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# Pressure

**Pressure **in meteorologic terms is the pressure exerted by the weight of the atmosphere. Pressure is affected by a variety of factors including **Air Temperature**, **Altitude**, and **Humidity****. **Pressure also plays an integral role in several other meteorologic processes available to be modeled in HEC-HMS including **Evapotranspiration**, **Relative Humidity**, **Longwave Radiation**, and **Snowmelt.**

The **Pressure Method **included in the Meteorologic Model is required when certain types of **Evapotranspiration Methods **or **Longwave Radiation Methods **are used. Two methods **Gridded Pressure** and **Specified Barograph **are available for adding pressure data to the Meteorologic Model. More detail about each method is provided in the following sections.

## Gridded Pressure

The most common use of the **Gridded Pressure Method **is to utilize radar-based pressure estimates. Using additional software, it is possible to develop a gridded representation of pressure data or to use output from atmospheric models. If it is used with a transform method other than** ModClark**, an area-weighted average of the grid cells in the subbasin is used to compute the thermograph for each subbasin.

The **Gridded Pressure Method **includes a **Component Editor **with parameter data for all subbasins in the Meteorologic Model. The **Watershed Explorer **provides access to the **Pressure Component Editor. **

The **Component Editor **for all subbasins in the Meteorologic Model includes the selection of a data source. Gridded Pressure data must be stored as a pressure grid before it can be used in the Meteorologic Model. The data may be from radar sources or could be the result of complex calculations exterior to the program. Regardless, the grid data must be stored as a temperature grid. Only pressure grids already defined will be shown in the selection list. If there are many different gridsets available, you may wish to choose a gridset from the selector accessed with the **Grid **button next to the selection list. The selector displays the description for each gridset, making it easier to select the correct one.

The **Time Shift **can be used to correct for pressure grids stored with a time zone offset. All calculations during a simulation are computed assuming an arbitrary local time zone that does not observe summer time (daylight savings in the United States). Set the shift to zero if all the time-series and grid data is referenced in same local time zone. If other data sources such as observed discharge or precipitation are referenced in local time and the pressure grid is in referenced to a different time zone, select the correct shift so that the pressure data will match the rest of the data.

## Specified Barograph

The **Specified Barograph Method **includes a** Component Editor **with parameter data for all subbasins in the Meteorologic Model. The **Watershed Explorer **provides access to the **Pressure Component Editors. **

The **Component Editor **for all subbasins in the Meteorologic Model includes the **Time-Series Gage of Pressure **for each subbasin. A **Pressure Gage **must be selected for a subbasin. The current gages are shown in the selection list.

## Interpolated Pressure

Gridded data better captures the meteorology temporally and spatially across a watershed when compared to gage measurements at a single point. However, hourly or sub-hourly gridded meteorology products are not always available, particularly for historic events of interest to the modeler. In these cases, a gridded dataset can be created by interpolating point observations recorded at weather stations. The **Interpolated Pressure Method** allows the user to develop an interpolated pressure grid based on point pressure gage data.

The **Interpolated Pressure Method **includes a **Component Editor **with parameter data for all subbasins in the Meteorologic Model. The **Watershed Explorer **provides access to the **Shortwave Component Editor. **The **Component Editor **for each subbasin in the Meteorologic Model is used to enter parameter data that defines the** Interpolation Method** and to select radiation gages to use in the interpolation. The **Interpolation Method **can be selected from the dropdown menu. An interpolation method must be selected. The user has four interpolation options briefly summarized below.

The **Inverse Distance** interpolation method assumes the weight, or influence, of a gage is equal to the inverse of its distance from the interpolated cell. The **Inverse Distance Squared** interpolation method assumes the weight of a gage is equal to the inverse of the square of its distance from the interpolated cell. The **Nearest Neighbor **interpolation method simply assigns the nearest value to the cell center of interest without considering values of other nearby points. **Bilinear **interpolation within HEC-HMS relies on triangulation of the irregularly spaced gage locations. Based on the gage coordinates, a Triangulated Irregular Network (TIN) is created to represent the gage network in the basin model coordinate system. This TIN defines triangles, where each gage is a corner of one or more triangles. Given this TIN, a value at any given point is computed by first identifying the triangle in which that point falls, then interpolating within that triangle using Barycentric Coordinates. You must use three or more gages and the gages need to bound all grid cells.

Select **Pressure ****Gages** in the dropdown rows under the Gage column. The **Radius of Influence** is an optional parameter and represents the maximum interpolation distance. Beyond its radius of influence, the gage will not affect the cell values of the interpolated grid. Pressure Gages used in this method must be loaded in as **Time-Series of Pressure Data **with defined latitude and longitude information.

An interpolated pressure grid will be created once the simulation has been computed. Once the compute is complete the interpolated grid will be saved with the same name as the **Meteorologic Model **in **DSS** format in the HEC-HMS project directory.

## Barometric Pressure

The **Barometric Pressure Method **implements the atmospheric pressure algorithm described within Follum et al. (2015). The method calculates the atmospheric pressure at a specific location (grid cell or subbasin) by altering standard atmospheric pressure at sea-level (101.325 kPa) using a defined lapse rate. If the basin model is not georeferenced and/or doesn't have an associated terrain model, these calculations will proceed assuming the location is at sea-level.

The **Barometric Pressure Method**** **includes a **Component Editor **with parameter data for all subbasins in the Meteorologic Model. The** Watershed Explorer **provides access to the **Barometric Pressure ****Editor**, as shown in the following figure.

The **Barometric Pressure ****Component Editor **is shown in the following figure. The user must select a lapse method and provide an appropriate parameterization.