Tex. Water Code § 55.519

The board may invest sinking funds of the district in bonds of the United States, the State of Texas, any county, any incorporated city or town, any independent school district, or any school district authorized to issue bonds, or they may invest the funds in irrigation or water improvement bonds. The board may not purchase any bonds which under their terms would mature subsequent to the maturity date of bonds for which the sinking fund was created.