California Code, Water Code - WAT § 75475.5

The commissioners shall determine whether the district should be divided into more than one zone of benefit.

If the commissioners determine that the district shall be divided into more than one zone, the zones shall be given a numerical designation starting with Zone 1, which shall be the zone which receives the greatest proportional benefit.  The benefit received by Zone 1 shall be assessed at 100 percent, and the benefit received by the other zones shall be expressed in terms of relatively smaller percentages as the benefits compare with those received by Zone 1.

The commissioners shall prepare a map showing the boundaries of the zones and their respective benefit percentages, and a report explaining the basis for their determination.