Transient Simulations in MODFLOW

Explore the key aspects of transient simulations in MODFLOW such as storage coefficients, initial conditions, stress periods, and the Changing Head Boundary package. Learn about the importance of storage parameters, LPF package options, defining initial conditions, and utilizing model-generated heads for consistent results. Gain insights into steady-state and transient model conditions, along with practical examples illustrating water table changes over time.

• Transient Simulations
• MODFLOW
• Storage Coefficients
• Initial Conditions
• Groundwater Modeling

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1. CE 547 BRIGHAM YOUNG UNIVERSITY Transient Simulations

2. Lecture Objectives Know what storage coefficients are required by MODFLOW Understand the importance of proper initial conditions for transient simulations Understand stress periods and time steps in MODFLOW Learn about the Changing Head Boundary (CHD) package

3. Storage Parameters MODFLOW requires either a primary storage coefficient or both a primary and secondary storage coefficient Depends on: Layer type Flow package used

4. LPF Package Confined Specific Storage Convertible Specific Storage Specific Yield Note: Similar set of options for BCF and HUF packages

5. Defining Initial Conditions For steady state models: The initial conditions represent a guess at the starting heads and don t generally have a big impact on the solution. For transient model: The initial conditions represent the state of the aquifer at the beginning of the simulation Starting head values should be carefully selected Arbitrary head values or heads interpolated from observation wells should NOT be used

6. Model-Generated Heads "Use of model-generated head values ensures that the initial head data and the model hydrologic inputs and parameters are consistent. If the field-measured head values were used as initial conditions, the model response in the early time steps would reflect not only the model stresses under study but also the adjustment of model head values to offset the lack of correspondence between model hydrologic inputs and parameters and the initial head values." Anderson & Woessner, Applied Ground Water Modeling

7. Steady State Initial Condition Head at a point in the model domain Arbitrary starting head Steady-state solution compatible with BC for transient simulation Time

8. Example Water table lowers with time

9. Flat Water Table Initial Condition

10. Steady State Initial Condition

11. Initial Conditions Recent versions of MODFLOW include an option to automatically run first time step as a steady state solution. This takes care of the problem automatically.

12. Stress Periods & Time Steps Few stress periods, many time steps Few time steps, many stress periods

13. Time Steps Must choose appropriate distribution of time step sizes: Too short -> long simulation times, large solutions Too long -> model error

14. Time Step Rule of Thumb Where: t = the time step increment S = storage coefficient a = representative cell dimension in x and y. T = transmissivity

15. Changing Head (CHD) Package Original method for defining specified heads works for static (steady state) heads only - IBOUND Array - Starting Heads Array CHD later added to allow for transient specified head boundary conditions

16. CHD Package For each stress period, beginning and ending head is assigned