Groundwater Modeling 3: Transient Models

Daene C. McKinney, Professor The University of Texas Austin

Eusebio Ingol Blanco, Teaching Assistant

Remember Your Old Model?

• Find the file that you used for the example model from the previous class

• Or recreate it by reviewing the overhead slides from that assignment.

Example • Boundaries

– North & South: No-flow

– East & West: Constant-head

• Layer 1 – unconfined (13 m) – Kh = 5x10-3 m/s; Kv = 5x10-4 m/s

– Porosity = 0.25

• Layer 2 – confined (5 m) – Kh = 1x10-3 m/s; Kv = 1x10-4 m/s

– Porosity = 0.25

No-flow Boundary

No-flow Boundary

Con

sta

nt H

ea

d B

ou

nd

ary

(h =

9 m

)

Co

nsta

nt H

ea

d B

ou

nd

ary

(h

= 8

m)

Pumping Well

60

0 m

600 m Adapted from Chiang, W-H and W. Kinzelbach, Processing Modflow: A

Simulation System For Modeling Groundwater Flow and Pollution, 1996

Pumping Well

Layer 1

Layer 2

5

13

10 m

-3 m

-8 m

N

Results – Remember?

To get contours of water

table, check this box

Always check the box for

Cell-by-Cell flow

Transient Groundwater Models

• Transient models simulate changes over time – Necessary when boundary conditions vary with time (e.g., pumping

rates, recharge, river stage, etc.)

• Stress period: – Period of time during which boundary conditions are constant

– Stress periods can have multiple time steps

– Boundary conditions can change at the beginning of a stress period

Stress period

Time Steps

Stress period

Time Steps

Stress period

Time Steps

Time

Pumping and boundary

conditions can change e.g., one month

e.g., one day

Transient Model

• Convert you steady-state model to a transient model

• Open your model

• Select: Model MODFLOW Package Options

• Select: Basic Package Tab

• Uncheck: Steady-State checkbox

• Enter: Number of stress periods = 12

• Select: Days

• Select: OK

• Do you want to copy data? Select: Yes

• Do you want to set up stress periods? Select: Yes

Stress Periods • Use 12 stress periods, one for each day for 12 days

• Enter – Length of each stress period (= 1 day)

– Number of time steps (= 1)

– Time step multiplier (= 1.0)

Pumping Conditions

• Select: Layer 1

• Select: BC Well

• Select: BC Modify Layer

• Uncheck: “Steady-state Boundary Condition”

• Press: “Transient Data”

Uncheck this box

Press this button

Pumping Conditions • Change the pumping rates from “m3/sec” to “m3/day”

• Enter: Starting and Ending Stress Period Numbers

• Enter: Q = 0 m3/d for stress periods 1, 2, and 3

• Enter: Q = -159,840 m3/d for stress periods 4 - 12

• Repeat for layer 2: Q = 0 m3/d in stress periods 1, 2 and 3; Q = -1296 m3/d in periods 4 - 12

Hydraulic Conductivity • Change the hydraulic

conductivity values from “m/sec” to “m/day”

• Select: Props – Hydraulic Conductivity

• Select: Props – Property Values – Database

• Enter:

– Zone 1: • Kx = 432 m/d • Ky = 432 m/d • Kz = 43.2 m/d

– Zone 2: • Kx = 86.4 m/d • Ky = 86.4 m/d • Kz = 8.64 m/d

Storage/Porosity

• Select: Props – Storage/Porosity

• Select: Props – Property Values – Database

• Enter:

– Zone 1 and Zone 2:

• Ss = 0.0001

• Sy = 0.15

• Porosity = 0.15

Leakance

• Select: Props – Leakance

• Select: Props – Property Values – Database

• Enter: – Zone 1 and Zone 2:

• Leakance = 864 m/day

Monitoring Wells • Select: AE Well

• Select: Row 15, Column 24

• Select: Top layer = 1, and Bottom Layer = 2

• Change: Pumping rate = 0.0

• Check box: Monitor Head/Concentration vs. Time

• Select: Well Name button

– Enter MW-1

• Select: OK

Add another

Monitoting Well

(MW-2) in cell

(15, 18)

Model with Monitoring Wells

Run Simulation • Select: Calculator button

• Would you like to process the results? Select: Yes

• Select: Cell-by-cell flows

Switching Between Time Steps

• There are several options that allow switching between time steps in a transient run:

– Import Results (Plot->Import Results) Ctrl+Shift+I

– Next Time Step (Plot->Next Time Step) Ctrl+Shift+N

– Previous Time Step (Plot->Previous Time Step) Ctrl+Shift+P

• Selecting the next time step will bring in the next set of heads, drawdowns, and cell-by-cell flows

Results – Stress Period 12