Estimating hydraulic properties and residence times of unconfined aquifers

Abstract

When detailed groundwater data are unavailable to determine unconfined aquifer characteristics, lumped parameter methods are used to estimate hydraulic properties (initial water table depth, D, saturated hydraulic conductivity, k₀, for a given value of drainable porosity, f) and residence time (drainage time scale), τ. Current procedures require the construction of recession-slope plots (first derivative of recessional flows versus flow) and their interpretation to estimate the parameters in two of the three expressions of the Boussinesq formulation. A novel procedure is proposed to estimate D and k₀ and, separately, τ based on the maximum recession constant, a parameter which is estimated using an objective approach that is easily automated. Another novel aspect of the proposed approach is the recognition that the variability in D and τ is a measure of aleatory uncertainty and, therefore, the results can be specified in terms of non-exceedance cumulative frequencies to represent catchment behaviour under different climatic conditions. The methods are applied to five very different catchments, and estimated values of D, k₀ and τ are compared with field or modelled estimates using current methods.