Similar to Specified Pyrgeometer method, the Interpolated Shortwave method within HEC-HMS requires the user input radiation gages of observed solar radiation data within the study area. Unlike Specified Pyrgeometer, 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 radiation 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 shortwave radiation grid.
Required Parameters
Radiation Gages used in this method must be loaded in as time-series of radiation datawith defined latitude and longitude information. The basin model should be georeferenced so gages can be accurately applied to subbasins.