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

• Why model?

• What are models?

• Groundwater modeling protocol

• Governing equations

• Input/output data

• Model scales

• Uncertainty

• Interdisciplinary issues

• Modeling difficulties

• Modeling options

July 18, 2005

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

July 18, 2005

Modeling ProtocolDefine PurposeDefine Purpose

Write or Choose Code

Collect Field Data Conceptual Model

Mathematical Model

Numerical or Analytical

Verify Code

Model Design

Calibration

Prediction/Sensitivity Analysis

Verification

Presentation of Results

Postaudit

No

Yes

Field Data

Yes

No

Collect Field DataJuly 18, 2005

Governing Equations

• Flow Model

• Transport Model

July 18, 2005

Rt

hS

z

hhK

zy

hhK

yx

hhK

x zyx

Mcn

ccn

q

t

cRcv

xx

cD

x e

bs

e

sdi

iiij

i

'

Dispersion Advection Sorption Source/Sink

First OrderDecay

MatrixDiffusion

Fluid continuity Storage Sources/sinks

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

July 18, 2005

Input/Output Data

• Transport model input requirements– 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

July 18, 2005

Input/Output Data• Transport model

output– Contaminant

concentrations over space and time

– Contaminant breakthrough curves at specified locations

July 18, 2005

Model Scales

• Groundwater flow and transport models span scales from the sub-centimeter to 100’s of kilometers

July 18, 2005

Model Uncertainty

• Predictions made by groundwater flow and transport model are inherently uncertain

• The uncertainty stems from numerous sources– 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

July 18, 2005

Model Uncertainty• Since model uncertainty always exists modelers should strive to

quantify it impact on model predictions

• Methods typically used to quantify uncertainty include:– Taylor’s series approximations (e.g. first order second moment techniques)

– Monte Carlo analysis

– Statistical evaluation of spatial variability in input parameters

– Bayesian techniques

July 18, 2005

Input Parameter PDF 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:– Geochemists– Geologists– Surface water hydrologists– Statisticians– Ecologists– Biologists– Soil scientists– Heath physicists

July 18, 2005

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

July 18, 2005

Modeling Difficulties

• Defining a purpose and associated objectives

• Estimating groundwater recharge

• Assigning boundary conditions

• Calibrating

• Estimating the source term for transport models

July 18, 2005

Available Groundwater Models

July 18, 2005

ModelSaturated

FlowUnsaturated

FlowSolute

Transport

Complex Geochemical

Reactions

Surface/Subsurface Coupling

Pre/Post Processor

Density Dependence

Heat Transport

Three-dimensional

MODFLOW X X XMODFLOW-SURFACT X X X XMT3D X XFEHM X X X X X X X XMIKE-SHE X X X X X XGSFLOW X X X X X XFEFLOW X X X X X X XTOUGH2 X X X X X XHYDRUS X X X X XSUTRA X X X X X XPHREEQC X X XGROUNDWATER VISTAS XGMS XVISUAL MODFLOW X

Processes