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Author Topic: Orographic precipitation and drainage  (Read 3925 times)

JoeJoe

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Orographic precipitation and drainage
« on: July 15, 2014, 10:06:41 am »

I initially posted this on the wiki discussion board, but that seems to be rather poorly frequented so I'll ask again here.

So, to quote myself:


When OROGRAPHIC_PRECIPITATION is turned on, not only elevations have an influence on rainfall, but drainage does as well.
I created a completely flat world with min and max rain set to 50 and OROGRAPHIC_PRECIPITATION turned on. Below are the drainage and rain maps.

Spoiler (click to show/hide)

As you can see, when the drainage values are higher than 50, patches of stronger and weaker rainfall are created, shifted a bit east. When OROGRAPHIC_PRECIPITATION is turned off rain is completely uniform across the entire map, as expected. I find this rather unintuitive, because I would expect that when there is more surface water, i.e. lower drainage values, more water will evaporate and come down somewhere else.

So, does anyone with more knowledge about geography than me know why this is? Or is this probably a bug?
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cephalo

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Re: Orographic precipitation and drainage
« Reply #1 on: July 15, 2014, 01:47:36 pm »

I find it strange that you would get those patterns on a flat world. I wonder if elevations are generated before any explicit map data is read in and applied.
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PerfectWorldDF World creator utility for Dwarf Fortress.

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JoeJoe

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Re: Orographic precipitation and drainage
« Reply #2 on: July 15, 2014, 02:27:51 pm »

I find it strange that you would get those patterns on a flat world. I wonder if elevations are generated before any explicit map data is read in and applied.

I'm not sure I understand what you mean to say.
If it was somehow dependent on some elevation information that doesn't make it into the final map, then it wouldn't correlate with the drainage map so closely.

EDIT:

I genned a couple more worlds and here's something interesting: As expected, with orographic precipitation turned on, mountains create rain shadows. But rainfall over oceans is completely uniform, unless the drainage is higher than 50 or something. Look at these pictures.

I'll submit a bug report for now, see if it gets cleared up.
« Last Edit: July 15, 2014, 02:52:30 pm by JoeJoe »
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cephalo

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Re: Orographic precipitation and drainage
« Reply #3 on: July 15, 2014, 02:43:46 pm »


I'm not sure I understand what you mean to say.
If it was somehow dependent on some elevation information that doesn't make it into the final map, then it wouldn't correlate with the drainage map so closely.

Does it? I'm not seeing that in the screenshots.
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PerfectWorldDF World creator utility for Dwarf Fortress.

My latest forts:
Praisegems - Snarlingtool - Walledwar

JoeJoe

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Re: Orographic precipitation and drainage
« Reply #4 on: July 15, 2014, 02:54:32 pm »

ah, you ninja'd my edit. you can see, that the rain shadows only exist in areas where the drainage is high (probably when it's higher than 50).
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cephalo

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Re: Orographic precipitation and drainage
« Reply #5 on: July 15, 2014, 03:13:52 pm »

Ok, I see it now.

Here's my guess. Since Drainage is normally influenced by elevation (I'm not actually sure if that's true in vanilla world gen), maybe the idea was to sort of fake the orographic effect to look natural, but not necessarily accurate, when all the maps were generated together. I have always felt that the orographic effect was very loosely connected to the other maps if at all.

I used a different system for this in PerfectWorldDF because it's useful to change the prevailing wind direction etc.
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PerfectWorldDF World creator utility for Dwarf Fortress.

My latest forts:
Praisegems - Snarlingtool - Walledwar

JoeJoe

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Re: Orographic precipitation and drainage
« Reply #6 on: July 15, 2014, 03:21:10 pm »

Hm, yeah I guess that kind of makes sense. Higher drainage --> higher elevation --> more rainfall. Then again, many of the regions actually have less rainfall.

Might be worth investigating whether drainage can be influenced by elevation at all or if they are generated completely independently.
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Melting Sky

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Re: Orographic precipitation and drainage
« Reply #7 on: July 16, 2014, 06:43:41 am »

Is the orthographic precipitation only occurring over the land? Those even patches of precipitation are the oceans?

Are we looking at large worlds in these pics?
« Last Edit: July 16, 2014, 07:03:20 am by Melting Sky »
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JoeJoe

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Re: Orographic precipitation and drainage
« Reply #8 on: July 16, 2014, 07:26:41 am »

All small worlds. The first world is, if I recall correctly, entirely with an elevation of 200 (I know for sure that there are no oceans). And for the second one, I first tried to create a world consisting entirely of ocean but DF needs to place civilizations so I created a world with mostly ocean (see the elevation map). As you can see, the rainfall is uniform over the land.
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Melting Sky

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Re: Orographic precipitation and drainage
« Reply #9 on: July 17, 2014, 08:36:20 am »

That really is quite odd.  :o
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Toady One

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Re: Orographic precipitation and drainage
« Reply #10 on: July 17, 2014, 12:35:42 pm »

There is a check in there for drainage>=50.  I think it was something to do with differentiating hills/plains (which drainage does currently -- it's sort of a catchall variable until we have more information), but it doesn't have enough other biome checks for that to work (so it might trigger at glaciers for example), and we don't really have a proper notion of hills (or localized elevation events of any kind aside from volcanic ones) yet anyway.

On the other hand, until we have more information, I think leaving it out makes stuff look too uniform -- the main effect is to break up the "hilly" forests rather than the "flat" wetlands.  This is better accomplished by using soil conditions and actual local elevation effects, but we need those to exist first.
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JoeJoe

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Re: Orographic precipitation and drainage
« Reply #11 on: July 17, 2014, 01:05:03 pm »

An answer by the almighty Toad himself! Thanks a lot for clearing things up.
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