Applicability of Effective Parameters for Unsteady Flow in Nonuniform Aquifers

As a rule, a groundwater aquifer consists of materials whose characteristics vary in space. On account of the uncertainty in determining these characteristics and their variation in the field and also for the sake of parsimony, it is often desirable to assume that the aquifer may be considered homogeneous and uniform; in other words, the prototype is modeled by an equivalent homogeneous and uniform aquifer with a set of effective parameters, which displays the same flow characteristics. To study the applicability of this concept, a Monte Carlo technique was used in the simulation of unsteady gravity drainage from a large unconfined aquifer. For given criteria of hydraulic equivalence applied to the outflow rate, effective parameters (hydraulic conductivity and drainable porosity) were obtained, hose variation was studied. It was found that only under certain conditions is it possible to define a single set of effective parameters to reproduce the median outflow hydrograph. The effective parameters were found to depend on both the distribution of the hydraulic conductivity and its spatial correlation.