Transient Groundwater Modeling for Efficient Simulation

Groundwater Modeling 3 Transient Models Groundwater Hydraulics Daene C. McKinney 1

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. See file: 07_Groundwater_Modeling_2.pptx 2

Example Pumping Well 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 Layer 1 10 m 13 Layer 2 -3 m -8 m 5 No-flow Boundary N Constant Head Boundary (h = 9 m) Constant Head Boundary (h = 8 m) Pumping Well 600 m No-flow Boundary 3 Adapted from Chiang, W-H and W. Kinzelbach, Processing Modflow: A Simulation System For Modeling Groundwater Flow and Pollution, 1996 600 m

Results Remember? To get contours of water table, check this box Always check the box for Cell-by-Cell flow 4

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 e.g., one day Time Steps Time Steps Time Steps Time Stress period Stress period Stress period Pumping and boundary conditions can change e.g., one month 5

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 6

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) 7

Pumping Conditions Select: Layer 1 Select: BC Select: BC Layer Uncheck: Steady-state Boundary Condition Press: Transient Data Well Modify Uncheck this box Press this button 8

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 9

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 10

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 11

Leakance Select: Props Leakance Select: Props Property Values Database Enter: Zone 1 and Zone 2: Leakance = 864 m/day 12

Monitoring Wells Select: AE 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 Well Add another Monitoting Well (MW-2) in cell (15, 18) 13

Model with Monitoring Wells 14

Run Simulation Select: Calculator button Would you like to process the results? Select: Yes Select: Cell-by-cell flows 15

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 16

Results Stress Period 12 17

Monitoring Wells Row 7 Column 9 Select: Plot Hydrograph - Monitoring Well Select: Your Monitoring Wells Row 2 Column 9 18