Mans Impact on Future Great Salt Lake Elevations

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• Mans Impact on Future Great Salt Lake Elevations

Craig W. Miller, P.E. Utah Division of Water
Resources AWRA Conference May 15, 2007
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Lake Facts
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Lake Facts (Arnow and Stephens, 1990)
1987
1963
1972
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Great Salt Lake in Local News
Correcting Misconceptions

• May 9, 2002
• Salt Lake Countys thirsty lawns may spur an
  environmental casualty at the Great Salt Lake.
• ..the diversion of 250,000 acre feet, or 20
  percent of the Bear Rivers flow, would drop the
  lakes water level anywhere from 1 to 6 feet,
  depending on annual precipitation. .. consulted
  more than 20 academics and specialists in
  disciplines ranging from water and wetland
  management to ornithologists and environmental
  consultants at the United States Geological
  Survey before writing her report.

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Concept

• Diversion ? Depletions
• In Agriculture, a lower proportion diversions
  return to the system than occurs with municipal
  diversions. The difference between what is
  diverted and what returns is depletion.

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Lake Elevation-Why be concerned?

• Lake stage and volume are tied to salinity.
• Mineral extraction industries
• Brine shrimp harvest
• Recreation, tourism
• Riparian vegetation, marshes, wetlands
• Flood damage
• The lake can threaten important transportation
  lifelines.
• Bird refuge, flyway
• Industries
• Recreation
• Newly exposed mud flats likely contaminated
• A lake with stable surface elevation is easier to
  manage

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A model was created to examine the effect of
additional depletions on the Great Salt Lake

• The Great Salt Lake is a terminal lake
• Over time inflow (surface and groundwater)outflow
  (evaporation)
• If inflows decrease or increase a new stable
  elevation of the lake is determined for
  equilibrium.

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Terminal Lake Elevation Stabilization
Increased or Decreased Lake Area
Increased or Decreased Inflow
Previous Lake Area
At Equilibrium Difference in Inflow Difference
in Area x Net Evaporation
Rule of thumb for Great Salt Lake 100,000 AF of
long term additional inflow or depletion yields
.75 feet elevation difference
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Shoreline expression
From Shoreline Superelevation by Genevieve
Atwood, 2006
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Another Look at Elevation Change
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Today
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Does Man Affect Lake Elevations?
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Depletion 1.4 MAF from Estimation of Water
Surface Elevation Probabilities and Associated
Damages for the Great Salt Lake, USU, 1979
Limit of Volume/Elevation Table
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Depletion 1.2 MAF Water Resources, 2007
Limit of Volume/Elevation Table
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Inundation map
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Traditional Data
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Residential DepletionsLower Than Agriculture
Rainfall on roof either enters storm sewer or
shallow groundwater system
Indoor water use depletes little water
Lawn evaporation lower than alfalfa
Rainfall on hardscape evaporates initially and
then runs off
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What Changes in Agricultural Conversion?

• Residential
• About half of area is hardscaped (.25 acre
  lots)
• Landscape depletion somewhat less than alfalfa
  depletion
• Indoor depletions are relatively small
• Net depletions are around half.

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Rough Salt Lake City Residential Depletion Budget
(.25 acre lots)
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Practical Considerations

• In depletion results shown later, the effects of
  hardscape are ignored for residential,
  commercial, and institutional development.
• Outdoor depletions were calculated using an
  equivalent area of turf.
• Not all municipal and industrial development is
  on agricultural land.

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Commercial Water UsersMay Add Water to the System
Rainfall on roof either enters storm detention
shallow groundwater system or storm sewer
Indoor water use depletes little water
Some landscapingwater depletion
Hardscape evaporates little water. Runoff may
feed directly into storm sewers and then into a
water body.
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What Changes in Agricultural Conversion?

• Commercial
• Often mostly hardscape.
• Small landscaped areas.
• Net depletions small (depending upon the
  business).

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Rough Salt Lake City Commercial Water Use Budget
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Institutional DepletionsLower Than Agriculture
Rainfall on roof either enters storm sewer or
shallow groundwater system
Indoor water use depletes little water
Lawn evaporation lower than alfalfa
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What Changes in Agricultural Conversion?

• Institutional (includes parks, schools, churches)
• Large landscaped areas (depletions)
• Large hardscaped areas (small or no depletions)
• Indoor depletions relatively small

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Rough Salt Lake City Institutional Depletion
Budget
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Municipal Development in Salt Lake County
Land Area Summary
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Agricultural Depletionsby Basin (Acre-Feet)
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Cautions

• Development pressures may reduce irrigation
  demands below what is shown (Denos and Larsen,
  2007).
• Future agricultural demands will depend on.
• Development patterns.
• Open space regulations in the future.

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Denos and Larsen, 2007
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Denos and Larsen, 2007
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Municipal and Industrial Depletionsby Basin
(Acre-Feet)
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Basin Imports (Acre-Feet)
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Total Basin Depletions (Acre-Feet)
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THE FINAL NUMBER!
37,000 More AF/Year To the Great Salt Lake!
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Theres no simple relationship
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Conclusions and Observations

• Development through 2050 will have a subtle
  effect upon lake elevations.
• Landscaping practices will affect depletions
• Conversion of irrigated agriculture
• Climate change may loom as a much larger concern.
• Water quality will be affected by increased
  municipal usage.

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Thank You! Questions?