The REW model is a complex hydrological simulation tool, which is designed and developed for the simulation of the entire hydrological cyle of a watershed system, underlain by a regional aquifer, that extends beyond the topographic boundaries.
The tool this can be used for a series of hydrological studies, which look at different components of the hydrological cycle and at processes that play a role at different time scales. For example it can be used for event-based studies, such as the response of a watershed to an extreme hydrological event, ot the behaviour of the hydrological system under forcing conditions that are changing over loger time periods. Examples of possible applications and hydrological studies are 1) hydrological water balance, 2) rainfall-runoff studies, 3) groundwater recharge and development studies, 4) impact of climate change on the hydrological cyle.

The solution of the system of Ordinary Differential Equations (ODE's) and Richard’s Equation, including the groundwater flow module are performed by optimized C++ code. The preparation of input data can be facilitated by using appropriate GIS software such as for example ArcView®. The following section describes the hydrological processes simulated by the model.

Spatial discretization of the landscape into modelling units

In the REW model a watershed is partitioned into a series of discrete spatial units called Representative Elementary Watersheds (REWs). REWs are defined by performing an analysis of the catchment topography and constitute a set of the interconnected elements that are organized around the tree-like structure of the stream channel network, as shown in Figure 1.

REWs constitute 3D regions, with a vertical prismatic mantle surface defined by the REW boundaries. The REW boundaries coincides with the topographic divides. They delineate a well-defined area of the land surface that captures the precipitation.
The contour of a REW mantle surface coincides with the shape of the ridges defining a subcatchment. A schematic representation of REW is depicted in Figure 2.

A REW is delimited by the atmosphere at the top and by an impermeable layer at the bottom. The impermeable layer can be either defined by a horizontal surface or can be given by interpolation of bedrock depth for a series of irregualr points.