Atmospheric (or barometric) pressure is the pressure exerted by all gases in moist air. It is strongly influenced by elevation and varies with local meteorological conditions. Generally, atmospheric pressure decreases with increasing altitude. At sea level, typical values are around one atmosphere (1 atm). The watershed display unit system selection does not affect how atmospheric pressure data is displayed.

The Meteorological Dataset Editor provides three data source options for entering atmospheric pressure: Constant Value, DSS, and Elevation (see Met Data Sources – Overview). Unique to atmospheric pressure, users can select Elevation (measured in feet, ft, or meters, m) to approximate the pressure based on the site's physical elevation. The available units for atmospheric pressure include: atmospheres (atm), bars (bar), kilopascals (kPa), millibars (mb), millimeters of mercury (mm-Hg), inches of mercury (in-Hg), and pounds per square inch (psi).

For instructions on entering atmospheric pressure using the Constant Value or DSS data source options, refer to Met Data Sources – Overview.

Elevation

If a time series of atmospheric pressure is unavailable, it may be estimated based on site elevation. Although using a time series is preferred, if none is available, the local atmospheric pressure can be estimated from the reference elevation of the meteorological station (set in the Watershed Setup module via the Station Editor, as described in the section Define a Water Quality Station). The reference elevation should correspond to the average land surface elevation. Note: The reference elevation is set in the Watershed Setup module and cannot be modified within the Meteorological Dataset Editor.

To set the atmospheric pressure to be estimated during the simulation compute:

  1. From the ResSim main window, navigate to the Water Quality module and select Meteorological Data from the Water Quality menu. The Meteorological Dataset Editor opens.

  2. In the Meteorological Dataset Editor, select the desired meteorological dataset from the navigation bar (e.g., Met_Dataset1). Choose the appropriate view configuration (either the Station or Variables radio button). In the selection tree, select the meteorological station and the atmospheric pressure parameter of interest.

  3. In the editable data panel, from the Data Source dropdown list, select Elevation (Elevation Data Source – Available for Atmospheric Pressure – Example). The editable panel updates based on the selected data source.

  4. Enter a description (optional, but recommended) in the Description box.

The Elevation box, Units, and plot window automatically update to display the reference elevation (set in the Watershed Setup module via the Station Editor). This reference elevation cannot be modified within the Meteorological Dataset Editor. For example, as shown in (Elevation Data Source – Available for Atmospheric Pressure – Example), the elevation for the selected station is 489 feet (ft).


Elevation Data Source – Available for Atmospheric Pressure – Example

Click Apply to save the entered value before switching to a different data source, meteorological parameter, station, or dataset. Click OK to save the data and close the Meteorological Dataset Editor.