How much pressure in the mains will a 100,000-gallon reservoir develop compared to a 1-million gallon reservoir at the same elevation?

The pressure developed in water mains at a given elevation is determined primarily by the height of the water column above the point of measurement, rather than the total volume of water in the reservoir. This gives rise to the principle that pressure is a function of the elevation of the water above a reference point, typically expressed in terms of feet of water.

Since both reservoirs—the 100,000-gallon and the 1-million-gallon—are at the same elevation, the height of the water column translates to the same hydrostatic pressure exerted at the mains. The pressure is independent of the size of the reservoir; both would generate identical pressure as long as the elevation remains constant. Thus, regardless of whether there's more or less water in the reservoir, the pressure at the mains will remain the same for both.