Improving Regional Groundwater Models with Transmissivity Observations

Hydraulic-conductivity estimates in groundwater-flow models typically are constrained by a range for each hydrogeologic unit. These often-times wide ranges are derived from interpretations of many aquifer tests binned by hydrogeologic unit. Uncertainty is added to these ranges where hydraulic-conductivity estimates derived from aquifer tests use contributing thicknesses that differ from simulated thicknesses in a numerical model. Transmissivity observations from individual aquifer tests constrain model calibration better than hydraulic-conductivity ranges assigned to hydrogeologic units because simulated transmissivity and aquifer-test results are compared directly. Transmissivity comparisons require that simulated thicknesses and hydraulic conductivities for the volume investigated by the aquifer test be extracted from a model and integrated into a simulated transmissivity. Transmissivity observations have been ignored primarily because sampling simulated transmissivities is mechanically painful from complex models. A suite of programs called T-COMP has been developed to sample simulated transmissivities easily from regional MODFLOW models. Transmissivities of model cells are sampled where drawdown exceeds a user-defined threshold. Sampled transmissivities of model cells are averaged within a layer and summed between layers. This computationally intensive process occurs in separate programs that are executed prior to model calibration. Simulated transmissivities can be sampled quickly during calibration because nodes and their fractional contributions have been defined.