Tex. Water Code § 60.041

Each bid submitted on real property to be sold or leased for more than 50 years under Section 60.040 shall be accompanied by a certified check, cashier's check, or bidder's bond with a responsible corporate surety authorized to do business in Texas. The check or bond shall be in an amount equal to five percent of the bid price for the real property or 100 percent of the first rental payment under the lease and shall guarantee that the bidder will perform the terms of the bid if it is accepted by the district.