(a) The authority may use, distribute, and sell, outside the authority, any water the authority controls, stores, preserves, conserves, develops, or acquires if the board determines that adequate provision can be made to continue to serve the water requirements inside the authority.
(b) Notwithstanding Subsection (a), the authority may not enter into any agreement that contemplates or results in the removal from the watershed of the Guadalupe and Blanco Rivers and their tributaries of any surface water of the authority necessary to supply the reasonably foreseeable future water requirements for municipal uses inside the watershed during the next 50 years, except on a temporary, interim basis.