I think the use cases UML models can specify the functional requirements of a service.

You can use models abstracting details related to the solution domain. UML diagrams can be used for that, for instance, use case diagrams or class diagrams. You cannot bring information about the solution domain to the problem domain modeled at the CIM level. This is the concept. To apply such concept you can use any model. Usually, the adopted model-driven approach defines which models you have in each level.

You can use UML model Specially Business UseCase. for detail see the following paper:

http://v1.wcsit.org/media/pub/2012/vol.2.no.1/A%20New%20Method%20for%20Generating%20CIM%20Using%20Business%20and%20Requirement%20Models.pdf

http://ceur-ws.org/Vol-376/paper5.pdf

I Suggest to use Component diagram from UML. It is easy to understand that web services as a Component to access its interfaces I/O. An Independent Webservice may represented as Component that accessed by other Components or Service.

http://www.ibm.com/developerworks/architecture/library/ar-logsoa/