If aa_int_cnt = 0, the model will print average annual output for the entire period. If aa_int_cnt > 0, the end years of the print intervals have to be specified by the user by listing them in chronological order in the same line as aa_int_cnt.

Over the past 20 years, the Soil and Water Assessment Tool (SWAT) has become widely used across the globe. Various applications of the model have revealed limitations and identified model development needs. Numerous additions and modifications of the model and its individual components have made the code increasingly difficult to manage and maintain. In response to these issues and in order to face present and future challenges in water resources modeling, the SWAT code has undergone major modifications over the past few years, resulting in SWAT+, a completely restructured version of the model. Even though the basic algorithms used to calculate the processes in the model have not changed, the structure and organization of both the code (object based) and the input files (relational based) have been modified significantly. This is expected to facilitate model maintenance, future code modifications, and foster collaboration with other researchers to integrate new science into SWAT modules. Additionally, SWAT+ provides a more flexible spatial representation of interactions and processes within a watershed.

SWAT was developed by USDA-ARS and Texas A&M scientists.

SWAT+ is able to end a simulation at any day of the year. This option can for example be useful, if the user wishes to simulate hydrological years instead of calendar years.

All SWAT+ input files are free format and space delimited.

The first line in each input file is reserved for a title. If the files were written using the SWAT+ Editor, the title will specify the name of the file, the version of the SWAT+ Editor, and the date and time the file was written. While this line is required, it is not read in by SWAT+ and may be modified or left blank. The title is limited to 80 spaces.

The second line in all SWAT+ input files except for file.cio is reserved for the header, i.e. the names of the variables listed in the file. Some files will have additional header lines (e.g., print.prt).

SWAT+ is able to begin a simulation at any day of the year. This option can for example be useful, if the user wishes to simulate hydrological years instead of calendar years.

Some simulations will need a warm-up or equilibration period. The use of a warm-up period becomes more important as the simulation period of interest shortens. For 30-year simulations, a warm-up period is optional. For a simulation covering 5 years or less, a warm-up period is recommended.

Soil water contents of the soil layers can be printed for all HRUs in the same file by setting obj_typ_no = 0. For all other outputs, a separate object print record with a unique name has to be defined for each object.

This parameter specifies the interval within the specified printing period.

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

Please refer to the Theoretical Documentation for a description of the use of this parameter in the mineralization calculations.

Sediment routing is a function of peak flow rate and mean daily flow. Because SWAT originally could not directly calculate the sub-daily hydrograph due to the use of precipitation summarized on a daily basis, this variable was incorporated to allow adjustment for the effect of the peak flow rate on sediment routing. This factor is used in the MUSLE equation and impacts the amount of erosion generated in the HRUs.

The phosphorus percolation coefficient is the ratio of the solution phosphorus concentration in the top 10 mm of soil to the concentration of phosphorus in the percolate.

Root density is greatest near the surface, and plant nitrogen uptake in the upper portion of the soil will be greater than in the lower portion. The depth distribution of nitrogen uptake is controlled by n_uptake, the nitrogen uptake distribution parameter.

The importance of the nitrogen uptake distribution parameter lies in its control over the maximum amount of nitrate removed from the upper layers. Because the top 10 mm of the soil profile interacts with surface runoff, the nitrogen uptake distribution parameter will influence the amount of nitrate available for transport in surface runoff. The model allows lower layers in the root zone to fully compensate for lack of nitrate in the upper layers, so there should not be significant changes in nitrogen stress with variation in the value used for n_uptake.

Normal flow is defined as the streamflow when the channel is at 0.1*bankfull depth. This parameter is only important if channel routing is simulated using the Muskingum routing method.

Sediment routing is a function of peak flow rate and mean daily flow. Because SWAT originally could not directly calculate the sub-daily hydrograph, this variable was incorporated to allow adjustment for the effect of the peak flow rate on sediment routing. This variable impacts channel degradation.

This parameter controls the amount of pesticide removed from the surface layer in runoff and lateral flow relative to the amount removed via percolation. This parameter is only important if pesticide transport is simulated.

The weighting factor is a function of the wedge storage. This parameter is only important if channel routing is simulated using the Muskingum routing method.

This parameter defines the fraction of field capacity water content above which denitrification takes place. Denitrification is the bacterial reduction of nitrate (NO3) to N2 or N2O gases under anaerobic (reduced) conditions. Because SWAT+ does not track the redox status of the soil layers, the presence of anaerobic conditions in a soil layer is defined by this variable. If the soil water content calculated as a fraction of field capacity is ≥ denit_frac, then anaerobic conditions are assumed to be present and denitrification is modeled. If the soil water content calculated as a fraction of field capacity is < denit_frac, then aerobic conditions are assumed to be present and denitrification is not modeled.

The evaporation coefficient is a calibration parameter for the user that was created to allow reach evaporation to be dampened in arid regions. The original equation tends to overestimate evaporation in these areas.

All soils in the watershed will be initialized to the same fraction. If sw_init = 0.0, the model will calculate it as a function of average annual precipitation.

The phosphorus soil partitioning coefficient p_soil is the ratio of the soluble phosphorus concentration in the top 10 mm of soil to the concentration of soluble phosphorus in surface runoff.

The primary mechanism of phosphorus movement in the soil is by diffusion. Diffusion is the migration of ions over small distances (1-2 mm) in the soil solution in response to a concentration gradient. Due to the low mobility of solution phosphorus, surface runoff will only partially interact with the solution P stored in the top 10 mm of soil.

In large routing units with a time of concentration greater than 1 day, only a portion of the surface runoff will reach the main channel on the day it is generated. SWAT+ incorporates a surface runoff storage feature to lag a portion of the surface runoff release to the main channel.

This parameter controls the fraction of the total available water that will be allowed to enter the reach on any one day. For a given time of concentration, as surq_lag decreases in value more water is held in storage. The delay in release of surface runoff will smooth the streamflow hydrograph simulated in the reach.

Normal flow is defined as the streamflow when the channel is at bankfull depth. This parameter is only important if channel routing is simulated using the Muskingum routing method.

This parameter controls the amount of nitrate removed from the surface layer in runoff relative to the amount removed via percolation.

The fraction of residue that will decompose in a day assuming optimal moisture, temperature, C:N ratio, and C:P ratio.

This file lists the names of all input files used in a simulation run. The files are grouped in different categories and there is one line for each category. The first column lists the names of the categories. The number of columns per line depends on the number of files in a category. Most files are required for all SWAT+ runs, i.e. they have to be listed in file.cio and the corresponding file has to be present in the TxtInOut folder of the SWAT+ project. There are also some optional files that are only required for specific SWAT+ applications. The category names and their files are listed below.

Simulation

Basin

Climate

3. wind-dir.cli (currently not used)

Connect

4. gwflow.con (a description of the gwflow module and all related input files will be added asap)

Channel

2. channel.cha (currently not used)

3. hydrology.cha (currently not used)

4. sediment.cha (currently not used)

Reservoir

Routing Unit

4. rout_unit.dr (currently not used)

HRU

Export Coefficient

1. exco.exc

2. exco_om.exc

3. exco_pest.exc (currently not used)

4. exco_path.exc (currently not used)

Recall

1. recall.rec

Delivery Ratio

1. del_ratio.del (currently not used)

2. dr_om.del (currently not used)

3. dr_pest.del (currently not used)

4. dr_path.del (currently not used)

Aquifer

Herd

1. animal.hrd (currently not used)

2. herd.hrd (currently not used)

3. ranch.hrd (currently not used)

Water Rights

1. water_allocation.wro

Link

1. chan-surf.lin

2. aqu_cha.lin

Hydrology

Structural

HRU Databases

Operation Scheduling

Land Use Management

Change

1. cal_parms.cal

2. calibration.cal

3. codes.sft

4. wb_parms.sft

Initial

3. om_water.ini

4. pest_hru.ini

Soils

3. soils_lte.sol (currently not used)

Conditional

2. res_rel.dtl

3. scen_lu.dtl

4. flo_con.dtl

Regions

3. ls_reg.ele

4. ls_reg.def

The last five rows in file.cio are used to specify the climate file directories if these are stored in a folder other than the project TxtInOut folder.

Landscape Unit

A Landscape Unit is a collection of HRUs. A Landscape Unit can be equivalent to a subbasin, a floodplain or upland unit, or a grid cell with multiple HRUs. Landscape Units are only used for output. Two input files are required to define which HRUs belong to which Landscape Unit: 1) landscape elements and, 2) landscape define. The elements file includes HRUs and their corresponding LSU fraction and basin fractions. The define file specifies which HRUs are contained in each LSU.

The print.prt file is formatted differently than most other SWAT+ input files (see figure below). In line three, there are several variables for controlling the time period to be printed. In line five, the user can specify the number of print intervals for average annual output. In line seven, the user can select to have output files printed in a specific file format (in addition to the default *.txt output files). In line 9, the user can control the printing of outputs for soils and management as well as flow duration curves. In lines 11 to 90, there is a list of outputs for different spatial levels and objects that can be printed at daily, monthly, yearly, and average annual time steps. A description of the output files is provided in the section.

The table below lists the objects that are listed in the first column of this section of the print.prt file. For each of these objects, output can be printed at daily, monthly, yearly, and average annual time steps by entering y (=yes) or n (=no) in the following four columns.

Output can be printed for the following types of objects:

The following hydrographs can be printed:

Seibert and McDonnell (2002) suggested the use of “hard” and “soft” data for multi-criteria model calibration. Hard data are defined as measured time series, typically at a point (e.g., streamflow, groundwater levels, or soil moisture) that is commonly used in regression-based calibration techniques. Soft data are defined as information on individual processes within a balance that may not be directly measured in the study area, may be an average annual estimate, and may entail considerable uncertainty. Examples of soft data include regional estimates of baseflow ratios or ET, average depths of groundwater tables, average annual runoff coefficients for various land uses, annual rates of denitrification from research plots found in the literature, event mean concentrations, nutrient/sediment export coefficients, sediment deposition from reservoir sedimentation studies, average crop/vegetation LAI, and county crop yields (Arnold et al., 2015).

Soft calibration procedure

Seibert and McDonnell (2002) argued that soft data represent a new dimension to the model calibration process that could: (1) enable dialog between experimentalists and modelers, (2) be a formal check on the reasonableness and consistency of internal model structures and simulations, and (3) specify realistic parameter ranges often ignored in today’s automatic calibration routines.

The object.prt file is commonly used to:

• Print daily channel outflow to compare to observed streamflow a stream gage

• Print daily flow to a file that can be read in as a point source from another SWAT+ simulation

The only timestep output can be printed at using this file is daily. A description of the output files is provided in the section.

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

There are three files that control the settings for a simulation run:

• specifies the land area and the total area (including ponds and reservoirs) of the watershed and the counts of all spatial objects in a simulation,

• controls the simulation time period and time step, and

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: .

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 , , , , and , respectively.

SWAT+ is able to run at the following time steps:

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

SWAT+ offers considerable flexibility with regard to the configuration of a watershed. The elements of a watershed are defined as spatial objects:

• Landscape Unit

• Routing Unit

• HRU

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

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

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 is the mean daily minimum temperature for the month (ºC), is the daily minimum temperature on day in month (ºC), and is the total number of daily minimum temperature records for month .

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

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

where is the standard deviation for daily minimum temperature in month (ºC), is the daily minimum temperature on day in month (ºC), is the average daily minimum temperature for the month (ºC), and is the total number of daily minimum temperature records for month .

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

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

where is the standard deviation for daily maximum temperature in month (ºC), is the daily maximum temperature on day in month (ºC), is the average daily maximum temperature for the month (ºC), and is the total number of daily maximum temperature records for month .

The average or mean total monthly precipitation is calculated as

where is the mean monthly precipitation (mm H2O), is the daily precipitation for day in month (mm H2O), is the total number of records in month used to calculate the average, and is the number of years of daily precipitation records used in calculation.

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.

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 . 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 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.

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 is the mean daily solar radiation for the month (MJ/m2/day), is the total solar radiation reaching the earth’s surface on day in month (MJ/m2/day), and is the total number of daily solar radiation records for month .

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.

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 atmo.cli file is formatted differently than most other SWAT+ input files and its structure 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.

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

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.

Numerous methods exist to calculate potential evapotranspiration. Three of the most widely-used ones are included in SWAT+, the Priestley-Taylor, Penman/Monteith, and Hargreaves equations. The codes for the three methods are listed in the table below. If a method other than Priestley-Taylor, Penman/Monteith, or Hargreaves is recommended for the area, in which the watershed is located, the user can calculate daily PET values with the recommended method and import them into SWAT+ (using in ).

There are two channel water routing methods available in SWAT+:

This variable is used to calculate values for .

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.

Papers Daniel, Tássia

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.

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 is the mean daily maximum temperature for the month (ºC), is the daily maximum temperature on day in month (ºC), and is the total number of daily maximum temperature records for month .

If crack = 1, the crack volume potential is controlled by in .

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.

This parameter controls plant uptake of phosphorus from the different soil horizons in the same way that controls nitrogen uptake.

Phosphorus removed from the soil by plants is taken from the solution phosphorus pool. The importance of the phosphorus uptake distribution parameter lies in its control over the maximum amount of solution P removed from the upper layers. Because the top 10 mm of the soil profile interacts with surface runoff, the phosphorus uptake distribution parameter will influence the amount of labile phosphorus available for transport in surface runoff. The model allows lower layers in the root zone to fully compensate for lack of solution P in the upper layers, so there should not be significant changes in phosphorus stress with variation in the value used for p_uptake.

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 .

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 is the mean daily dew point temperature for the month (ºC), is the dew point temperature for day in month (ºC), and is the total number of daily dew point records for month . Please refer to the SWAT+ Theoretical Documentation for the equations used to convert dew point to relative humidity.

Many studies have shown that after an application of soluble P fertilizer, solution P concentration decreases rapidly with time due to reaction with the soil. This initial "fast" reaction is followed by a much slower decrease in solution P that may continue for several years (Barrow and Shaw, 1975; Munns and Fox, 1976; Rajan and Fox, 1972; Sharpley, 1982). In order to account for the initial rapid decrease in solution P, SWAT+ assumes a rapid equilibrium exists between solution P and an "active" mineral pool. The subsequent slow reaction is simulated by the slow equilibrium assumed to exist between the "active" and "stable" mineral pools. The algorithms governing movement of inorganic phosphorus between these three pools are taken from Jones et al. (1984).

Equilibration between the solution and active mineral pool is governed by the phosphorus availability index. This index specifies the fraction of fertilizer P which is in solution after an incubation period, i.e. after the rapid reaction period.

A number of methods have been developed to measure the phosphorus availability index. Jones et al. (1984) recommends a method outlined by Sharpley et al. (1984) in which various amounts of phosphorus are added in solution to the soil as K2HPO4. The soil is wetted to field capacity and then dried slowly at 25°C. When dry, the soil is rewetted with deionized water. The soil is exposed to several wetting and drying cycles over a 6-month incubation period. At the end of the incubation period, solution phosphorus is determined by extraction with anion exchange resin.

The P availability index is then calculated as:

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.

General watershed attributes are defined in two basin input files, and . These attributes control a diversity of physical processes at the watershed level. Users can use the default values set by the interfaces or change them to better reflect what is happening in a given watershed.

Papers Xuesong, Armen

Vadas & White (2010)

The Unit Hydrograph method is only relevant when simulating at a sub-daily timestep and = 0.

This coefficient allows the user to control the rate of denitrification.

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

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