Subsurface Hydrology

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# Subsurface Hydrology - PowerPoint PPT Presentation

Subsurface Hydrology. Unsaturated Zone. Water table. Saturated Zone . R = P - ET - RO. ET. ET. P. RO. E. R. R / P = ?. Water Table. Groundwater. Processes we might want to model Groundwater flow calculate both heads and flow

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Presentation Transcript
Subsurface Hydrology

Unsaturated Zone

Water table

Saturated Zone

R = P - ET - RO

ET

ET

P

RO

E

R

R / P = ?

Water Table

Groundwater

Processes we might want to model

• Groundwater flow
• calculate both heads and flow
• Solute transport – requires information on flow (velocities)
• calculate concentrations

v = K I / n

Darcy’s law

Conceptual Model

A descriptive representation

of a groundwater system that incorporates an interpretation of the geological & hydrological conditions. Generally includes information about the water budget.

Toth Problem

Water Table

Groundwater

divide

Groundwater

divide

AQUIFER

Impermeable Rock

Steady state system: inflow equals outflow

Types of Models

• Physical (e.g., sand tank)
• Analog (electric analog, Hele-Shaw)
• Mathematical

Types of Models

• Physical (e.g., sand tank)
• Analog (electric analog, Hele-Shaw)
• Mathematical

Components of a Mathematical Model

• Governing Equation (Darcy’s law + water balance eqn) with head (h) as the dependent variable
• Boundary Conditions
• Initial conditions (for transient problems)

Toth Problem

Water Table

Groundwater

divide

Groundwater

divide

AQUIFER

Impermeable Rock

Steady state system: inflow equals outflow

Toth Problem

Water Table

Groundwater

divide

Groundwater

divide

Laplace Equation

Impermeable Rock

Types of Solutions of Mathematical Models

• Analytical Solutions: h= f(x,y,z,t)
• (example: Theis eqn.)
• Numerical Solutions
• Finite difference methods
• Finite element methods
• Analytic Element Methods (AEM)

Topo-Drive

Finite element model of a version of the Toth Problem for regional flow in cross section.

Finite difference models

• may be solved using:
• a computer program or code (e.g., a FORTRAN program)

MODFLOW

•  a computer code that solves a groundwater flow model using finite difference techniques
• Several versions available
• MODFLOW 88
• MODFLOW 96
• MODFLOW 2000
• MODFLOW 2005

Steps in Groundwater Modeling

• 1. Model Design
• Calibration (History matching)

Define Purpose

Conceptual Model

Field Data

Mathematical Model

Numerical Method-Computer Code

Model Design

Grid, Boundary and Initial Conditions, Parameterization

Calibration and Sensitivity Analyses

Predictive Simulation

Presentation of Results

Postaudit

Define Purpose

Conceptual Model

Field Data

Mathematical Model

Numerical Method-Computer Code

Model Design

Grid, Boundary and Initial Conditions, Parameterization

Calibration and Sensitivity Analyses

Predictive Simulation

Presentation of Results

Postaudit