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Introduction to MODFLOW

Introduction to MODFLOW. Part 2. Conductance. In general, conductance = KA/  L. Also, leakance = K v /thickness resistance (an analytic element term) = thickness/K v. Vertical Conductance Terms. MODFLOW uses special terms (VCONT) for

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Introduction to MODFLOW

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  1. Introduction to MODFLOW Part 2

  2. Conductance

  3. In general, conductance = KA/L Also, leakance = Kv/thickness resistance (an analytic element term) = thickness/Kv

  4. Vertical Conductance Terms MODFLOW uses special terms (VCONT) for vertical conductance, which is required in 3D problems. Modflow.exe requires the user to supply values for the VCONT arrays. GW Vistas computes the values in the VCONT arrays for you. VCONT = Kv/z = Conductance/(xy) = “leakance”

  5. See handout from Domenico & Schwartz for MODFLOW’s FD equation.

  6. Examples of solution techniques that combine matrix solution with iteration: IADI (see chapter 5 of W&A) SSOR* SIP* PCG2* *Used in MODFLOW

  7. m+1 Note: m is time level and n is iteration level. t m m-1 Schematic of solution procedure

  8. SSOR

  9. The 3D eqn. with an implicit approximation generates a coefficient matrix with 7 diagonals

  10. SSOR The coefficient matrix has 5 diagonals

  11. Types of Layers (LAYCON array) Confined Unconfined Convertible (Useful to think in terms of a layer transmissivity.)

  12. Conductance Confined layer transmissivity = (KRi,j-1/2,k)(vk) Unconfined layer transmissivity = (KRi,j-1/2,k)(hi,j,k-BOTi,j,k)

  13. Input of Grid Spacing • MODFLOW asks for values of horizontal grid spacing • (i.e., r and c) but not vertical grid spacing (v). • Vertical grid spacing (v) is calculated by MODFLOW • from user supplied values of the bottom (BOT) • of each layer. (GwVistas asks for both top and bottom of • each layer.) TOP BOT

  14. Note that a top unconfined layer actually has no top. The head value in the top layer could potentially rise to infinity. Land surface elevation is not used in MODFLOW, except in the ET package.

  15. Wooster example from Domenico & Schwartz

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