|This article is about an older version of DF.
Evaporation in DF occurs when liquids (water or magma) are very shallow. Also, outdoors, murky pools can quickly evaporate when the outdoor environment is very hot - this can be dangerous, leaving your fortress with no drinking water for wounded dwarves. Building a cistern quickly after embark is critical on such maps. Flowing water, rivers and brooks, do not (significantly?) evaporate.
If a tile has liquid, either water or magma, at a depth of 1 (also referred to as 1/7) for too long, that liquid will slowly disappear. (A tile of depth 1/7 liquid resting on a full 7/7 liquid tile below it will also evaporate in this fashion.) Magma will evaporate more slowly than water[Verify] - compounded by its reduced flow rate due to lack of pressure, this can slow or even prevent long diversions of liquids (well over 100 tiles for magma), as at some extreme point the flow and the evaporation occur at the same rate. One solution is to install floodgates or other barriers at regular intervals, allow that section of the passage to fill, and then move on to the fill the next section.
Outdoor water of any depth will evaporate (and possibly disappear completely) during hot or dry seasons. Outdoors, deeper water seems to lose water at a slower rate; over a couple of seasons, a depth 2 pool has been observed to vanish completely while a depth 5 pool didn't lose any water at all.