If simulation and optimization models are the heart of DSSs, then database management systems are perhaps the brain. Today's systems provide comprehensive facilities for storing, retrieving, and manipulating data, which are often essential to the decision-making process. Two common models are the relational database, which relates information in a tabular way so that the rules of relational algebra can be applied, and the geographic database (or geographic information system), which relates information pertaining to fundamental map features such as points, lines, and polygons. Recently, the ``geo-relational'' data model has been introduced to provide a link between the relational and geographic data models ( Reitsma et al. [1994]).
Of all the computer technologies being integrated into DSSs, geographic information systems (GIS) is perhaps the fastest growing. In 1993, a total of about 130 papers were presented at two specialty conferences on the application of GIS in hydrology and water resources ( Kovar and Nachtnebel [1993]; Harlin and Lanfear [1993]). GIS is an especially appealing technology for DSSs because it can be used not only for information management ( Weghorst et al. [1992]), but also for spatial analysis and the visualization of model output. Thus, a GIS by itself could function as a DSS, provided that spatial analyses alone provided adequate support in the decision making process (an example of such a problem is given by Xiang and Stratton [1993]).