Any ground water charge levied pursuant to this part shall be in addition to any general tax or assessment levied within the district or a zone or zones thereof; provided, that in any fiscal year, the sums raised by any ground water charge levied by the district in any zone or zones, shall not produce funds for district purposes that would exceed such amount as is deemed necessary by the district board to be used in furtherance of district purposes in the replenishment, augmentation, and the protection of water supplies for users within the district or a zone or zones thereof.