Transcript Basic Groundwater Modeling Concepts

Interdisciplinary Modeling of Aquatic Ecosystems
Curriculum Development Workshop
July 18, 2005
Groundwater Flow and Transport Modeling
Greg Pohll
Division of Hydrologic Sciences
Desert Research Institute
July 18, 2005
Outline
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Why model?
What are models?
Groundwater modeling protocol
Governing equations
Input/output data
Model scales
Uncertainty
Interdisciplinary issues
Modeling difficulties
Modeling options
Why Model?
• Prediction
– Solve management problems
• When will contaminated groundwater move off site
• How much water can be pumped
• Interpretation
– Data synthesis
– Studying system dynamics
• Hypothesis testing
– Testing new theories and conceptual models
July 18, 2005
What is a Model?
• A model is anything that represents an approximation of a
field situation
• Models include:
– Mathematical models
• Numerical
• Analytical
– Physical models
• Sand tank
• A model is a simplified version of a real system and the
phenomena that take place within it
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Modeling Protocol
Define Purpose
Collect Field Data
Conceptual Model
Mathematical Model
Numerical or Analytical
Write or Choose Code
No
Verify Code
Yes
Model Design
Calibration
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Verification
Yes
Prediction/Sensitivity Analysis
Presentation of Results
Collect Field Data
July 18, 2005
Postaudit
Field Data
Governing Equations
• Flow Model
 
h   
h   
h 
h


 K xh    K yh    K zh   S  R
x 
x  y 
y  z 
z 
t
Fluid continuity
Storage Sources/sinks
• Transport Model

xi


b
c  
c qs
 Dij
 
cvi   Rd  cs    c 
t ne
ne
 xi  xi

Dispersion
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Advection
Sorption
Source/
Sink

c'   M

First Order
Decay
Matrix
Diffusion
Input/Output Data
• Flow model input data
requirements
– Defining hydrostratigraphic
units
– Fluid sources (e.g. recharge,
interbasin flow)
– Fluid Sinks (e.g. ET, pumping)
– Boundary conditions (e.g.
specified flow, specified head,
head-dependent)
– Model grid geometry
– Time stepping information
– Hydraulic Parameters
– Initial hydraulic head
distribution
July 18, 2005
Input/Output Data
• Flow model output
– Hydraulic head values over space and time
– Groundwater fluxes over space and time
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Input/Output Data
• Transport model input requirements
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Fluid velocities
Initial distribution of contaminants
Sources and sinks for contaminants
Boundary conditions
Dispersion coefficients
Effective porosity
Decay and/or reaction coefficients
Contaminant loading functions
Input/Output Data
• Transport model
output
– Contaminant
concentrations
over space and
time
– Contaminant
breakthrough
curves at specified
locations
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Model Scales
• Groundwater flow and transport models span scales from
the sub-centimeter to 100’s of kilometers
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Model Uncertainty
• Predictions made by groundwater flow and transport
model are inherently uncertain
• The uncertainty stems from numerous sources
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Conceptual model errors
Lack of information on the spatial and temporal variability
Errors due to inaccurate boundary condition specification
Numerical solvers can impart errors, especially for transport
solutions
– Parametric uncertainty in model input parameters
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Model Uncertainty
• Since model uncertainty always exists modelers should strive to
quantify it impact on model predictions
• Methods typically used to quantify uncertainty include:
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Taylor’s series approximations (e.g. first order second moment techniques)
Monte Carlo analysis
Statistical evaluation of spatial variability in input parameters
Bayesian techniques
Input Parameter PDF
July 18, 2005
Model
Prediction PDF
Interdisciplinary Issues
• As groundwater models become more and more complex,
so does the need for integrated modeling teams and models
• Some of the disciplines that are typically required to
effectively build complex models include:
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Geochemists
Geologists
Surface water hydrologists
Statisticians
Ecologists
Biologists
Soil scientists
Heath physicists
Interdisciplinary Issues
• Groundwater models are beginning to be coupled to other
types of models to investigate various types of problems
– Surface/subsurface models (e.g. GSFLOW, MIKE-SHE)
– Flow and geochemical reaction models (e.g. FEHM, PHREEQC)
– Risk assessment
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Modeling Difficulties
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Defining a purpose and associated objectives
Estimating groundwater recharge
Assigning boundary conditions
Calibrating
Estimating the source term for transport models
Available Groundwater Models
Processes
Model
MODFLOW
MODFLOW-SURFACT
MT3D
FEHM
MIKE-SHE
GSFLOW
FEFLOW
TOUGH2
HYDRUS
SUTRA
PHREEQC
GROUNDWATER VISTAS
GMS
VISUAL MODFLOW
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Saturated
Flow
Unsaturated
Flow
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Solute
Transport
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Complex
Surface/Subsurface Pre/Post
Geochemical
Coupling
Processor
Reactions
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Density
Dependence
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Heat
ThreeTransport dimensional
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