I got a farm room set up with a floodgate inlet and a grate that lets over flow drain to another farm room below. I've learned (at least for the size of my farm) how much water to let in before pulling it so both levels get 1 water everywhere.
What I can't figure out is just how much irrigation the land needs. I think I managed to survive 3 years (dunno how to check that) but I was never sure exactly when my plots needed water. So I started thinking: Do they just need to be irrigated once to get the muddy soil in and then never again??
The other thing I tried was connecting the floodgates to a water pressure (set to 2 exactly) plate. I found that the gates kept opening and closing every time the water on the plate switched between 1 and 2 units of water. This lead to dwarves cancelling the drink water task they were gonna do because the water disappeared (i.e., the floodgate closed. They were gonna go up the floodgate tunnel and take a sip of the river from the bottom!). I can see now that making the plate react only to 1-3 water would have stopped the constant switching, but I was never sure if it was going to simply close the doors and keep them closed after I had initiated a flooding with a lever. I think it just kept toggling them open and closed.
It would be handy to be able to specify whether a unit of pressure activates, deactivates or does nothing, rather than every pressure plate (and lever) doing both actions.
Anywho, so how DO we know when to irrigate?