How do we find total volume of water for a certain catchment?

Think of catchments as a way to model non-flow water processes like precipitation, evapotranspiration and infiltration. While there is certainly a volume of water in the catchment, because of the modeled processes, the volume itself won’t tell you much about the actual water yield. However, in the results section, there are several options to view run off, the primary being Land Class Inflows and Outflows (the first item in the Catchments menu).

The first two sections of the Hydrology Module in the WEAP tutorial are all about catchments.