The third datum we generate for a system (main world) is the hydrographics percentage. This doesn’t mean water; on some worlds this could be another surface liquid (Titan, for example, has lakes of liquid methane).

Do note that it explicitly means “liquid”. A world covered partially or entirely by ice should have a hydrographic digit “0” if no liquid oceans are present.

The Hydrographic stat is determined by 2d6-7, adding the world size code. Modifiers are:

  • Size 0 or 1: Hydrographics 0
  • Atmosphere code 0, 1, A, B, or C: -4
  • If atmosphere is not D, or F (the rules state “of a type thick enough to retain water”): -2 for “hot” temperature; -6 for “roasting”.

I am not quite sure about the reasoning behind some of that, but it’s just as well since the “hard science” rules I am using replaces (adds? the rules aren’t even clear on that) this with the following:

  • If Size is 3–4 and Atmosphere is A the DM is –6.
  • If Atmosphere is 0–1 the DM is –6.
  • If Atmosphere is 2–3, B or C the DM is –4.

This still seems broken: Assume a vacuum world with a size of 6 (Size 5, 2d6-7+5, roll a 2). Roll 12-7+5 for Hydrographics, results in 10, negative DM 6 meaning 40% liquid water. This could be avoided if the presence of water included ice, but as I am trying not to change what the UWP means, I can’t really make that call.

A different approach

Atmospheric pressure is vital to determining whether or not liquid surface water exists on a world. The reason hydrographics is not based off of the atmosphere, I think, is because the atmo digit doesn’t just encode atmospheric pressure, it also encodes composition – and to differing amounts for different values, too! An exotic atmosphere is not more likely to produce surface water, for example. Unfortunately, conversely, similar size doesn’t necessarily mean similar atmospheric pressure. We need not look further than Venus for proof of that.

Roll 2d6-2. Do not add size directly, instead use the following modifiers:

  • Atmosphere 0 or 1: Hydrographics is set to 0.
  • Atmosphere 2-3 (very thin): -4
  • Atmosphere 4-5 (thin): -2
  • Atmosphere 8-9 (dense): +2

We’ll disregard planetary temperature and weird atmo codes for now. This gives us the following basic distribution:

Hydrographics probabilities by type of atmosphere. y-axis is probability, x axis is hydrographics digit

Is this realistic? I have no idea, but it seems reasonable plausible to my un-educated eyes and it’s certainly a smoother system than theTraveller default.

A quick sanity check seems to work out:

  • Mercury: Size 3, Atmo 0: Automatic Hydro 0.
  • Venus: Size 8, Atmo A: No surface water possible due to temperature.
  • Earth: Size 8, Atmo 8: Hydro 7. 11.11% chance for that, which works just as well.
  • Mars: Size 4, Atmo 1: Automatic Hydro 0.
  • Europa: Size 2, Atmo 0: Automatic Hydro 0, but surface is covered in ice.
  • Titan: Size 3, Atmo A: At most a “few percent” are covered by lakes, which is a hydro of 0.

Odd Atmospheres

Surface liquids for atmospheres A, B and C are difficult to judge for a layman like myself.

Atmosphere D (high pressure oxy-nitro) should probably be rolled as “dense”. Despite the high pressure the world is still very earth-like or it would be classified with atmo digits A-C.

Atmosphere E (thin atmosphere that collects in fissures and other depressions) should likely have an automatic 0-1, maybe 2 at most. Anything higher defeats the idea behind this world type. That would be a DM of -8.

Atmosphere F is a completely special case, it’s basically a catch-all for “everything that’s very exotic”. Set Hydro to 0 as a safe default, then roll according to actual conditions.


Planets that are very hot or very cold will likely have little or no liquid water. I’ll tackle that point in the next post. Meanwhile – if you have any feedback, I’d appreciate it!

5 thoughts on “Hydrographics”

  1. The Hydro number doesn’t always means a liquid liquid. It can also be a frozen liquid. Since the Trade Class Ice Capped (Ic = Atm 0-1, Hyd 1+) exists, frozen liquids are allowed in the rules.

    Quite confusing, I would say. But it may be important to know that if you melt some ice, you might get usable unrefined fuel for your ship.

    1. A frozen liquid is not a liquid, it’s a solid. 😉

      Good point about the IC trade code.. It would indeed be much easier if the Hydrographics just included surface ice. Unfortunately, that is not the case, at least not in Mongoose Traveller:

      “The Hydrographic characteristic measures the amount of surface liquid, and hence the amount of dry land on the world.”

      It’s further complicated because MgT assumes negative DMs for hot temperatures, but not for cold temperatures. The rules really are a mess, and I wonder why nobody spotted all these glaring problems pre-release.

  2. As a PS – MgT can’t make Europa if we assume it should have a significant hydro digit (frozen water ice surface)

    2d6-7 for hydro, +2 for world size, -4 or -6 atmo DM.. makes 2d6-9 or -11.

    More headaches.

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