Problem

How is the "Flow Stored" calculated in the Calculation Summary generated after computing a model? What is the difference between Flow Supplied, Flow Demanded and Flow Stored?

What is the difference between Flow (In) and Flow (Out) for tank results?

Solution

For each time step in an EPS simulation (or for the single time step in a steady state), WaterCAD/WaterGEMS displays the mass balance of flow supplied, flow demanded, and flow stored.

The "Flow Supplied" is the total flow rate out of water sources. This is mainly reservoirs, but also included negative demands (inflows). Reservoirs that are filling will experience negative flow supplied. In a system with only demands, the flow supplied will be equal to the flow demanded. In a system with downstream tanks, the reservoir flow supplied is based on energy balance - the outflow required in order for those known HGL boundaries to be true, while also supplying demands.

The "Flow Demanded" is the total demand at that time (flow leaving the system), which could be from fixed demands at nodes, pressure dependent
demands, emitter coefficients on hydrants, etc.

The "Flow Stored" is the net inflow into tanks at that time. For example, if you have just one tank which has only outflow at that time, the flow stored will be negative, at that amount. If it has only inflow at that time, the flow stored will be positive.

Therefore: Flow Stored = Flow Supplied - Flow Demanded

In other words, if more flow is coming from reservoirs than is demanded, the surplus goes into tank storage. If more flow is demanded than is supplied by reservoirs, the deficit is supplied by tank storage.

For tanks, if the Flow(In Net) result is greater than zero, then the flow into the tank is greater than the flow out, and thus the tank is filling and the level will rise. If the Flow (In Net) is less than zero, then the flow into the tank is less than the flow out and thus the tank is draining and the level will drop. The "Flow (Out net)" will always have the same magnitude as Flow (In net), but with opposite sign.

Overall Balance in the "All Time Steps" row

For an Extended Period Simulation, the top row of the Calculation Summary shows "All Time Steps". This represents the average of all reporting timesteps. (all of the time steps displayed in the Calculation Summary, which may be hiding some calculated timesteps) This can also be interpreted as the overall balance, over a period of time.

Note that if you use a constant reporting timestep, there may be calculated timesteps between the reporting times, which will not be included in the calculation of the average value shown in the "All time steps" row. Also, in a model with irregular timesteps (such as a one hour calculation timestep and a few intermediate timesteps inserted at odd times like 13.15 hours), all values are included in the calculation of the average.

For example in a system with reservoirs, demands and tanks, if the volume in the tanks at the end of the simulation is equal to the volume in the tanks at the beginning of the simulation, then the "All time steps" flow stored will be zero while the flow supplied will equal the flow demanded (since the only source of new water comes from reservoirs or negative demands). This also means that in a model with multiple tanks, the "all time steps" flow stored may be close to zero even if some tanks drain and end at a lower elevation, because other other tanks may be filling and ending at a higher elevation.

Note that the "all timesteps" value will be a more accurate overall balance with a smaller timestep. So, if you are using a larger timestep and/or using "constant" reporting timesteps, consider changing the reporting time step to "All" and using a smaller calculation timestep.