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
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