(federal government must compensate two regional water authorities for diversion of water for alleged environmental preservation purposes because such diversion breached existing water contracts between Bureau of Reclamation and water districts; contracts only allow government to deliver less water than contractually required only when circumstances are beyond control of federal government and Act of Congress (1992 Central Valley Project Improvement Act) is within control of the federal government; court did not rule on whether government must compensate water contractors for underdeliveries resulting from attempts to protect endangered species).