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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"?

44

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.

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

  • Addendum: you need more if you're breeding than if you are maintaining an existing population; it's something of an extreme case of retention. But in that case you're liable to be even closer to trouble; losing only 20% of your water might be unsupportable, depending on the exact nature of the ecosystem being supported. – geekosaur Jun 4 '11 at 2:58
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    Thanks for the response. It makes sense that they'd run out fast, but I'm still finding the timeline a little hard to believe. By rationing, you could easily avoid using maybe 80% of your normal water usage, at least for a few weeks, before you start to run into problems. You just need a few glasses a day to drink, take quick showers less often, and let a percentage of the plants die (like you said, not as critical), and I'd expect you could easily stretch those few days into a few weeks or even months, assuming the recycling process is still running on the water you do have. – Joe Enos Jun 4 '11 at 3:18
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    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. – geekosaur Jun 4 '11 at 3:27
  • Great, thanks for the explanation. – Joe Enos Jun 4 '11 at 3:58
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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).

Graph of the made-up numbers

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).

Second graph of the made-up numbers

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

Third graph of the made-up numbers

  • Awesome, thanks for the math. – Joe Enos Jun 4 '11 at 7:37
  • @geekosaur made a perfect explanation of the logic, you made a perfect explanation of the maths, +1 for both of you! – Samuel Herzog Jun 4 '11 at 8:40
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    Also, as roughly 50% of the water storage capacity (the ability to store water, not the water itself) of Galactica was lost it would be reasonable to assume that for a short time (couple of days to a week maybe) that at least some of Galactica's ability to recycle water was compromised as well, putting a further strain on the fleet as they could no longer turn around the dirty water in the same amount of time. – Xantec Jun 5 '11 at 5:50
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With deference to the series writers and the people above who have done the "numbers" on this, the Water episode really doesn't "fly." Water, while scarce on some planets, isn't that scarce in space. There are comets, ice moons and planets which have vast resources of water throughput the universe. They could ave easily found a source of water, transformed it to a liquid state and then transferred it aboard. After all, how did they get water on board a ship which was built in space in the first place?

While it made for great drama, in reality, water in space (especially freshwater) isn't difficult to obtain. Had the Galactica simply located even a small comet, they could have easily replenished their stores and had enough left over for the remainder of the fleet to last months or even years.

  • Locating even a small comet is not that easy since space is big and they don't have months to spare. – DeadMG Feb 25 '17 at 19:59
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Well math aside I'm watching it right now and the presidential aid said 1/3 of the water supply is 16,000 people. Seem to be a big error seeing they show the white board with the population 1 or two time a episode before this with a population at this point I believe under 5k

  • Are you sure? I may be thinking of later episodes, but it seemed to hover around 40k – colmde Dec 7 '16 at 10:04
  • According to Wikipedia, there were 47,973 survivors in the opening credits. – Joe Enos Dec 7 '16 at 14:15

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