Example 8
Multiple Rain Gages and Weighting in HEC-HMS
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.
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.
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)
Example 8 Multiple Rain Gages
• Ash Creek Example 3 – Import basin model– Import meteorological model– Import time series model– Verify run, then save.
Example 8 Multiple Rain Gages
• Verify run, then save
Example 8 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
Example 8 Multiple Rain Gages
• Create two gages in Time Series manager– 14W– 15W
Example 8 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
Example 8 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”
Example 8 Multiple Rain Gages
• Select which gages will be applied to Subbasin-1
Example 8 Multiple Rain Gages
• Select which gages will be applied to Subbasin-1
Example 8 Multiple Rain Gages
• Enter gage weights – in this case depth weights– Time weights are set to 1.0
Example 8 Multiple Rain Gages
• Run the model and examine output
Example 8 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