Interdisciplinary Modeling of Aquatic Ecosystems Curriculum Development Workshop July 18, 2005 Groundwater Flow and Transport Modeling Greg Pohll Division of Hydrologic Sciences Desert Research Institute July 18, 2005

Outline Why model? What are models? Groundwater modeling protocol Governing equations Input/output data Model scales Uncertainty Interdisciplinary issues Modeling difficulties Modeling options July 18, 2005

Why Model? Prediction –Solve management problems When will contaminated groundwater move off site How much water can be pumped Interpretation –Data synthesis –Studying system dynamics Hypothesis testing –Testing new theories and conceptual models July 18, 2005

What is a Model? A model is anything that represents an approximation of a field situation Models include: –Mathematical models Numerical Analytical –Physical models Sand tank A model is a simplified version of a real system and the phenomena that take place within it July 18, 2005

Modeling Protocol Define Purpose Write or Choose Code Collect Field DataConceptual Model Mathematical Model Numerical or Analytical Verify Code Model Design Calibration Prediction/Sensitivity Analysis Verification Presentation of Results Postaudit No Yes Field Data Yes No Collect Field Data July 18, 2005

Governing Equations Flow Model Transport Model July 18, 2005 Dispersion Advection Sorption Source/ Sink First Order Decay Matrix Diffusion Fluid continuityStorageSources/sinks

Input/Output Data Flow model input data requirements –Defining hydrostratigraphic units –Fluid sources (e.g. recharge, interbasin flow) –Fluid Sinks (e.g. ET, pumping) –Boundary conditions (e.g. specified flow, specified head, head-dependent) –Model grid geometry –Time stepping information –Hydraulic Parameters –Initial hydraulic head distribution July 18, 2005

Input/Output Data Flow model output –Hydraulic head values over space and time –Groundwater fluxes over space and time July 18, 2005

Input/Output Data Transport model input requirements –Fluid velocities –Initial distribution of contaminants –Sources and sinks for contaminants –Boundary conditions –Dispersion coefficients –Effective porosity –Decay and/or reaction coefficients –Contaminant loading functions July 18, 2005

Input/Output Data Transport model output –Contaminant concentrations over space and time –Contaminant breakthrough curves at specified locations July 18, 2005

Model Scales Groundwater flow and transport models span scales from the sub-centimeter to 100’s of kilometers July 18, 2005

Model Uncertainty Predictions made by groundwater flow and transport model are inherently uncertain The uncertainty stems from numerous sources –Conceptual model errors –Lack of information on the spatial and temporal variability –Errors due to inaccurate boundary condition specification –Numerical solvers can impart errors, especially for transport solutions –Parametric uncertainty in model input parameters July 18, 2005

Model Uncertainty Since model uncertainty always exists modelers should strive to quantify it impact on model predictions Methods typically used to quantify uncertainty include: –Taylor’s series approximations (e.g. first order second moment techniques) –Monte Carlo analysis –Statistical evaluation of spatial variability in input parameters –Bayesian techniques July 18, 2005 Input Parameter PDFModelPrediction PDF

Interdisciplinary Issues As groundwater models become more and more complex, so does the need for integrated modeling teams and models Some of the disciplines that are typically required to effectively build complex models include: –Geochemists –Geologists –Surface water hydrologists –Statisticians –Ecologists –Biologists –Soil scientists –Heath physicists July 18, 2005

Interdisciplinary Issues Groundwater models are beginning to be coupled to other types of models to investigate various types of problems –Surface/subsurface models (e.g. GSFLOW, MIKE-SHE) –Flow and geochemical reaction models (e.g. FEHM, PHREEQC) –Risk assessment July 18, 2005

Modeling Difficulties Defining a purpose and associated objectives Estimating groundwater recharge Assigning boundary conditions Calibrating Estimating the source term for transport models July 18, 2005

Available Groundwater Models July 18, 2005