understanding ground water modeling gary johnson donna cosgrove idaho water resources research...
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Understanding Ground Water Modeling
Gary Johnson Donna Cosgrove
Idaho Water Resources Research Institute
University of Idaho
Idaho Falls
Idaho Water Resources Research Institute
• Goal: link up water research and education needs with Idaho university researchers and funding
• National program – an institute in each state
• Housed at U of I
• Objective: Unbiased research and evaluation
Outline
• Modeling temperature and altitude
• Comparison with ground water model
• Ground water model description
• Model calibration
• Using the model (scenarios)
• What a model will and will not do
Modeling Temperature and Altitude
The model analogy
Altitude
MeanTemp
2000 4000 6000 8000 10000
30
35
45
50
MODEL:Temp=58.4 - .0032 Altitude
Model fits a relationshipTo measurements or observations
Model can yield a prediction or estimate an effect
SCENARIO: What is the effect of climbing 1000 feet higher on the mountain ?
Altitude 2000 10000
30
35
45
50
3.2 degreesTemperature drops by 3.2 degrees
MODEL RESULTS:
Concern: Will it really cool by exactly 3.2 degrees as you climb
another 1000 feet ?
Probably not. But it is your best estimate
Ground Water Flow Model Input/Output Comparison
Altitude/Temp Model
Model Input:Altitude
Model Output:Temperature
Ground Water Flow Model
Model Input:Recharge and discharge at all locations for time frame of interest
Model Output:•Aquifer water levels•Spring discharge & river gains and losses
Ground Water Flow Model Construction Comparison
Altitude/Temp Model
Fitting the model (line) to measurements of altitude and temperature
Ground Water Flow Model
Fitting the model: model calibration to water level measurements and river gains and losses
Ground Water Flow Model Scenario Comparison
Altitude/Temp Model
Scenario: What is the effect of climbing another 1000 feet in altitude ?
Ground Water Flow Model
Scenario: What is the effect on spring discharge of pumping 100 AF at a given location ?
Ground Water Flow Model Comparison
Altitude/Temp Model
Concern: Will temperature fall exactly 3.2 degrees as you climb 1000 feet ?
No – but best estimate
Ground Water Flow Model
Concern: Will spring discharge decline by exactly the modeled amount in response to pumping?
No – but best estimate
Ground Water Model Description
Model Ins and Outs
Aquifer RechargePrecipitationCanalsS.W. Irrig.StreamsTrib. Valleys
Aquifer DischargePumping
AquiferHead
Exchange WithSurface Water
ComputerModel
Inside the Black Box
• Thousands of equations
• Aquifer boundary representations
• Aquifer property estimates
ComputerModel
Greatest Challenges
• Estimating Inputs (Recharge and Discharge)
• Estimating Aquifer Properties– Ability to transmit water (transmissivity)– Ability to store water (storativity)– River/Spring interconnection with the aquifer
Addressing Challenges: 1) Estimating Recharge and Discharge
Approach: Lots of data collection coupled with reasonable assumptions
PrecipitationSoil characteristicsIrrigated areasEvapotranspirationDiversions and return flowsPumping ratesManaged recharge
Variation with location(each model cell)
Variation with time(every 6 months from1980-2002)
Example: Evapotranspiration
Addressing Challenges: 2) Estimating Aquifer Properties
“Model Calibration”
Similar to fitting line through elevation/temperature data
Altitude
MeanTemp
2000 4000 6000 8000 1000030
35
45
50
Good Model
Bad Model
Model Calibration
MODELEstimates of
aquifer recharge and
discharge
Simulated water levels and spring discharges
1980-2002
Measured water levels and spring discharges
1980-2002
ComparisonAdjust estimates of aquifer properties
Automated
Snake Plain Model Calibration
Matching change in water level in a well
Snake Plain Model Calibration
Matching Measured Discharge At Springs
Why not perfect match ?
• Model is an approximation of reality– e.g. assume everything is uniform throughout
entire aquifer thickness (2-dimensional)
• Recharge and discharge are best scientific ESTIMATES
• Aquifer properties are best scientific ESTIMATES
Using the Model(Scenarios)
Basic Goals of Scenarios
• Discriminating effects of specific activities or eventse.g. What is the effect of ground water pumping
on spring discharge ?
• Prediction (Future Conditions)e.g. What will aquifer water levels and spring
discharge be in 20 years if we continue our current practices and have average weather ?
Discriminating Effects
Time
SpringDepletion
PumpingStarts
0
Results show only the effect of specified pumping on a specific reach
Prediction
Time
SpringDischarge
Past Future
Requires that we estimate future weather conditions
Numerical Superposition
• Effects of individual events can be summed to determine total effect
• Model can be used to evaluate effects of changes in recharge and pumping, without consideration of other events
Example: effect of pumping 100 AF from the aquifer is the same whether it is a wet year or a dry year
Analogy: going up 1000 feet in altitude drops the temperature by 3.2 degrees whether you are at 3000 feet altitude or 6000 feet altitude
Superimposing Effects(Superposition)
Time
SpringAccretion
PumpingStops
Time
SpringDischarge
Past Future
WithPumping
WithoutPumping
Bottom Line: What the model can do
The model will provide best estimates of– Changes in aquifer water level in a region– Changes in spring discharge and river gains and
losses for a reach along the Snake River
Given: Estimates of changes in recharge or discharge
What the model won’t do• Automatically determine changes in recharge and
discharge associated with some change in land or water management– Can be lots of work– Wrong input yields wrong answer
• Provide “point specific” estimates of aquifer water level and spring discharge
• Provide exact solutions• Assess injury in a legal sense• Assess economic impact of alternatives
How to contact us
• Gary Johnson(208) [email protected]
• Donna Cosgrove(208) [email protected]
• Idaho Water Resources Research Institute(Idaho Falls)http://www.if.uidaho.edu/~johnson/ifiwrri/