Complicated? A single full 1*10 ditch with a 7 square moisture spread would moisten 350 squares on its own. Double the moisturizing range, and it would be 1092 squares... for just one unremarkable channelette that even an untrained miner can dig out faster than cats breed. A channel replaces a lot of manual labour by the dwarves with a bit of planning by the player, which is just the effect we need. If you consider the bucket brigade a more economical use of dwarfpower, that'll still be available as well.
Considering their depth, most irrigation (or drainage) ditches afk are just the height a dwarf would drown in. A dwarf that can't swim nor pull himself up on the edge, that is. While the dwarves wouldn't get much bang for their buck by learning to swim (short legs, ya know), grabbing the edge is a trick they will master soon enough. And that's assuming a full ditch: a climate that needs irrigation ditches will usually see them no more than at height 3-4.
Finally, why shouldn't plants be able to use the moisture from non-irrigation ditches?
The problem comes when this essentially just turns into, from a gameplay standpoint, simply one more spike on the learning curve. Yes, the more experienced players can easily handle automated pressure-plate-based water channels fed by water reserves, but quite a few new players find nothing but Fun when they start messing with water.
As it stands, the people who have plenty of experience with water already will spend about 3 more minutes designing a resevoir, and then the entire mechanic of needing to water crops will be completely meaningless - they have jumped that hurdle once, and never will have to do it again.
The new players, however, now have to learn how not to kill themselves making an irrigation "ditch" deep enough to drown their dwarves within.
It doesn't wind up adding anything to the play experience (basically being just building the exact same farm you've always built, but with a trench down the middle this time, and one more trick with water resevoirs), while punishing those not yet familiar with the system. While there's plenty of ways that new players can kill themselves, not being able to do something as basic as FEEDING themselves without tunneling a resevoir and building a floodgate-and-pressure-plate system to provide
precisely 4/7 water is a little extreme. At the very least, a bucket brigade would wind up actually causing more labor to be consumed, which addresses the complaint of allowing a single legendary farmer to feed a full fortress, without requiring any more work than what the fort's well already takes.
This is where I have to go back to saying that "realism" should not be the (sole) objective - that it should be what the player is expected to do with the system that should be the goal.
Getting players to have a chance to set up crop rotation schedules to maximize gain, but still be able to work on the fly, so long as they have extra farmland that they can leave fallow lets more experienced players have a little something to focus in on, while letting less experienced players slide by with somewhat sloppy work. Players can go on and modify and extend such a farm to suit their needs.
Filling a trench with water once, then never touching it again, however, is just a way to punish a lack of forethought, which isn't the same thing as rewarding further planning. It loses all meaning once built, except as a roadblock to further farm expansion (you'll have to dig that trench further to make your farms larger, and that will mess up your precise trench-flooding mechanism). It hinders instead of enables.
I'm all for adding complexity to a system that is too simple... but keep in mind that even for all the complexity of modelling the exact melting points of various materials in this game, the only thing that it really comes out meaning to players is "Build anything that touches magma with bauxite mechanisms instead of other stones", and you're through.
This, then, comes back to what it should mean to the player to have water required to run a farm. I'm a little conflicted on the subject of a special irrigation ditch and a matching pump or sluice gate, but it would afford the player a menu where they could set it for the farmers to fill or empty irrigation ditches, depending upon the requirements of the crops in question. Even better would be if you could simply shovel over irrigation ditches the way you can remove ramps, without having to build walls on the floor below to "undig" a trench. That would allow for more easily extensible farmlands.
Still, even that, I have to wonder if simply letting it go by bucket brigade wouldn't be better, as it would at least be giving out a little more labor for the whole thing (better if watering gave experience, instead of just being another hauling task), making feeding a fortress take at least a half dozen dwarves.
