40d:Water pressure

Water pressure is a phenomenon apparently exhibited by some, but not all, water in Dwarf Fortress (Murky pools seem pressureless unless connected to another water source, but this is probably because pressure is only transmitted by 7/7 water).

Water which is pumped up high into an enclosed tower and then allowed to flow to the bottom, at least if pumping continues, will have tremendous pressure at the bottom of the tower. This causes the water, if released, to move with remarkable speed and to even flow up stairs, ramps, or channels. Any water which flows down will tend to flow back up, until it has equalized.

Dwarf Fortress does not model air pressure or surface friction, and magma does not have pressure.

Water Pressure Dangers

It is easy to flood your fortress accidentally by not accounting for water pressure. For example:

• It is safe to dig out a cistern one level below a murky pool, and to channel above a few tiles of the cistern so that your dwarves can get water from it without having to go outside.
• It is safe to refill a murky pool with water from a pump or brook/river/etc on the same level.
• It is not safe to do both to the same pool! The water from the pump/brook/river/whatever will fill the pool to 7/7, and will then pressurize the water in the cistern, which will then flow up out of your channels and flood your fort.

Mitigating the Dangers

Hatches placed over channels prevent water pressure from spewing water up out of the channels, and still allow the tiles to be used as a water source (and possibly still for fishing too).

Pumps can be used to eliminate pressure. If you allow water to fall two levels down into an enclosed area, and then pump it from one level above that, the water which comes out of the pump should have 0 pressure, rather than 1 pressure. You can even deliver water to the second-lowest level of your fortress safely in this manner. Of course, there is a downside - you still have to power the pumps. Due to the source water's pressure, the pump must be powered instead of dwarf-run (the tile the dwarf would stand on is filled by the pressurized water). Additionally, the power cannot be hooked up to the walkable tile of the pump, because the water will simply flow directly up it! Your vertical axles or gear assemblies should be placed above the unwalkable tile of the pump, and there must not be a channel over the walkable pump tile. (Water can only flow straight upward, not up and to the side at the same time)

Water Pressure Theory

What follows here are several rules which specify how water pressure affects water's behavior. These were written to fit experimentally observed behavior as best as possible. (There used to be two models here, but the second one couldn't be reconciled with the observed results of the water-overpressure-in-river experiment)

Current Best Pressure Model

• 1. Water normally has 0 pressure.
• 2. If a water source attempts to create water but cannot because the output tile is already 7/7, IFF there is also 7/7 water below the output tile, it will become pressurized to pressure level 1.
• 3. Sourced 7/7 water tiles with more than 0 pressure first attempt to generate water in (potentially distantly) connected low-water (below 7/7) tiles on the same level. (It is unclear what pressure such generated water starts with).
• 4. If sourced 7/7 water tiles with more than 0 pressure cannot generate water with rule 2, they instead will generate water above themselves if there is room for it. (Such generated water appears to start with 1 less pressure than the water below it)
• 5. If sourced 7/7 water tiles with more than 0 pressure cannot generate water with rule 2 or rule 3, they instead transmit their pressure level to lower-pressured adjacent water tiles, and to 7/7 water tiles below themselves (and possibly 7/7 tiles above?).
• 6. Pressure which is transmitted downward is increased by 1.
• 7. Pressure which is transmitted upward is decreased by 1.
• 8. Pumps do not transmit pressure. Water which comes out of pumps has 0 pressure. (Note that you must ensure that output water does not contact input water, or pressure WILL be transmitted through the connection)
• 9. Water tiles which drop below 7/7 density become 0 pressure.
• 10. Pressurized water moves much faster than 0 pressure water, with each level of pressurization seemingly providing greater speed. It also seems to increase the rate at which water is created.
• 11. 'Sourced' water means water which is connected via 7/7 tiles to a water source, which includes the edge-of-map tiles where water enters the map, actively pumping pumps which have water to pump, and possibly aquifers.

Anomalies

• If you drop a fixed amount of water several levels down a shaft into another room, you can end up with more water than you started with. ^v33g
• Pumps can turn into seemingly infinite water generators (though they can be stopped by blocking their input with a hatch, or by disconnecting their power).^v33g The above model currently appears to match this behavior (possibly not in all circumstances which may cause it).
• A "u-bend bug" has been reported, which seemingly generates infinite amounts of water without pumps.^v33g[Verify]
• It is also possible for water which lacks an endless source to exhibit overpressure and follow the overpressure rules. This is not a bug, but something which doesn't fit the current model. For example, if you take a 4x4 room of 7/7 water and drop it down a 1x1 shaft with a 3x1 tunnel at the bottom, which connects to another empty shaft which leads straight up, the water will fill the 3x1 and then equalize height in both shafts.

Possible Experiment: From a pump, fill a cistern which is several levels lower. Shut off the pump and the higher level tiles with hatches. Open other hatches above the cistern, combine water with unpressurized water, and see what happens.

Movies of pressure experiments, and save files for them

• [1] - Showing that pressure is not transmitted through non-7/7 tiles.
• [2] - Showing that pumps output 0-pressure water even from a high-pressure source, that water will not flow up and to the side at the same time (has to flow straight up), and a few other things
• [3] - Pump turned into infinite water generator, but still provided useful information on how overpressure causes upward flooding.