It also presents challenges in establishing economically efficient water pricing mechanisms. In these cases an irrigation project has the characteristics of a public good, in which several water users can simultaneously consume the services of the project. Finally, the flowing nature of water and its changing physical characteristics as it passes among states in the hydrologic cycle produce high transaction costs in establishing and enforcing property rights to develop, allocate, and use water. In these cases, water services, especially environmental values produced by leaving water in its natural state, may emerge as a common property resource, for which the complete opportunity costs of its use may not face the user.