Comparison of the SCS Curve Number Method to the Baseflow Separation Method for Determining Runoff o

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Comparison of the SCS Curve Number Method to the
Baseflow Separation Method for Determining Runoff
over the San Marcos Basin
My Rainfall Runoff Project

• Lauren Schneider
• CE394K.2 Surface Water Hydrology
• Dr. Maidment
• 4/28/05

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Overview

• Purpose and Goals
• Background What are Curve Numbers?
• Data Gathered
• Methodology
• Curve Number Calculation
• Base Flow Separation
• Derivation of CN from real data
• Results
• Conclusions and Future Work

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Purpose and Goals

• Purpose to compare the SCS CN method for
  calculating runoff to runoff data
• Goals
• Determine CNs for each watershed from land cover
  and hydrologic soil group data
• Separate base flow for stream gage data to get
  runoff data
• Derive CNs from runoff data
• Compare CNs

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What are Curve Numbers?

• A curve number is an indicator of land
  impermeability
• Uses Land Use and hydrologic soil data
• Used to relate rainfall (in.) to runoff (in.)
• CN can be weighted for a watershed
• Generally used for small areas

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Data Collection San Marcos Basin
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Goal 1 Determine Watershed CN
Blanco River at Wimberley
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Goal 1 Determine Watershed CN
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Goal 2 Base Flow Separation

• Volume of runoff must be separated from stream
  gage data
• Base flow is the discharge NOT associated with a
  rainfall event
• Used BFI program

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BFI Software

• A program for determining base flow
• Input comes directly from USGS stream gage data
• BFI (base flow)/(total flow) for the period
• Only significant parameter is N

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BFI results

• The following graphs show the results of
  separating base flow
• Stream gage data from 1995
• Monthly runoff data was calculated from the BFI
  results for the purpose of deriving the
  watersheds CN

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Base Flow Blanco at Wimberley
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Base Flow Blanco Near Kyle
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Base Flow SM at SM
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Base Flow SM at Luling
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Precipitation Data

• Monthly precipitation data was interpolated over
  the watersheds

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Goal 3 Derive CN from Data

• Monthly runoff and precipitation was used to find
  CNs for each watershed

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Results

• Base flow separation provided runoff/CN
• Runoff/CN overestimated using land cover and soil
  group
• Large variation from month to month

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Conclusions

• Using CN to predict runoff for a large watershed
  might give significant error
• CN table used might overestimate
• Runoff data from base flow separation not
  dependable for losing streams

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Future Work

• Research other CN tables
• Use raster calculation instead of weighted CN
• Use larger N for base flow separation
• Determine a way to account for recharge from
  stream
• Analyze more years of data so that a statistical
  analysis can be performed