29. If a water-supply increasing the value of such holding is afterwards restored to the said land otherwise than at the cost of the tenant, the rent of the tenant may be enhanced, in respect of the increased value of such land due to the restored water-supply, to an amount not exceeding that at which it stood immediately before the abatement. Such enhancement shall be on account only of the restored water-supply, and shall not effect the liability of the tenant to enhancement of rent on any other grounds.