Just a suggestion, but I wouldn't mind seeing dwarves use various tricks to gain water in maps that it is scarce. My initial thoughts were rain barrels, for example, or to allow pits to fill in with water in rainy environments.
This would add a certain flare to tropical environments -- imagine dwarves having to set up bucket brigades to bail out sections of the fortress that have flooded with rainwater.
In my mind, the way to do that would be to keep track of "soaking wet" sections of the map (or start using decimal points to keep track of smaller amounts of water), and maybe use the flow code to find lower sections. The idea being that the dwarves could make a pit outside and let water naturally fill it over time.
In addition, in humid environments (swamps, rainy days, etc) water simply shouldn't evaporate away, even if it's only a small, "1 unit" amount of water.
This would also allow lakes to refill over time, and even for for "flash floods" to occur (when a natural lake overflows, for example).
Rain Barrels would use a similar system -- you'd place them like a building, and when they are rained on they would slowly fill with water. You could accelerate the process by putting them next to a wall or building (the game would presume that the dwarves have set up rain gutters) or even a floor grate system overhead.
I picture dwarves setting up an underground reservoir and filling it with rainwater collected with barrels at random.
In other environments, dwarves could use other tricks to get water. For example, in a desert environment, they could use leather and a barrel to make water traps in the sand -- mostly bury the barrel, put the leather on top of the pit, then a stone on top of the leather. The sand "sweats" over time, slowly filling the barrel. Farmer workshops could process water from plants, such as cacti or certain fruit. In a glacier, dwarves could burn wood to melt ice -- I picture a "magma ice smelter" (or even a standard magma smelter) being used to make barrels of water for dwarven use.
Thordain Oakenleg cancels Smelt Ice, needs empty barrel.
[ November 24, 2007: Message edited by: KiTA ]