62 
♦ At the national level, decisions concerning the guidelines and constraints for initiating an 
environmental sustaining project for the whole country are taken. These decisions are based on the 
information provided by the regional level, which are used to screen the underlying basin in order to 
locate and identify degraded areas, which are ranked according to the rate of degradation, and their 
social and economical impact. At this level, political factors are likely to be considered and even could 
overrule other considerations. The regional information, referring to the positive and negative impacts 
of the new management on each watershed, are used for new decision making processes affecting 
particular or interrelated scenarios. 
♦ At the regional level, the concern is to select the optimum combination of management scenarios to 
produce maximum, positive environmental impacts on the whole watershed. The scenarios and their 
impact on the underlying area are aggregated and quantified at the national level for deciding their 
acceptability. Subcatchments are also ranked at the regional level. The rank is used to prioritize further 
analysis and protection measures. Scenarios with highest ranks are subject to implementation at the 
local level. 
♦ At the local level, highest rank scenarios are implemented and monitored, and their results are used to 
provide feedback to the regional level for modifying and improving the scenarios. 
According to this functionality, the SDSS should maintain the following capabilities: 
a) Ability to retrieve, process, format, display, and store data sets and information using current 
technology and appropriate models that help managers at each level in their processes of monitoring, 
analysis and management of watersheds. 
b) Ability to communicate with the different hierarchical management levels for the required interchange 
of data, knowledge, and decisions. This possibility would be an extended capability of the SDSS that 
gave rise to the name of Multi-Level Spatial Decision Support System, MLSDSS as is proposed by 
[Radwan et al., 1995]. 
Provided that each discipline and level of decision making has its requirements and objectives to achieve in 
a particular watershed, there underlying databases plays various roles and consequently the real world is 
modeled differently. Our task is then to link them in order to provide information and data sharing. There 
are two main aspects which are involved in order to provide this link: 1) design of the components of the 
SDSSs as well as the way they should be linked; 2) design a mechanism for sharing information and data 
between heterogeneous databases of the component SDSSs. The first aspect was achieved by providing an 
architecture which is based on the client/server mechanism, [Boar, H.B., 1993]. The latter, which is the 
focus of this paper is achieved by designing a mechanism based on concepts from federated databases. In 
the next section, aspects of heterogeneity in spatial databases are shown. Then, a proposal for solving this 
heterogeneity is introduced.