. Daily precipitation values of 0 mm are included in the standard deviation calculation.

This parameter quantifies the symmetry of the precipitation distribution around the monthly mean. The skew coefficient is calculated as

where is the skew coefficient for precipitation in the month, is the total number of daily precipitation records for month , is the amount of precipitation for day in month (mm H2O), is the average precipitation for the month (mm H2O), and is the standard deviation for daily precipitation in month (mm H2O). Daily precipitation values of 0 mm are included in the skew coefficient calculation.

This parameter is calculated as

where is the average number of days of precipitation in month , is the number of wet days in month during the entire period of record, and is the number of years of record.

The probability is calculated as

where is the probability of a wet day following a wet day in month , is the number of times a wet day followed a wet day in month for the entire period of record, and is the number of wet days in month during the entire period of record. A wet day is a day with > 0 mm precipitation.

The probability is calculated as

where is the probability of a wet day following a dry day in month , is the number of times a wet day followed a dry day in month for the entire period of record, and is the number of dry days in month during the entire period of record. A dry day is a day with 0 mm of precipitation. A wet day is a day with > 0 mm precipitation.

If all twelve months are < 1.0, the model assumes the data provided is relative humidity. Relative humidity is defined as the amount of water vapor in the air as a fraction of saturation humidity. If any month has a value > 1.0, the model assumes the data provided is dewpoint temperature.

Dew point temperature is the temperature at which the actual vapor pressure present in the atmosphere is equal to the saturation vapor pressure. This value is calculated by summing the dew point temperature for every day in the month for all years of record and dividing the sum by the number of days:

where $μdew_{mon}$ is the mean daily dew point temperature for the month (ºC), $T_{dew,mon}$ is the dew point temperature for day $d$ in month $mon$ (ºC), and $N$is the total number of daily dew point records for month $mon$. Please refer to the SWAT+ Theoretical Documentation for the equations used to convert dew point to relative humidity.

This value represents the most extreme 30-minute rainfall intensity recorded in the entire period of record.

This value is calculated by summing the average or mean wind speed values for every day in the month for all years of record and dividing the sum by the number of days:

where is the mean daily wind speed for the month (m/s), is the average wind speed for day in month (ºC), and N is the total number of daily wind speed records for month .

This value is calculated by summing the total solar radiation for every day in the month for all years of record and dividing the sum by the number of days:

where $μrad_{mon}$ is the mean daily solar radiation for the month (MJ/m2/day), $H_{day,mon}$ is the total solar radiation reaching the earth’s surface on day $d$ in month $mon$ (MJ/m2/day), and $N$ is the total number of daily solar radiation records for month $mon$.

The precipitation data files contain the observed precipitation input data. They are named by the user and must have the file ending *.pcp. There must be one file per station used in the simulation. As in all SWAT+ input files, the first line in the precipitation data files is reserved for user comments. The second line contains the column headers for the third line, which lists basic information about the station.

Starting in the fourth line, the year, Julian day, and precipitation amount are listed. There are no headers for these columns.

If the user wishes to run simulations at a sub-daily time step, precipitation data has to be provided at the simulation time step. Currently, the model is able to run at an hourly time step. Smaller time steps have not been tested yet. Three additional columns need to be included in the hourly precipitation files:

The pcp.cli file lists the names of the precipitation data files used in the simulation. The first line is reserved for user comments. The second line is reserved for the column header "filename". The user can list as many precipitation data file names as needed for the simulation. Only one file name should be listed per line. All file names listed in weather-sta.cli must be listed here. For every file name listed in pcp.cli, a file with that name must be provided by the user that contains the precipitation data measured at the station.

The temperature data files contain the observed temperature input data. They are named by the user and must have the file ending *.tmp. There must be one file per station used in the simulation. As in all SWAT+ input files, the first line in the temperature data files is reserved for user comments. The second line contains the column headers for the third line, which lists basic information about the station.

Starting in the fourth line, the year, Julian day, and the maximum and minimum temperatures are listed. There are no headers for these columns.

The tmp.cli file lists the names of the temperature data files used in the simulation. The first line is reserved for user comments. The second line is reserved for the column header "filename". The user can list as many temperature data file names as needed for the simulation. Only one file name should be listed per line. All file names listed in must be listed here. For every file name listed in tmp.cli, a file with that name must be provided by the user that contains the temperature data measured at the station.

The relative humidity data files contain the observed relative humidity input data. They are named by the user and must have the file ending *.hmd. There must be one file per station used in the simulation. As in all SWAT+ input files, the first line in the relative humidity data files is reserved for user comments. The second line contains the column headers for the third line, which lists basic information about the station.

Starting in the fourth line, the year, Julian day, and the relative humidity are listed. There are no headers for these columns.

The hmd.cli file lists the names of the relative humidity data files used in the simulation. The first line is reserved for user comments. The second line is reserved for the column header "filename". The user can list as many relative humidity data file names as needed for the simulation. Only one file name should be listed per line. All file names listed in must be listed here. For every file name listed in hmd.cli, a file with that name must be provided by the user that contains the relative humidity data measured at the station.

The solar radiation data files contain the observed precipitation input data. They are named by the user and must have the file ending *.slr. There must be one file per station used in the simulation. As in all SWAT+ input files, the first line in the solar radiation data files is reserved for user comments. The second line contains the column headers for the third line, which lists basic information about the station.

Starting in the fourth line, the year, Julian day, and solar radiation are listed. There are no headers for these columns.

The slr.cli file lists the names of the solar radiation data files used in the simulation. The first line is reserved for user comments. The second line is reserved for the column header "filename". The user can list as many solar radiation data file names as needed for the simulation. Only one file name should be listed per line. All file names listed in must be listed here. For every file name listed in slr.cli, a file with that name must be provided by the user that contains the solar radiation data measured at the station.

The wind speed data files contain the observed precipitation input data. They are named by the user and must have the file ending *.wnd. There must be one file per station used in the simulation. As in all SWAT+ input files, the first line in the wind speed data files is reserved for user comments. The second line contains the column headers for the third line, which lists basic information about the station.

Starting in the fourth line, the year, Julian day, and wind speed are listed. There are no headers for these columns.

The wnd.cli file lists the names of the wind speed data files used in the simulation. The first line is reserved for user comments. The second line is reserved for the column header "filename". The user can list as many wind speed data file names as needed for the simulation. Only one file name should be listed per line. All file names listed in weather-sta.cli must be listed here. For every file name listed in wnd.cli, a file with that name must be provided by the user that contains the wind speed data measured at the station.

SWAT+ is able to read in monthly, yearly, and average annual atmospheric deposition values. Reading in daily values is currently not an option in SWAT+. The time step of the atmospheric deposition data has to be specified by the user in codes.bsn and the time step of the data in atmo.cli has to match the specified time step.

The structure of the file atmo.cli varies slightly depending on the time step of the data. As in all SWAT+ input files, the first line is reserved for user comments. The second line contains the column headers for the third line, which lists basic information about the atmospheric deposition stations.

Below, there will be 5 lines for each station included in the atmospheric deposition file. In the first of these, the name of the station will be specified. It is followed by 4 lines of data:

1. Wet deposition of ammonia nitrogen

2. Wet deposition of nitrate nitrogen

3. Dry deposition of ammonia nitrogen

4. Dry deposition of nitrate nitrogen

The number of values listed in the data lines depends on the number of months or years data is available for.