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Subsurface Hydrology

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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Subsurface Hydrology

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  1. Subsurface Hydrology Unsaturated Zone Water table Saturated Zone

  2. R = P - ET - RO ET ET P RO E R R / P = ? Water Table Groundwater

  3. 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

  4. 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.

  5. Toth Problem Water Table Groundwater divide Groundwater divide AQUIFER Impermeable Rock Steady state system: inflow equals outflow

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

  7. Electric analog model of Long Island, New York

  8. Hele-Shaw Model

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

  10. 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)

  11. Toth Problem Water Table Groundwater divide Groundwater divide AQUIFER Impermeable Rock Steady state system: inflow equals outflow

  12. Toth Problem Water Table Groundwater divide Groundwater divide Laplace Equation Impermeable Rock 2D, steady state

  13. 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)

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

  15. Finite difference models • may be solved using: • a computer program or code (e.g., a FORTRAN program) • a spreadsheet (e.g., EXCEL)

  16. 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

  17. Steps in Groundwater Modeling • 1. Model Design • Calibration (History matching) • Prediction (of heads)

  18. 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

  19. 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

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