Try using Design Pattern: Associated Items
You have already used a good naming convention for your items, so implementing it should be quite easy.
Basically, you rule could be (provided you put all Irrigation*_Valve items in a group Irrigation_Valves):
rule "PumpStation2 update flow and volume"
when
Item PumpStation2_Meter received update
then
Irrigation_Valves.members.filter([item|item.state == ON]).forEach [ item |
postUpdate(item.name + "Volume", PumpStation2_Meter.state as Number)
postUpdate(item.name + "Flow", PumpStation2_Flow.state as Number)
]
end
Note: untested and written on my phone, might be syntax errors, but should demonstrate the basic principles.