Comparing the Storage Efficiencies of the Highland

Lakes and the Proposed LCRA-SAWS Project Lower Colorado

River Reservoirs

Andrew JuddSemester Project for CE 394K.2

Surface Water HydrologyInstructor: Dr. Maidment

University of Texas

• 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

INTRODUCTION

Source: www.LCRA.org

Source: www.regionk.org

• 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

OBJECTIVE

Research Project: Compare storage efficiencies of Highland Lakes and proposed Lower Colorado reservoirs using historical data

Source: www.lcra.org

Storage System Flows

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

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

Evap & Precip Data Acquisition

http://hyper20.twdb.state.tx.us/Evaporation/evap.html

Regional Boundaries projected ontoGIS map of Lower Colorado Region

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

ac-ft cf acres sfBuchanan Buchanan 875,566 3.81E+10 22,335 9.73E+08

Inks Inks 0 0 837 3.65E+07Wirtz LBJ 0 0 6,534 2.85E+08

Starcke Marble Falls 0 0 611 2.66E+07Mansfield Travis 1,131,650 4.93E+10 18,622 8.11E+08

Miller Austin 21,725 9.46E+08 1,599 6.97E+072,028,941 8.84E+10 50,538 2.20E+09

Volume to Surface Area

Ratio

Highland Lakes Summary

40.1Total

Storage Volume Surface AreaDam Lake

ac-ft cf acres sfColorado 50000 2.18E+09 3333 1.45E+08Wharton 50000 2.18E+09 3333 1.45E+08

Matagorda 50000 2.18E+09 3333 1.45E+08

County

Summary of Proposed Lower Colorado Reservoirs

Volume to Surface Area

Ratio

15.0

Storage Volume Surface Area

Proposed Lower Colorado Reservoirs

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

Source: LCRA & http://www.givlerengineering.com/jnres.htm

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)

Reservoir TWDB RegionUpper Highland

(Buch, Inks, LBJ, Marble)Lower Highland(Travis, Austin)

Colorado 811Wharton 811

Matagorda Average(911,912)

TWDB Regions Used in Analysis

709

Average(709,710)

Highland Lakes Lower Colorado0.996 0.996

AVERAGE STORAGE EFFICIENCY

Storage Efficiency Analysis (cont.)

Regional Evaporation correlated more than Precipitation

Regions 709 and 710 Evaporation Correlation

y = 0.9501x

R2 = 0.9091

024

68

1012

141618

0 5 10 15 20

Region 709 Evaporation (in)

Reg

ion

710

Eva

po

rati

on

(in

)

Regions 709 and 710 Precipitation Correlation

y = 0.9974x

R2 = 0.7378

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14

Region 709 Precipitation (in)

Reg

ion

710

Pre

cip

itat

ion

(in

)

Regions 911 and 912 Evaporation Correlation

y = 0.9295x

R2 = 0.8908

0

2

4

6

8

10

0 2 4 6 8 10 12

Region 911 Evaporation (in)

Reg

ion

912

Eva

po

rati

on

(in

)

Regions 911 and 912 Precipitation Correlation

y = 0.9809x

R2 = 0.6715

0

24

68

1012

1416

18

0 5 10 15 20

Region 911 Precipitation (in)

Reg

ion

912

Pre

cip

itat

ion

(in

)

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

TWDB Region Water Balance (in)709 -1168710 -948811 -366911 -575912 -202

Regional Evaporation/Precipitation Balance (1954-2002)

Highland Lakes 0.006Lower Colorado 0.016

Storage Efficiency Standard Deviation

Storage Efficiency Analysis (Pt 2 of Pt 2)

Highland Lakes 0.006Lower Colorado 0.016

Storage Efficiency Standard Deviation

Storage Efficiency

0.950

0.960

0.970

0.980

0.990

1.000

1.010

1.020

1.030

1.040

Jan-90 Jan-91 Jan-92 Jan-93 Jan-94 Jan-95 Jan-96 Jan-97 Jan-98 Jan-99 Jan-00 Jan-01

Date

Eff

icie

ncy

Highland Lakes

Lower Colorado

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