DF has lots of different types of rock, but they aren't differentiated on an feature:
porosity.
Sandstone and most other sedimentary rocks are porous. If you dump water on them, they'll soak it up pretty quickly. Igneous rocks are not porous, but in their natural state they will have fissures that will leak to some extent. Either kind of rock can also contain fluids and water-soluble minerals that can taint water.
My point is that if you really want to store water underground, then you ought to build a proper cistern. Dig a deep pit, and then line it with worked non-porous stone. Otherwise, there should be a fair chance that 1) the earth will soak up all your water, or 2) minerals and other fluids will contaminate your water.
This works really well for DF, because cisterns really were historically part of siege warfare. Just as water resources determine where people build cities, and available water determined where they could build fortresses. Cistern water can come from an aquifer, channeled surface water, or rainwater. Though efficient use of rainwater would require some extra work to keep the soil from soaking it up.
Filling your cistern should take time, just like stockpiling food, and the size of your cistern should partly determine how long you can hold out in a siege, as intelligent enemies should move immediately to cut off or poison outside water supplies.
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Topic: Stone Porosity and Cisterns (Read 1508 times)