(1) The department may calculate the reservoir capacity in acre-feet, in absence of detailed data, as 0.4 times the vertical height in feet from the downstream toe of the dam to the maximum normal operating pool times the water surface area at that level in acres.

(2) Reservoir capacity due to excavation in the impoundment area is excluded from capacity calculations.

(3) This rule applies to multiple or a series of dams erected on a valley, basin, coulee, or ravine where each reservoir is less than 50 acre-feet, but all are operated singly as one reservoir containing more than 50 acre-feet. Whether the series is operated as a single reservoir will be a question of fact in each instance. Factors to consider include:

(a) whether all dams or reservoirs are under the same ownership and control;

(b) whether all dams or reservoirs operate as a unit (i.e., if one of the dams or reservoirs suffers a change, it will in most instances affect one or more of the other dams or reservoirs); or

(c) whether the dams or reservoirs are physically proximate and on the same valley, basin, coulee, or ravine.