Calibration studies on a dual porosity unsaturated flow code (DCM3D)

Abstract

A dual continuum, three-dimensional, isothermal ground-water flow code for unsaturated, fractured, porous media (DCM3D) was calibrated against two test cases; a laboratory block study and data from a monitored field location at the Apache Leap Research Site. Variably saturated water flow in the matrix as well as in the fracture system are described with two separate Richards' equation. Flow between the respective continua is simulated by means of a first order rate equation. Relative permeabilities are calculated using the van Genuchten characteristic relation. The formulation leads to a coupled system of nonlinear partial differential equations which are solved numerically using an integrated finite difference technique. Model calibrations were developed from existing data and supplemented with estimates of parameters not supported by previous research. Estimates included; initial conditions, fracture porosity, fracture van Genuchten parameters (a, m and Or) and the transfer factor. DCM3D proved able to reproduce flow behavior from both test cases. However, given the degrees of freedom, a unique solution was not found. Therefore, an evaluation on the conceptual understanding of flow in each test case was not possible. Sensitivity runs on fracture parameters showed fracture saturated intrinsic permeability and fracture porosity to be the most sensitive parameters relating to travel time. Increases in model complexity dramatically increased run times. Parameters which had the greatest effect on run time where the fracture a and fracture (}r.