Hammer V8i Reservoir elevation issue

Question

Acually, I am a little bit confused about "elevation" of reservoir attibutes, and I checked the information http://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/23776.elevation-and-elevation-inletoutlet-invert-fields-reservoir-properties and the answer is that "elevation" is water surface elevation. however, normally, there are top water level, mean water level and bottom water level, therefore, 
 Can somebody help me to understand what is meaning of water surface elevation (elevation) in reservoir attibutes? Thank you very much.

Scott Kampa · Accepted Answer

Hello Chenfei, 
 
Reservoirs are considered to be an infinite source of flow, so they will never empty. Therefore, the "Elevation" field representing the water surface elevation of the reservoir will never change. The "Elevation (Inlet/Outlet Invert)" field is the invert elevation of the pipe connected to the reservoir. As mentioned in the link you referenced, this is used in HAMMER to determine the pressure at the connection between the reservoir and the pipe. 
 
If you need to represent a source where the water surface elevation might change, you would need to use a tank. Tanks include maximum and minimum elevations, as well as an initial elevation. Over the course of a simulation, the water elevation in a tank will change. 
 
However, if you want to continue to use the reservoir, note that the elevation will not change because reservoirs are considered to be an infinite source of flow. 
 
Regards, 
Scott