MODFLOW-2000, the U.S. Geological Survey Modular Ground-Water Model -Documentation of the Hydrogeologic-Unit Flow (HUF) Package

This report documents the Hydrogeologic-Unit Flow (HUF) Package for the groundwater
modeling computer program MODFLOW-2000. The HUF Package is an alternative
internal flow package that allows the vertical geometry of the system hydrogeology to be defined
explicitly within the model using hydrogeologic units that can be different than the definition of
the model layers. The HUF Package works with all the processes of MODFLOW-2000. For the
Ground-Water Flow Process, the HUF Package calculates effective hydraulic properties for the
model layers based on the hydraulic properties of the hydrogeologic units, which are defined by
the user using parameters. The hydraulic properties are used to calculate the conductance
coefficients and other terms needed to solve the ground-water flow equation. The sensitivity of
the model to the parameters defined within the HUF Package input file can be calculated using
the Sensitivity Process, using observations defined with the Observation Process. Optimal values
of the parameters can be estimated by using the Parameter-Estimation Process. The HUF
Package is nearly identical to the Layer-Property Flow (LPF) Package, the major difference being
the definition of the vertical geometry of the system hydrogeology. Use of the HUF Package is
illustrated in two test cases, which also serve to verify the performance of the package by
showing that the Parameter-Estimation Process produces the true parameter values when exact
observations are used.