This paper describes our first step towards the realization of complex and large scale cross-organization virtual observatories by presenting a new semantically-enhanced "Sensor Network as a Service" (SNaaS) framework, which can repurpose existing sensor networks as needed and aggregate and fuse heterogeneous sensors into new virtual sensors in near-real-time. The architecture of this system allows users to create virtual sensors in a Web 2.0 collaborative map interface. Components of the system are highlighted in the paper including a semantically enhanced streaming data toolkit, virtual sensor ontologies and management middleware. Case studies are presented which can allow users to create new virtual rain gages based on the NEXRAD (Next Generation Weather Radar) data stream with or without in-situ rain gages on demand in a Chicago urban watershed testbed. The resulting virtual sensor data streams then can be published in multiple formats including a SWE-compliant one so that external SWE-compliant users and applications can seamlessly query and integrate them.