1. Multiple Rain Gages and Weighting in HEC-HMS
Example 10
Multiple Rain Gages and Weighting in HEC-HMS

2. Purpose
Illustrate the steps to use multiple rainfall gages on a watershed.
Multiple gage inputs to a sub-basin
Input weighted by gage weights, in the example determined by Theissen polygons.

3. Learning Objectives Reinforce use of HEC-HMS Graphical User Interface.
Reinforce the concepts of “Projects” as a data-storage paradigm.
Explore different meterological model concepts:
Initial loss and constant rate loss model
SCS Unit Hydrograph transformation model
User-specified hyetograph.

4. Problem Statement
Simulate the response of the Ash Creek watershed at Highland Road for a 5-year for the May 1978 storm using the two recording rain gages as the input.
Treat the entire watershed as a single sub-basin.
Each gage contribution is proportional to their Theissen weights (determined elsewhere)

5. Example 8 Multiple Rain Gages
Ash Creek Example 3
Import basin model
Import meteorological model
Import time series model
Verify run, then save.

6. Example 10 Multiple Rain Gages
Verify run, then save

7. Example 10 Multiple Rain Gages
Examine the rainfall source file
Two gages
14-W
15-W
Theissen weights are unreported
14-W = 0.12
15-W = 0.88
should work for this example

8. Multiple Rain Gages
Create two gages in Time Series manager
14W
15W

9. Multiple Rain Gages
Copy the date from the external file (Excel) into the respective gages.
Pay attention to non-uniform time, use the fill feature as needed
Use chart to look for gaps in input

10. Multiple Rain Gages Once gage data is entered, save.
Meteorological model is next, we need to change the model to allow multiple gages.
Select “Gage Weights”

11. Multiple Rain Gages
Select which gages will be applied to Subbasin-1

12. Multiple Rain Gages
Select which gages will be applied to Subbasin-1

13. Multiple Rain Gages Enter gage weights – in this case depth weights
Time weights are set to 1.0

14. Multiple Rain Gages
Run the model and examine output

15. Summary
Illustrate how to use multiple gage data and assign by weights each gages contribution to rainfall on a sub-basin.
Example can be extended to multiple sub-basins.
Gages need not be physically present on the watershed, but should be close by (same hemisphere?)
More exotic approaches are mentioned in Chapter 11, HEC-HMS user manual.
Weighting methods require external data preparation and weight determination