Comparing the Storage Efficiencies of the Highland Lakes and the Proposed LCRA-SAWS Project Lower Colorado River Reservoirs

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Comparing the Storage Efficiencies of the
Highland Lakes and the Proposed LCRA-SAWS Project
Lower Colorado River Reservoirs

• Andrew Judd
• Semester Project for CE 394K.2
• Surface Water Hydrology
• Instructor Dr. Maidment
• University of Texas

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INTRODUCTION

• Lower Colorado River Authority (LCRA)
• Project Centers around water transfer agreement
• 150,000 ac-ft/yr to San Antonio
• Income to LCRA for improving water supply system
  and conservation
• One project component is 3-4 off-channel storage
  reservoirs for San Antonio supply

Source www.regionk.org
Source www.LCRA.org
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OBJECTIVE

• Majority of regional water supply storage in
  Highland Lakes
• Lower Colorado Reservoirs to supply water to San
  Antonio
• Without reducing storage volume in Highland Lakes

Source www.lcra.org
Research Project Compare storage efficiencies of
Highland Lakes and proposed Lower Colorado
reservoirs using historical data
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Storage System Flows
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Storage Efficiency
Efficiency Equation (per time segment) in this
analysis (per month)

• Ratio of surface flux volume per time to total
  reservoir volume
• Constant reservoir volume and surface area
  assumed
• Somewhat valid for Highland Lakes
• Less valid for proposed Lower Colorado reservoirs

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Data Requirements and Sources

• Evaporation and Precipitation
• NARR good for many points over one time, not
  for single point over many times
• NCDC has plenty of precipitation data, very
  little evaporation data
• Texas Water Development Board (TWDB) has
  regional measurement of monthly precipitation and
  evaporation quantities (1954 onward)

• Physical Characteristics of Reservoirs
• Highland Lakes LCRA provides data on website
• Lower Colorado Reservoirs obtained through
  request to LCRA

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Evap Precip Data Acquisition
Regional Boundaries projected onto GIS map of
Lower Colorado Region
http//hyper20.twdb.state.tx.us/Evaporation/evap.h
tml
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Dimensional Data Acquisition

• All areas and volumes converted to feet
• Only 3 of the 6 Highland Lakes provide water
  storage
• Higher volume to surface area ratio for Highland
  Lakes
• Surface fluxes have greater effect on Lower
  Colorado Reservoirs

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Proposed Lower Colorado Reservoirs

• Reservoirs to have Ring Dike Structure
• Capture excess flows in Colorado River (pumping)

Source LCRA http//www.givlerengineering.com/jn
res.htm
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Storage Efficiency Analysis

• Using Excel, compiled regional monthly
  evaporation and precipitation data
• Evaporation/Precipitation balance
• Efficiency Equation for each month (1954-2002)
• Computed monthly storage efficiency for each
  storage system (weighted according to volume)

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Storage Efficiency Analysis (cont.)

• Regional Evaporation correlated more than
  Precipitation

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Storage Efficiency Analysis (Part 2)

• Same storage efficiency, but different surface
  area to volume ratios
• Summed E P (1954-2002)
• Highland Lakes has greater difference between
  evaporation and precipitation
• Average storage efficiency not capturing full
  picture
• Variability of storage efficiency also important
  factor
• Calculated standard deviation of storage
  efficiency

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Storage Efficiency Analysis (Pt 2 of Pt 2)
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Conclusions

• Highland Lakes and Proposed Lower Colorado
  Reservoirs have comparable storage efficiencies
• However, the storage efficiency of the Lower
  Colorado Reservoirs would be more variable due to
  greater sensitivity to climatic conditions