Idaho Water Issues Background and Science Focus: Snake River Plain Aquifer

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Idaho Water IssuesBackground and ScienceFocus
Snake River Plain Aquifer

• Dr. Gary S. Johnson
• Dr. Donna M. Cosgrove
• Mr. Bryce A. Contor
• Dr. John Tracy
• Idaho Water Resources Research Institute
• University of Idaho

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Topics for today

• Hydrology Basics
• Conjunctive Management Challenges
• Snake Plain Example
• Aquifer Management Opportunities

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The aquifer is a large, leaky tank
recharge varies dramatically
Aquifer
Springs
Output is more constant
Characteristics Flow Recharge and
Discharge Water Budget Storage
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Aquifer Flow
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Aquifer Flow
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Aquifer Recharge

• Water entering the aquifer is called Recharge
• Common sources of recharge
• Infiltrating precipitation
• River or lake-bed seepage
• Subsurface inflows from nearby aquifers
• Seepage from human activities
• Storm drains
• Irrigation

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Aquifer Discharge

• Water leaving the aquifer is called Discharge
• Common sources of discharge
• Spring discharge
• Seepage to rivers or lakes
• Wetlands
• Subsurface outflows to adjacent aquifers
• Ground-water pumping
• Note that rivers can be a source of recharge or
  discharge

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Water Budget

• Since we cannot create or destroy water, a basic
  law exists
• Inflow Outflow /- Change in Storage
• It is like a bank account, if we spend more money
  than we make, our account balance goes down

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Aquifer Storage

• Aquifer storage is the volume of water stored in
  the aquifer
• We can detect increases in aquifer storage by
  rising aquifer water levels
• Changes in storage impact connected surface water
• Long term decreases in storage indicate aquifer
  mining

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Well Water Levels
Seasonal Variation Long-term Change
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River/Aquifer Interconnection

• A gaining river or lake is said to be
  hydraulically connected
• Springs and seeps discharge to rivers and lakes
• The gain is not at a constant rate
• As aquifer elevation rises, the gain increases
• As aquifer elevation declines, the gain decreases
• If aquifer elevation is well below river
  elevation, river is a losing reach

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Conceptual Aquifer System
Recharge
Pumping
Discharge to River
BASIC TRUTH A gallon taken out of the aquifer
represents a gallon that never makes it to the
river ISSUE When, where depletion effects are
felt
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Conjunctive Management

• Conjunctive Management is the simultaneous
  management of the surface-water resource and the
  interconnected ground-water resource
• Not all aquifers are interconnected with rivers
  or lakes
• The Snake Plain Aquifer is intimately
  interconnected with the river

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Why is Conjunctive Management So Difficult?

• Water Law originally intended to manage surface
  water
• Effects are immediate, visible and down-stream
  only
• With ground-water, the effects exist but are more
  difficult to identify
• Effects are spatially distributed
• Effects are spread out over time

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COMPLICATION NO. 1
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COMPLICATION NO. 2 TIME ATTENUATED IMPACTS
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How do we sort this out?

• Ground-water model can predict the timing and
  location of impacts
• Identify arrival time of impacts
• Identify arrival locations
• Ground-water model is a numerical representation
  of a physical system
• Example Snake Plain Aquifer Model

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Some Snake Plain Background
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Snake Plain Aquifer Water Budget
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Water Budget on Snake Plain

• Approximately 6.5 million acre-feet (8,000 cfs)
  of water recharges the aquifer annually
• Discharge to Thousand Springs reach is 5,000 to
  6,000 cfs
• Discharge to American Falls reach is 2,500 to
  3,000 cfs
• We are actually water-rich

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History of Irrigation on Snake Plain

• Surface water irrigation started around 1890s
• Water table rose 10s to 100s of feet
• New springs emerged
• Earliest claims on spring flow in Thousand
  Springs started around 1920s
• Ground-water pumping introduced in 1950s
• Rural electrification
• Deep pump technology

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• Water levels on Plain peaked in 1950s
• Primarily declining since then
• Impacts of ground-water pumping
• Conversion from flood irrigation to more
  efficient sprinkler systems
• Enlargement of irrigated areas
• Drought
• The aquifer seems to be very responsive to drought

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Changes in diversions/time
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Changes in Ground Water Irrigated Acres Over Time
1 million acres 2 MAF/yr or 2,700 cfs
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Model is Translation of Physical System to
Numerical Model Representation
Hydrologic Properties
Aquifer Properties
Boundaries
Recharge/ Discharge
Starting Conditions
Model
Ending Water Levels
Spring Discharge River Gains and Losses
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Snake Plain Model Enhancement

• Completed over past four years
• Appropriation from state, contribution from Idaho
  Power, in-kind from USGS
• Collaborative process
• Multi-agency
• Consultants for water users
• Model calibrated to 22 years of data, with 15,000
  measured data points

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Steady State Water Table
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Comparison of Modeled to MeasuredAquifer Water
Levels
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Future Snake Plain Challenges

• Continued changes in irrigation practices
• Canal lining
• Continued conversion to sprinkler irrigation
• Land use changing
• Subdivisions taking over agricultural land
• Industry moving in
• Dairies moving in
• Societal desires changing
• Species protection
• Potential for impacts of climate changes

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Is this just an Eastern Idaho Problem?

• Snake Plain may be ahead of other basins in
  facing these challenges
• How we sort out the problems in the Snake Plain
  will have far-reaching impacts throughout the
  state
• Bear River Drainage
• Spokane Valley/Rathdrum Prairie Aquifer
• Treasure Valley Aquifer

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Toolbox for Aquifer Management

• Managed Recharge
• Aquifer storage of excess spring run-off in high
  water years
• Conservation Reserve Program
• Taking acres out of production
• Ground-water Banking
• Conversion of some ground-water irrigation back
  to surface irrigation

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Toolbox for Aquifer Management (contd)

• Partial or full curtailment of use during low
  water years
• Buy-out of some threatened water rights

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Summary

• Spring discharge and aquifer water levels are
  changing in response to
• Man-induced effects pumping and recharge
• Weather variability
• Demands exceed supplies
• Some technical tools available to minimize
  conflict
• State in the position of balancing priorities

Protection of Senior Rights
Full Economic Use
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More information?

• Gary S. Johnson
• Idaho Water Resources Research Institute
• University of Idaho
• 208-282-7985
• johnson_at_if.uidaho.edu
• Donna M. Cosgrove
• Idaho Water Resources Research Institute
• University of Idaho
• 208-282-7914
• cosgrove_at_if.uidaho.edu
• Bryce A. Contor
• Idaho Water Resources Research Institute
• University of Idaho
• 208-282-7846
• contor_at_if.uidaho.edu