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