A GIS-based Decision Support System (DSS) was developed to select the appropriate interpolation technique used in studying rainfall spatial variability. The DSS used the ArcView GIS platform by incorporating its spatial analysis capabilities, the programming language "AVENUE", and simple statistical methods. The system consists of a series of modules and can be applied in spatial studies of other hydrological parameters. A test case from the country of Switzerland is used to demonstrate the applicability of the system. This should aid in better input to hydrological models.