Hydraulic gradient comparison method to estimate aquifer hydraulic parameters under steady-state conditions

The hydraulic gradient comparison method is an inverse method for estimatio n of aquifer hydraulic conductivity (or transmissivity) and boundary conduc tance for a ground water flow model under steady-state conditions. This met hod, following formal optimization techniques, defines its objective functi on to minimize differences between interpreted (observed) and simulated hyd raulic gradients, which results in minimization of differences between obse rved and simulated hydraulic heads. The key features of this method are tha t (1) the derived optimality conditions have an explicit form with a clear hydrology concept that is consistent with Darcy's law, and (2) the derived optimality conditions are spatially independent as they are a function of o nly local hydraulic conductivity and local hydraulic gradient. This second feature allows a multidimensional optimization problem to be solved by many one-dimensional optimization procedures simultaneously, which results in a substantial reduction in computation time. The results of the numerical pe rformance testing on a heterogeneous hypothetical case confirm that minimiz ing gradient residuals in the entire model domain leads to minimizing head residuals. Application of the method in real-world projects requires rigoro us conceptual model development, use of a global calibration target, and an iterative calibration process. The conceptual model development includes i nterpretation of a potentiometric surface and estimation of other hydrologi c parameters. This method has been applied to a wide range of real-world mo deling projects, including the Rocky Mountain Arsenal and Rocky Flats sites in Colorado, which demonstrates that the method is efficient and practical .