The module FLUZO is developed to model flow through the unsaturated zone and to determine the effective groundwater recharge for FLAIRS or MODFLOW. FLUZO uses three factors to schematise the subsoil into a series of interconnected reservoirs. The vertical transport of water between these reservoirs are determined using the Van Genuchten relations. The factors are:

Factor 1: Physiological characterisation

The physiological characterisation of the unsaturated zone soil is based on a soil description method known as the Staring series. This series distinguishes two zones: the 'root zone' and the 'sub-soil'.

Factor 2: Land use

The second factor is land use. A database is available where for eighteen predefined types (crops, built up,etc) all relevant information, such as potential evapotranspiration, rainfall interception, leaf area index, are included as function of the growing season.

Factor 3: Meteorological conditions

The meteorological conditions are defined using observed precipitation and evaporation. FLUZO distinguishes two types of evaporation: open water (according to Penman) and crop-reference (according to Makkink).

Damping and retardation (thick unsaturated zone)

In areas with a deep groundwater table (for example in a mountainous terrain) the fluctuation of the effective groundwater recharge will not necessarily occur simultaneously with meteorological storm events. In these cases FLUZO takes damping and retardation into account.