Similar to the Specified Barograph Method, the Interpolated Pressure Method in HEC-HMS requires the user to input air pressure gages of observed atmospheric pressure within the study area. Unlike Specified Barograph, multiple gages can be applied to a single subbasin through interpolation. A radius of influence can be used to exclude gages beyond a certain distance from a subbasin node. Gage readings are then interpolated across nearby gages in accordance with the interpolation method chosen by the user:
Inverse Distance Weighted
Inverse Distance Squared
Nearest Neighbor
Bilinear
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. Three or more pressure gages must be loaded into the model and the gages need to bound all grid cells.
The result of this method is a computed, continuous atmospheric pressure gridset.
Required Parameters
Air Pressure Gages used in this method must be input as a time-series of atmospheric pressure data with defined latitude and longitude information. The basin model should be georeferenced so gages can be accurately applied to subbasins. An Interpolation Method must be chosen and then a Radius of Influence for each gage must be defined in kilometers or miles.