Basic Hydrology & Hydraulics: DES 601

Module 6Regional Analysis

Regional Regression Analysis• Regional analysis refers to the construction of

regression equations for a region by analyzing historical discharges on many streams within that region.

• These equations are statistical models of discharge based on certain explanatory variables, typically

AREASLOPESHAPE INDICESCLIMATE INDICES

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Watershed Characteristics• What characteristics influence runoff?

• Where you are• How large an area• Gradient

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Watershed Characteristics• What characteristics influence runoff?

• Width, shape• Elevation: minimum, maximum + slope• Roughness: Channels, overbanks• Geology and soils• Climate• Vegetation• Land use, including urbanization and

imperviousness• Controls: Dams, gates, diversions, channel

rectification

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Watershed Delineating• Topographic maps

• Hands-on methods• Marking directly on

map• Tracing using light

table• Computerized methods

• DEMs• GIS software• Semi-automated

delineation• Fully automated

delineation• Automated

measurements should always be verified

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Information Sources• USGS quadrangle maps• Aerial photos• Satellite imagery• NRCS soil surveys• Field surveys• Previous investigations

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NRCS county soils surveys

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Soil descriptions

WEB SOIL SURVEY:

http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm

http://soildata.tamu.edu/

http://soils.usda.gov/soils/technical/classification/scfile/index.html

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Learn More

Videos that show watershed delineation, and measuring some of the watershed characteristics.

http://ww.rtfmps.com/hec-hms-movies/Module-2/Video-2/

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Regional Regression Analysis• The equations are constructed by first fitting an

appropriate probability distribution to observations at a gaged location (station flood frequency).

• Then the station flood frequency curves are used as surrogate observations (at a specified AEP) to relate discharge to select geomorphic variables

• The “betas” are obtained by trying to make “epsilon” small, the AREA, SLOPE, and other watershed characteristics are the explainatory variables.

€

(v Q AEP − β0 − β1

v A − β2

v S 0L )2 =

v ε

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General forms of regional equations

• The resulting equations are then expressed in a power-law form for actual application

• Equation suites for some regions have been chronically problematic (produce inexplicable or inconsistent results)

• Traditional regionalization is probably unwise in Texas – too much variation in climate and terrain, too little data.

€

QAEP ,Estimate = β0(AREA)β 1 (SLOPE)β 2 (MAP)β 3

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Regression Equations in Texas• 1977 Massey and Schroeder (1977) USGS Water

Resources Investigations 77-110 Open File Report• 6 Regions and some undefined areas• Region 6 (trans-pecos Texas) used AREA and

MEAN ANNUAL PRECIPITATION as explanatory variables

• Region 3 (west-central Texas) used area as the only explanatory variable

• Analysis done using the “Bulletin 17B” process, which has since shown to perform erratically in much of Texas

• Variables used are AREA, SLOPE, and SHAPE, but their use is inconsistent within a suite of equations in some cases.

• Depreciated Method

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Regression Equations in Texas

• 1996 Asquith and Slade in• 11 Regions• 16 sets of equations (some regions are

broken at 32 square miles) 96 equations total

• Number of stations used for developing equations varies from 27 to 66

• SEE varies from 28% to 160%, with a mean of 66%.

• Depreciated Method

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Regression Equations in Texas

• 2009 Asquith and Roussel• Documented in HDM• 7 Equations for different AEP• Mean annual precipitation to account for

climatic variability• OmegaM used to account for location

(mapped value)• Area and Slope

• Current (2011) Suggested Method

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MAP

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OmegaEM

Regression Equations in Texas• 2009 Asquith and Roussel, Texas Regression

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Summary

• Regional analysis uses watershed characteristics obtained from maps, reports, etc. to estimate discharge at a location.

• Watershed characteristics are obtained from map and other sources – required a way to measure areas, lengths, etc.

• Regression equations relate these characteristics in a region to a discharge (and an AEP value).

• In Texas location is conveyed by the OmegaM parameter and climatic effect through the mean annual precipitation.

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