California Code, Water Code - WAT § 75214

If the improvement district assessment is insufficient to pay in full the cost of the improvements or pay the warrants issued for such improvements, an additional and supplemental assessment shall be made and levied upon all of the lands in the improvement district sufficient to pay such cost or warrants in full, and the procedure followed in making such additional and supplemental levy of assessment shall be substantially the same as the levy of the original assessment, but without the necessity of a petition.

If the proceeds from such additional and supplemental assessment exceeds the final amount necessary to pay such cost or warrants in full, the land paying such assessment shall be entitled to a credit of such excess amount upon the next succeeding district annual assessment levied upon such land.