In a steady state simulation, Only the initial water level/ elevation in the tank is considered for hydraulic analysis of the network. No volumetric analysis is done/ considered in Steady State Simulation (No consideration for Tank's Diameter). Hence, a tank is considered as an unlimited source of water at a fixed initial HGL. Hope that answers how tanks with lesser volume/ capacity satisfies more/ higher demands in steady state without any errors/ notifications.
However, if you want to understand the volumetric fluctuations in the tank, it is advisable that you model it in extended period simulation mode.
P.S. In order to make sure that the tank capacity is appropriate, the user must perform mass balancing of tanks for a certain demand and then determine the diameter accordingly. For existing tanks in the network, you can modify the Zone Boundary for the existing tank so that the demands in the revised zone will satisfy the mass balancing criteria. As a thumb rule, a tank's capacity (Vol.) is typically 1/3rd of the total demand on to it.
In addition to Tanay's response you can find out more about extended period simulation and tank operating range from the following articles;
Run an Extended Period Simulation (EPS)
Understanding Tank Operating Range
Hope this helps.
Bentley Technical Support
Thanks for the reply...I have another question...i have given the tank elevation lesser than the junction elevations...but still the flow is taking place and I don't have any pump near the tank... how is it possible for water to flow when tank is at lower elevation then junctions without any energy?
Thanks for the reply...what does warning status message Tank T-1 is emptying at 1m mean?
The notification means that tank becomes empty at 1 m as the water is drained out completely before it becomes full. You can add controls on pump -on& off, if you have in the model based on the tank levels. So that tank is continuously filled as per the levels.
Below technote explains how to setup controls in the model, please go through it.
Creating Controls - Conditions, Actions, and Control Sets (CONNECT Edition and V8i SELECTseries 6)
Since you are a new user, please go through the below learning resource of WaterGEMS/WaterCAD.
Bentley Technical Suppport