User:Jcreer/SWB

The Soil-Water-Balance (SWB) model has been developed by USGS to allow estimates of potential recharge to be made quickly and easily.

Required Input

Four gridded datasets are required:

• Hydrologic soil group
• land use/land cover
• Available soil-water capacity
• Surface-water flow direction

In addition to gridded land-surface data the model requires tabular daily climatological data (daily precip (inches), average air temperature [F], max air temp [F], min air temp [F]). Additional data may be required depending on model options: (1) daily average wind speed (m/s), (2) daily average relative humidity (%), (3) daily max relative humidity (%), (4) daily percentage of possible sunshine (%). Finally a lookup table must be supplied to assign curve numbers, interception values, rooting depths, and maximum daily recharge values.

Creating an SWB Simulation

To create an SWB simulation, do the following:

1. Right-click on the " Project" item the Project Explorer and select New Simulation | SWB.

Simulation Menu

Right-clicking on the simulation item in the Project Explorer will bring up the following menu command:

• Delete – Removes the simualation item.
• Duplicate Creates a copy for the simulation, including all parameters set in the model control.
• Rename – Allows changing the name of the simulation item.
• Model Control – Opens the Model Control dialog where the model run parameters are set.
• Model Check – Looks for potential errors in the model setup. Brings up the Model Checker dialog if issues are discovered.
• Export SWB – Generates files used in the model run.
• Launch SWB – Starts the SWB model run and opens the Simulation Run Queue dialog.
• Save, Export, and Launch SWB – Combines several other commands together. First, it saves the project file, then exports files for the model run, and finally launches the SWB model run.
• Properties – Opens the Properties dialog which displays the simulation properties and allows adding notes.

Model Control

The parameters for the SWB model run are set in the Model Control dialog.

Required Inputs

• Model Domain Definition – A flow direction grid is required as the domain. GMS requires this grid be a raster file loaded into the project. All other inputs must match the domain grid.
  • Domain raster definition – The Select button will bring up a Select Tree Item dialog where a raster in the project can be selected as the model domain.
• Growing Season – Specifies the duration of the growing season to determine interception.
  • Start day – The initial start time for the growing season.
  • End day – The ending time for the growing season.
  • Hemisphere – Either the "Northern Hemisphere" or the " Southern Hemisphere" can be selected.
• Climate Data – This section specifies the precipitation and temperature data.
  • Precipitation – Has options for entering a "Single station", "Arc grid", or "Surfer grid".
    • File prefix – If using an "Arc grid" or "Surfer grid", the prefix for the precipitation files needs to be entered. Using the 'Select button will bring up a browser where the folder containing the precipitation files can be selected.
  • Temperature – Has options for entering a "Single station", "Arc grid", or "Surfer grid".
    • TMIN file prefix – If using an "Arc grid" or "Surfer grid", the prefix for the minimum temperature files needs to be entered. Using the 'Select button will bring up a browser where the folder containing the temperature files can be selected.
    • TMAX file prefix – If using an "Arc grid" or "Surfer grid", the prefix for the maximum temperature files needs to be entered. Using the 'Select button will bring up a browser where the folder containing the temperature files can be selected.
  • Climate tabular files – This option must be set if using a single station for either Precipitation or Temperature.
  • File prefix – If using "single station", the prefix for the precipitation files in the climate folder needs to be entered.
• Flow Direction – A raster containing flow direction data is required.
  • Source raster – Clicking the Source button will bring up a Select Tree Item dialog where where a raster in the project can be selected for the flow direction.
• Soil Group – Can be set to use either a "Raster" or "Shapefile". Clicking the Select button will bring up a Select Tree Item dialog to specify when raster or shapefile in the project to use for the soil group. If a shapefile is used, the Shapefile field to convert to raster needs to specify which property in the shapefile to use for the soil group.
• Land Use – Can be set to use either a "Raster" or "Shapefile". Clicking the Select button will bring up a Select Tree Item dialog to specify when raster or shapefile in the project to use for the land use values. If a shapefile is used, the Shapefile field to convert to raster needs to specify which property in the shapefile to use for the land use values.
• Available Soil-water Capacity – Expresssed in units of inches of water per foot of soil. Can be set to use either a "Raster" or "Shapefile". Clicking the Select button will bring up a Select Tree Item dialog to specify when raster or shapefile in the project to use for the available soil-water capacity. If a shapefile is used, the Shapefile field to convert to raster needs to specify which property in the shapefile to use.
• Initial Soil Moisture – Expressed as a percentage (0–100) of the available water capacity. Can be either a "Constant" value or a "Raster". If using a raster, clicking the Select button will bring up a Select Tree Item dialog to specify when raster in the project to use.
• Initial Snow Cover – Expressed as water-equivalent value. Can be either a "Constant" value or a "Raster". If using a raster, clicking the Select button will bring up a Select Tree Item dialog to specify when raster in the project to use.
• Runoff Calculation and Routing Method – Can be set to "Downhill", "Iterative", or "None".
  • "Downhill" – This method sorts the grid cells to be either uphill or downhill.
  • "Iterative" – In this method, water is iteratively moved across the entire grid until it has infiltrated or left the grid.
  • "None" – Disables routing.
• Evapotranspiration Method – Method to estimate potential evapotranspiration.
  • "Thornthwaite-Mather" – Will produce lower estimates.
  • "Jensen-Haise" – Requires values to be entered for Albedo, Fraction of radiation on overcast days, and Fraction of radiation on clear days.
  • "Blaney-Criddle"
  • "Turc" – Requires values to be entered for Albedo, Fraction of radiation on overcast days, and Fraction of radiation on clear days.
  • "Hargreaves-Samani" – Suitable for use with gridded precipitation and air-temperature data.

Optional Inputs

• Adjusted Water Capacity – Calculates the adjusted water capacity independent of the model. Click the 'Select button for this will bring a Select Tree Item dialog where a raster can be selected to use for this value.
• Initial abstraction method – Method for calculating the initial abstraction within the NRS curve number. Can be set to "TR-55", "Hawkins", or "Not Specified".
• Specify iterative method tolerance – Can be increased to improve convergence at a potential cost of also increasing mass-balance errors.
• Ansi colored text – Generates output with color coding for positive and negative values.
• Frozen ground – This section sets options for continuous frozen ground.
  • Specify continuous frozen ground threshold values – Defines boundary between completely frozen soil and completely unfrozen soil.
    • Upper limit – Sets the upper boundary of the frozen ground threshold.
    • Lower limit – Sets the lower boundary of the frozen ground threshold.
  • Initial frozen ground index – Sets the initial continuous frozen ground index. Can be set to use a "Constant Value", "Raster", or "Not Specified".

Output Options

• Suppress screen output – Turns off the detailed mass-balanced information that is normally shown on the screen for each time step.
• Suppress daily files
• Suppress DISLIN messages
• Output grid file format
• Output grid filename prefix
• Output grid filename suffix
• Output Variables
  • Variable name
  • Daily output
  • Monthly output
  • Annual output
  • Copy/Paste Mode
  • Import
  • Export
  • Insert Above
  • Insert Below
  • Delete

External Links

• SWB website with USGS
• SWB User Manual