Similar to the Specified Dew Point Thermograph Method, the Interpolated Dew Point Temperature Method in HEC-HMS requires the user to input temperature gages of measured or computed dew point temperatures within the study area. Unlike the specified approach, 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 temperature 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 dew point temperature gridset.

Required Parameters

Temperature Gages used in this method must be input as a time-series of dew point 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.