Areal attributes are parameters that are applied over a polygonal area. Recharge, evapotranspiration, and hydraulic conductivity are all examples of areal attributes. Areal attributes are only associated with polygons, and not with arcs or points.
Attributes are assigned and edited by selecting a polygon or set of polygons and selecting the Attribute Table command by right clicking on the coverage in the data tree. Attributes can also be assigned to a single polygon by double-clicking the polygon. The Attribute Table command brings up the Polygon Attributes dialog. The main options are Recharge, and Evapotranspiration. Any combination of the three options may be assigned to a single polygon. Once an option is selected, the parameters required by the option must be entered.
When the conceptual model is converted to a grid-based numerical model using the Map -> MODFLOW and Map -> MT3DMS commands, the recharge and evapotranspiration parameters are discretized spatially to individual cells and temporally to individual stress periods.
For the spatial discretization, the centroid of each cell is compared with the polygonal zones and the zone containing the cell is determined. The attributes assigned to that zone are then assigned directly to the cell. In the case of recharge, if a cell is overlapped by more than one coverage, the recharge rate and the concentrations for the cell are determined as an aggregate of the values defined for the overlapping zones.
For temporal discretization, the temporal discretization techniques used for sources/sinks are used to assign the appropriate values to the stress periods.
When using polygonal zones, the values are constant across the entire polygonal area. While this may be appropriate for some models, in many cases it is more appropriate or accurate for the values to vary in a more continuous fashion. A sophisticated suite of tools for interpolating layer elevations is provided in GMS.
Related Links:
MODFLOW Conceptual Model Approach
Steps in Defining a Conceptual Model