I do surface farming by channeling down a "courtyard" which I then roof over, but that's really abusing the game's ability to grow surface crops even though they're under a roof (and the roof means the area isn't accessible to hostiles, rendering the area safe for the farmers).
I do this for that reason (though walls and a floor- roof overhead do the same), but additionally:
- Surface boulders that 'spoil' fields don't exist in the Z-1 earth layer, so I get 'pure' soils and no disconnections.
- The fields link directly with the dug-in-soil subsurface field-rooms (and can be converted from them, if I need another 'plot') simplifying the management[1] of the farm/farm stocks/farm-related workshops.
For drainage purposes, I'm wondering if the best idea is to arrange a 'well' (or more than one), if you can handily punch through a spot where the pump(s) will discharge away from the sucking zone.
My wells tend to be dug as a series of vertical 1x1 channels into a cistern. The well-site is designated a channel, the Z-1 is designated a channel too (once a miner has done these two, they are commited), these days with priority of designation to encourage staying there (and maybe an enclosing burrow assigned to
just the single miner currently being observed to be fed/'water'ed/bedded recently enough to be an imminent need) and they can then keep making a well-shaft down to the cistern level(s) without creating an awkward stairwell down which anybody/thing will path (if they didn't jump in at the wrong moment!). If you wish, you can even have the miner be an engraver, too, to smooth the walls from the inside (
mostly aesthetics, for my situation) between each new channel down.
Depending on whether I'm doing a multi-level cistern (I typically would if it's for water supply, but for you you'd just want a place to fill and spread out) I'd do something like ensure the bottom of the channel-tower leads to a (lower priority) 'escape tunnel'/upward ramp out to the side where the well-miner can finally (being released from the ęgis of this task burrow and confinement) go off and deal with hunger/thirst/tiredness they accumulated, and it lets in the other miner(s) to carve out the full rock-cistern in whatever way I wish to pursuade them to (and actual engravers to smooth off the edges and floor, because I'm like that). Then you put your pump feeding from river to 'well' hole, wall around the well-head (to prevent splashing) without preventing pump-dwarf access
or as part of the structure you overlay your levered-gear, the gear that supports the windmill (if you have wind!) and the windmill above that.
Properly constructed, with enough cistern space (which you should have arranged a further side-drain from, while dry, or possibly a water-smasher) pull (or repull, as you deactivated the levered-gear as soon as it was connected to not be pre-emptive) and water is slurped away from the sucking end. Hopefully enough of it, if it's being still fed from aquifer/river, to establish the floodgate(s)/barrier(s) you now know you need.
When all is done, successfully, dissassemble your pumphead (maybe reassemble a couple of tiles over if you have another slurp to do, above the prepared 'well' sink you had also dig into that cistern, or into another one - assuming you didn't multi-pump from the start) and seal it off unless you
want an outdoors well.
The underlying wellshaft may even be useful for putting underground wells in, by side-tapping your vertical shaft (at a level not itself flooded, NB!) and installing the block/bucket/chain there, to sup from the cistern. If you planned that far ahead, you can probably just remove the wall you used for a mid-well escape-hatch (if you had to send your miner down an uncomfortably long shaft, and thus extract them for a rest/replacement 20Z down or so...), but those a fiddly details right now...
You could also (intentionally or otherwise) empty the water down a shaft into
caverns. Knowing your underlying geology helps in working out whether you can do this digging without dropping your miner into previously unknown voids in the rock[2].
Oh, and because you have an aquifer, there's always just the drain-into-aquifer method. Or "portable drains" of various kinds.
Your big problem might be if you've already dug under and through the zone you might want to send water down. Dismantle anything in the way and then strategically wall off the vertical chute, then, perhaps, with the aim of reinstating t all back to normal rooms once you've done the emergency draining.
So many ways. Not sure what you would feel most capable of/happiest doing from scratch. But experimenting might be fun!
[1] Though vertical-farming (stacking)
does make some efficiencies. I may lay the stockpiles/etc in rock-dug rooms in the footprint below the soil layer(s) with stairwell access and if I have deep soil I can still have Z-(2+) soil-floor. Can't remember the time I last wetted rock!
[2] I tend to worldgen with several times more Zs between cavern layers, which gives me a lot of leeway to ensure its solid rock I'm creating my own voids in, occasionally I get caught out by an inter-cavern 'natural' tunnel, but sufficient ceiling-penetration of caverns tends to give me full visibility across my developmental footprint to know where's open...