Introduction

The Discretization method defines how a subbasin is discretized. This feature was introduced in HEC-HMS version 4.7.0. There are four Discretization types: Structured, Unstructured, File-Specified, and None. Prior to version 4.7.0, subbasin discretizations could only be specified through the use of a *.mod grid cell file, which was generated using HEC-GeoHMS.

HEC no longer provides technical support for HEC-GeoHMS. As of version 4.4.0, HEC-HMS contains GIS capabilities, including watershed delineation functions. 

One limitation of the *.mod grid cell file is that it does not use absolute spatial references. Grid cell locations are referenced from an arbitrary lower left corner. Structured and Unstructured discretizations provide spatial-awareness to spatially-discrete Subbasin elements. Advantages of the spatially-aware approach include the ability to view discrete elements, the ability to sample values from other geospatial data, and the ability to visualize results for discrete elements.

Additional tutorials are available that discuss the use of gridded boundary conditions with various discretization methods:

• Applying Gridded Precipitation to a Non-Georeferenced Project - Structured Discretization
• Applying Gridded Precipitation to a Non-Georeferenced Project - File-Specified Discretization
• Creating a Complex 2D Flow model within HEC-HMS - Unstructured and Structured Discretizations

Steps that can be taken to change a Subbasin element's Discretization from File-Specified to Structured are described below.

Instructions

1. Download terrain data from the U.S. Geological Survey National Elevation Dataset or another source.
2. Follow the instructions in Preparing Terrain Data for HEC-HMS using ArcMap or Preparing Terrain Data for HEC-HMS using ArcMap to process, reproject, and clip your terrain data and import the terrain data to HEC-HMS.
3. In HEC-HMS, create a Terrain Data element (Components | Create Component | Terrain Data) and select the *.tif file.
4. Select the Basin Model and in the Component Editor, select the newly created Terrain Data element, as shown below:
   
5. Click the save button (floppy disk ) or the Ctrl+ S keys on your keyboard and the terrain should appear in the map.
   
6. Select GIS | Preprocess Sinks and once that is finished, select GIS | Preprocess Drainage.
7. Make sure the Basin Model is selected and then click Parameters | Discretization | Change Method. You will be prompted with a dialog asking if you want to change the discretization method for all subbasins. Click Yes.
8. Select Structured from the drop-down menu and click Change.
9. The default Structured Discretization uses the Standard Hydrologic Grid (SHG) projection and a grid cell size of 2000 meters x 2000 meters. You can change the projection and cell size for all subbasins in the Global Editor by selecting Parameters | Discretization | Structured or for one subbasin at a time using the Component Editor.
10. Select GIS | Compute | Grid Cells to re-compute the grid cells.
    
    

    Once the grid cells have been computed, you can visualize them by clicking View | Map Layers and turning on the Discretization layer, as shown below.

     
11. The discretization information is saved in a *.sqlite file that is referenced by HEC-HMS.
    
    

    This *.sqlite file will automatically be linked to the appropriate subbasin(s).