understanding ground water modeling gary johnson donna cosgrove idaho water resources research...

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) 272-7885johnson@if.uidaho.edu

• Donna Cosgrove(208) 282-7914cosgrove@if.uidaho.edu

• Idaho Water Resources Research Institute(Idaho Falls)http://www.if.uidaho.edu/~johnson/ifiwrri/