The Hydrologic Information System (HIS) project by the Consortium of Universities for the Advancement of Hydrologic Science, Inc (CUAHSI) has as its mission development of cyber-infrastructure to support advanced hydrologic research and education. CUAHSI is an organisation representing more than a hundred US universities, supported by the US National Science Foundation. Involved in the project are several research universities, with as technology partner the San Diego Supercomputer Center. Over the last three years the CUAHSI HIS team has been researching, prototyping and implementing web services for the discovery and accessing of various hydrological data sources, and developing online and desktop applications for managing and exploring hydrological time series and other hydrological data.

WaterML
The HIS design follows the open services-oriented architecture model; that is, it relies on a collection of loosely coupled, self-contained services that communicate with each other and may be called up from multiple clients in a standard fashion. The core of the system is WaterOneFlow SOAP services for uniform access to heterogeneous repositories of hydrological observation data. The services follow a common XML messaging schema named CUAHSI WaterML which includes constructs for transmitting observation values and time series, and observation metadata including information about sites, variables and networks.

International Standards
The WaterML specification is available as an OGC Discussion Paper (document 07-041). The CUAHSI HIS team is working with OGC on harmonising it with OGC standards such as GML and Observations and Measurements (05-087) to make the next version of CUAHSI web services OGC-compliant. While the web services are already widely used for programmatic access to federal hydrological data repositories, we expect OGC compliance to dramatically increase their use and facilitate harmonisation with international standards. The goal is to encode the semantics of hydrological observations discovery and retrieval and implement the web services in a way that creates the fewest barriers for adoption by the hydrological research community. In particular this implies maintaining a single common representation for the key constructs returned on web-service calls, thus hiding structural, syntactic and semantic differences across observation networks.

HIS Server
The currently available services provide access to multiple types of US Geological Survey and Environmental Protection Agency data, as well as to data maintained in the CUAHSI observations data model (ODM) format. We work with US federal agencies to provide easy discovery of and unified access to their observation data repositories. Web services are accessed from different online and desktop (Matlab, ArcGIS, Excel) clients and several programming languages. The HIS Server organises observation databases, geographical data layers, data importing and management tools, and online user interfaces into a multi-tier application to serve both nationally and locally maintained observation data. At the time of writing, the HIS server software stack is being deployed at several hydrological observatory sites, including eleven NSF-supported WATERS test-beds.