In the BSG episode Water, how does losing 60% of the water cause such a crisis?

Question

I'm starting the new BSG series again, and just finished the early episode "Water". They mentioned that they have enough water to last for years. Shortly afterward, they lose 60% of the water, and suddenly they only have a few days of water left.

I'm sure there's some logic behind this having to do with their recycling process, but I must have missed it again - I had the same question when I first watched the episode.

How does 40% of "years" translate into "a few days"?

Answer 1

With efficient recycling you can get by with a little more (there will always be some loss — and retention as well; quoting a different SF series, carbon-based life is "ugly bags of mostly water") than just enough water to keep people, animals, plants, etc. alive for several days; you need long enough for enough of the water consumed to get back into the ecosystem and be recycled. Lose enough water, though, and you could easily get into a situation where you don't have enough water for all necessities over the period needed for that water to get back through recycling — and water is more critical than food; you can if necessary go a week or so without food, but only a couple days without water. And many plants are more sensitive than humans.

In a planetary ecosystem, you can do so; in an artificial ecosystem you run into a lot of things you need that you don't normally need to worry about (humidity, for one; if you don't maintain a certain minimum amount of atmospheric moisture, everyone/everything will need much more water than usual due to the severe evaporative water loss). And those plants you let die off might lead to knock-on hunger, or worse knock-on scurvy or the like. Artificial ecosystems have very little margin for error; there are many complex interactions, and even small imbalances can have severe knock-on results.

40% of the water supply not being enough is believable for a large, complex ecosystem.

Answer 2

The numbers in the episode are:

• 2.5 million JPs of water per week for the whole fleet.
• One third of the fleet relied on Galactica for water recycling, so about 0.8 million JPs needed each week, or roughly 0.1 million JPs per day.
• 10 million JPs lost in the explosion.
• Nearly 17 million JPs of water on Galatica, with "close to 100% effective" recycling.
• 7 million JPs leave Galactica 6 days supply, but when supplying the others only 2 days.

This isn't enough information to figure out exactly what the situation was, but numbers can be found that fit, for example (these aren't the only ones possible, but they do to demonstrate):

• Assume that the water lost by the ships that can't recycle well is negligible (i.e. they manage to get most of it to Galactica as waste water that she can recycle).
• Galatica needs 25% of the 0.1 million JPs per day (i.e. a disproportionately large amount given the population, because it's a battleship, and knowing this the makers built good recycling).
• It takes 5 days on average to do a waste water recycle (i.e. convert the waste into useable water).
• 6.5 million JPs are needed to recycle up to 1 million JPs of waste water (this water is used in the process, but at the end you're left with all the water clean, including the 6.5 million). The unknown recycling process could very well require a large supply of nearly-100%-clean water.

Before the explosion, we see that there's no danger of hitting the minimum level required (after several years the small amount lost to the not-quite-100% process and supporting the other ships would drop the level down to dangerous levels).

After the explosion, supporting the other ships, we see we're dead after 2 days, because we cannot recycle. (There's a month of water left, but no way to get more cleaned, and they were having a lot of difficulty finding new sources at this point).

After the explosion, not supporting the other ships, we see we're dead after 6 days.