In the water resources area the conjunctive use of hydrological models and GIS’s is 
already well spread out, but, generally the GIS’s are commercial. This paper 
presents the integration of the free and open Geographic Information System 
(OPENGIS) JUMP and a hydrological rainfall-runoff distributed model, composing a 
Decision Support System (DSS) for the water resources area. This DSS, named ARENA 
(Análises de Recursos Naturais, in portuguese), is made up of an OPENGIS, a 
georeferenced database and dialog modules, which allows the access to the 
hydrological module. In order to proceed the integration, the hydrological model 
equation’s were integrated to the geometric entities of the JUMP. This type of 
integration requires a deep understanding of the model as well as a good knowledge 
of the OPENGIS. In this case, the OPENGIS is based on standards of the Open 
Geospatial Consortium facilitates the comprehension of the JUMP sources codes 
developed in Java language. the implementation of the hydrological model was also 
based on Oriented Object (OO) concepts. Finally, the ARENA system was applied to 
the Gramame Watershed in Paraíba State (Brazil) and some spatial analyses were 
carried out, proving the convenience of conjunctive use of OPENGIS and models. Some 
limitations of the JUMP were also found, specifically with respect to the results 
output.