If water appropriated for irrigation purposes is not used by reason of compliance with crop control or soil conservation contracts with the United States, and in other cases of hardship as the board may by rule prescribe, the five-year forfeiture period applicable to water appropriated pursuant to the Water Commission Act or this code, and the forfeiture period applicable to water appropriated prior to December 19, 1914, shall be extended by an additional period of not more than 10 years or the duration of any crop control or soil conservation contracts with the United States if less than 10 years.