California Code, Water Code - WAT § 74462

If the divisions are based on population, the population of each as determined by the last published decennial federal census shall be substantially equal in population.  In no event shall the total population of any combination of divisions comprising a majority of the number of divisions of the district equal less than one-half of the population of the district.  In no event shall the largest division exceed the population of the smallest division by a ratio exceeding 2 to 1.

In establishing the boundaries of the divisions consideration may be given to the following factors:  topography, geography, cohesiveness, contiguity, compactness of territory, and community of interests of the divisions.