DES 606 : Watershed Modeling with

HEC-HMS

Module 8Theodore G. Cleveland, Ph.D., P.E

29 July 2011

Module 8: Average Rainfall

• Unit hydrographs are a tool used to explain the time re-distribution of excess precipitation on a watershed.

• Used for design purposes to produce discharge estimates at a location.

• Different unit hydrograph models– Empirical– Parametric

Module 8: Average Rainfall

• FHWA-NHI-02-001 Highway Hydrology– Chapter 2, Section 2.1; Chapter 3

• Examine spatial distribution of rainfall and averaging techniques.

• Examine how to put multiple gages into HMS and assign these gage depths to a particular watershed.

Module 8: Average Rainfall

• The total amount (measured as depth) of rainfall that occurs in a storm is the important input characteristic for describing the response of a watershed to rainfall.

Module 8: Average Rainfall

• There are a number of time-related and space-related factors that are used in explaining rainfall input.

• The four most important are: – Intensity (a rate: i.e. inches/hour)– Duration (a time: 15 minutes)– Frequency (a probability: 1%)– Average Depth (a length: inches)

• Actually better thought of as volume/area, but dimensionally it is a length.

Module 8: Average Rainfall

• HEC-HMS has precipitation input at “gages” that are assigned to basins.– The examples so far assume a single

gage is assigned to a sub-basin.

• HEC-HMS inputs are in depths, either incremental or cumulative– Intensity x Duration = Depth

• These computations are typically external to HMS.• Here duration is simply used as a time interval, but the

term really refers to an entire storm length and not some portion.

Module 8: Average Rainfall

• Watershed– Losses

– Transformation

– Storage

– Routing

• Precipitation– Meterology, Climate

• Runoff– Fraction of precipitation

signal remaining after losses

Spatial distribution – these precipitation arrows may not be identical.

Unless we wish to route hydrographs, need some way to “average” the input.

Module 8: Average Rainfall

• Averaging is used to generate uniform inputs for unit hydrograph applications– One implicit assumption of the UH is spatially

uniform input time series.

• Averaging avoids having to route hydrographs– Routing would probably be required on larger

watersheds.– If the data justify distribution, then would route

subdivided watersheds to capture storm patterns.

Module 8: Average Rainfall

• The entire volume of rainfall applied to an entire basin is called the precipitation volume– If the basin area normalizes this volume the

resulting value is called the equivalent uniform depth

• Methods to compute equivalent depth – arithmetic mean– theissen polygon network– iso-heyetal method

Module 8: Arithmetic Mean

• The mean value of all nearby gages is used– Not all gages actually on the

watershed

Module 8: Polygon Weighting

• A weighted mean based on polygon area is used– Not all gages actually on the

watershed

– Polygons formed using Theissen method

– Can use a minimum distance algorithm to semi-automatically generate the weights

Module 8: Polygon Weighting

• A weighted mean based on polygon area is used

Area ratios are called Theissen weights

Subarea A

Subarea A

Subarea C

Subarea C

Subarea D

Subarea D

Subarea E

Subarea B

Module 8: Iso-Hyetal

• A weighted mean based on iso-hyetal panel areas is used– Not all gages actually on the

watershed

– Areas formed by intersection of isohyetal contours and underlying drainage area

Module 8: Iso-Hyetal

• A weighted mean based on isohyetal panel areas is used

Module 8: Averaging Rainfall

• Theissen polygons and arithmetic mean are probably the most common because the weights are constants with respect to geography.– Arithmetic mean is easiest to automate

Module 8: Example 8

• Illustrate use of multiple gages on Ash Creek.– Known Theissen weights are

• 0.12 and 0.88

– Simulate using these known weights.

Summary

• Multiple rain gage data can be used to estimate an equivalent uniform depth

• Gage weights by a variety of methods– Arithmetic mean– Minimum distance (Theissen polygons)– Isoheyetal– Inverse distance methods

Summary

• HEC-HMS models multiple gages in the Meterological Model Manager

• Example 8 illustrated how to set-up multiple gages– Weights were supplied