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-   -   Can the hydrology of this world be made to work? (http://forums.sjgames.com/showthread.php?t=171500)

Michael Thayne 12-13-2020 02:47 PM

Can the hydrology of this world be made to work?
 
I'm working on a bronze-age fantasy setting based largely on ancient Egypt with a dash of Mesopotamia thrown in. I decided I didn't want the world to be too exact an analog for Earth, so I flipped a coin to determine the hemisphere (result: southern hemisphere) and rolled dice to determine the direction of the river flow (result: west to east). But otherwise, I've been using Egypt as my main inspiration for things like the physical environment and supportable population density, with a region resembling the Levant to the south, and some wooded mountains inspired by the https://en.wikipedia.org/wiki/Zagros_Mountains added at a point roughly equivalent to the Nile's first cataract.

I'm concerned that this may not work hydrologically, though. If there's nothing but open ocean to the East, the climate should be more like the Yangtze river valley than the Nile river valley. I think I can solve that by having my setting's Big River empty into a relatively narrow sea with another large land mass not too far to the east, so I can justify the east coast desert. But I think I still might have a problem with the Big River's basin not getting enough rainfall to produce the Nile-like floods necessary to support the population density. I'm thinking of maybe flipping east and west across the entire setting—or is there a way I can avoid that? The idea of making the setting's physical environment subtly weird in ways that make perfect logical sense once you have all the information is something that appeals to me, assuming I can make it work.

Hide 12-13-2020 04:56 PM

Re: Can the hydrology of this world be made to work?
 
Have you checked the peruvian Caral civilization? They were presumably contemporary with the Egyptian Pyramids, and were seated around 3 rivers. It might help you with your setting.

Agemegos 12-13-2020 07:02 PM

Re: Can the hydrology of this world be made to work?
 
I’m not very knowledgeable about hydrology, but I can think of two ways to get reliable annual flooding and two ways to get a desert for your river to flow through.

The flooding can depend either on snowmelt or on seasonal rains. The desert can be produced either by the high pressure belt in the horse latitudes or by a rain shadow. Mostly that’s easier with the river running between two latitudinal bands of climate (i.e. north or south), but you might like to look into the way that a seasonal analogue of the katabatic effect results in the Tibetan plateau producing the monsoons of South Asia.

Luke Bunyip 12-14-2020 03:00 AM

Re: Can the hydrology of this world be made to work?
 
Wot Agemegos said. The early civilisations along the Indus come to mind.

Michael Thayne 12-14-2020 09:47 AM

Re: Can the hydrology of this world be made to work?
 
Quote:

Originally Posted by Agemegos (Post 2358055)
I’m not very knowledgeable about hydrology, but I can think of two ways to get reliable annual flooding and two ways to get a desert for your river to flow through.

The flooding can depend either on snowmelt or on seasonal rains. The desert can be produced either by the high pressure belt in the horse latitudes

This was my initial thinking. But if you look at the prevailing winds around the latitudes most of the world's deserts are, they tend to run from east to west. This is why southern China is so much wetter than the Sahara, in spite of being at roughly the same latitude.

Quote:

or by a rain shadow.
This is sort-of what I'm thinking by having my not-Nile drain into a relatively narrow sea with significant land masses to the east of the sea. But it leaves the question of what generates the rain that feeds the not-Nile.

Quote:

Mostly that’s easier with the river running between two latitudinal bands of climate (i.e. north or south), but you might like to look into the way that a seasonal analogue of the katabatic effect results in the Tibetan plateau producing the monsoons of South Asia.
From what I understand though, monsoons mainly hit the east coast of Asia, due to the way the Coriolis effect causes winds blowing towards the equator to deflect to the west. This is why I say it might work better if I flipped west and east for the whole setting design.

Anaraxes 12-14-2020 10:52 AM

Re: Can the hydrology of this world be made to work?
 
You could also shift the river north or south, to change its position relative to this planet's analog of Hadley cells. Or if you really want to make it not an analog of Earth, spin the planet the other way around.

Anders 12-14-2020 11:10 AM

Re: Can the hydrology of this world be made to work?
 
Wouldn't it be easier to have the wind be the breath of the river god - one breath in and one breath out per year?

Michael Thayne 12-14-2020 12:04 PM

Re: Can the hydrology of this world be made to work?
 
Quote:

Originally Posted by Anaraxes (Post 2358140)
You could also shift the river north or south, to change its position relative to this planet's analog of Hadley cells.

Hmmm. So maybe something like the Patagonian desert, or Gobi desert? Though I think those are both noticeably colder than Egypt/Mesopotamia, where I'm drawing all my cultural inspiration from, and I'm not sure either ever hosted a river civilization. Plus, for reasons I'm somewhat unclear on, the Gobi doesn't extend all the way to the eastern coast of Asia. Maybe if that region were flatter the Gobi would extend further east?

On the whole, maybe I could get on board with this idea if I had a better sense of all the implications for the setting, but I'm not sure what those implications are.

Quote:

Or if you really want to make it not an analog of Earth, spin the planet the other way around.
This crossed my mind but I'm not sure it's coherent. What do "east" and "west" even mean independent of the planet's spin?

Quote:

Originally Posted by Anders (Post 2358142)
Wouldn't it be easier to have the wind be the breath of the river god - one breath in and one breath out per year?

That's not quite the flavor I want. That sort of thing implies "mythology all the way down", whereas I want a world that has a purely natural layer underneath all the supernatural stuff. Though could maybe see myself invoking a portal to the elemental plane of water, or a permanent weather control effect, somewhere in the highlands of the not-Nile's basin.

An option that hasn't been discussed is to have the not-Nile deflect either significantly north or significantly south once it's gotten ~100 miles beyond the "first cataract" / "Zagros mountains" region. But I'm concerned the results would be very unnatural looking on a bird's eye view of the map.

Michael Thayne 12-16-2020 02:45 PM

Re: Can the hydrology of this world be made to work?
 
Looking more at it, Bangladesh's summer monsoons do seem to blow from the southwest. Translating to the southern hemisphere, maybe having a monsoon from the northwest feeding my river basin would work?

ericthered 12-16-2020 03:53 PM

Re: Can the hydrology of this world be made to work?
 
You could use a double set of rain shadows: the winds blow in from the east, hit the first and lower set of mountains, then pass over a dry region before hitting a second set of higher mountains and getting completely wrung out.

As for turning the river north or south, could you tilt the river instead, giving it a slight incline into wetter latitudes?

You know, looking at a map... are the Tigris and Euphrates too crooked for you (if you mirrored them over the equator?)

Anaraxes 12-16-2020 07:00 PM

Re: Can the hydrology of this world be made to work?
 
Quote:

Originally Posted by Michael Thayne (Post 2358154)
What do "east" and "west" even mean independent of the planet's spin?

Not what I meant. Simply that if you spin the planet the other way around, it's the same as reversing the map east-west. The labels would also change (assuming the inhabitants find sunrise and sunset interesting), but the actual relative geography doesn't. Either the mountains are spinward of the river or they're anti-spinward.

Prevailing winds are a result of the Coriolis effect. Reverse the planet spin and you switch the direction of the Coriolis and thus prevailing wind direction relative to geographic features. Or you could flip the relative positions of the rivers and mountains (flip the map) while keeping the same rotation direction. Only if you do both reverses does the situation remain the same.

Assume the mountains start off antispinward of the rivers; other factors are such that prevailing winds come over the mountains so the rivers are in a dry region. Now, flip those mountains to spinward of the river. The planet's still spinning the same way, so the wind moves the same way, but now the wind crosses over the rivers first, then the mountains. Not dry. Now do the second flip, this time of the planet's spin. Now the mountains are antispinward of the river again, so the rain shadow is back. Either change alone will reverse your prevailing wind direction relative to the geography. Both together means no net change.

It doesn't matter to the existence of the rain shadow if the inhabitants also switch their words (which I agree they probably would, if they find sunrise as interesting as humans do), because that linguistic change doesn't flip the mountains to the other side of the river along with the planet spin or cause the wind to prevail in a different direction. The label is not the terrain.

But this point is really off in the weeds. The practical upshot is that it's merely a way you could save an existing map, assuming you had one already drawn and didn't want to redo it. (Or you scan it into Photoshop and reverse it left-to-right.) Or perhaps you want to save existing text that's chock full of references to this being "east" or "west" of that, and it'd be a chore to edit all those words to match changing planet spins.

Michael Thayne 12-17-2020 02:21 PM

Re: Can the hydrology of this world be made to work?
 
Quote:

Originally Posted by ericthered (Post 2358482)
You could use a double set of rain shadows: the winds blow in from the east, hit the first and lower set of mountains, then pass over a dry region before hitting a second set of higher mountains and getting completely wrung out.

As for turning the river north or south, could you tilt the river instead, giving it a slight incline into wetter latitudes?

You know, looking at a map... are the Tigris and Euphrates too crooked for you (if you mirrored them over the equator?)

Looking at a map of these rivers and their basin, Euphrates does change direction fairly dramatically somewhere around northern Syria, but that's not quite the same thing as having an L-shaped river basin, which is what I'd need here, I think.

Quote:

Originally Posted by Anaraxes (Post 2358499)
Not what I meant. Simply that if you spin the planet the other way around, it's the same as reversing the map east-west. The labels would also change (assuming the inhabitants find sunrise and sunset interesting), but the actual relative geography doesn't. Either the mountains are spinward of the river or they're anti-spinward.

Prevailing winds are a result of the Coriolis effect. Reverse the planet spin and you switch the direction of the Coriolis and thus prevailing wind direction relative to geographic features. Or you could flip the relative positions of the rivers and mountains (flip the map) while keeping the same rotation direction. Only if you do both reverses does the situation remain the same.

Assume the mountains start off antispinward of the rivers; other factors are such that prevailing winds come over the mountains so the rivers are in a dry region. Now, flip those mountains to spinward of the river. The planet's still spinning the same way, so the wind moves the same way, but now the wind crosses over the rivers first, then the mountains. Not dry. Now do the second flip, this time of the planet's spin. Now the mountains are antispinward of the river again, so the rain shadow is back. Either change alone will reverse your prevailing wind direction relative to the geography. Both together means no net change.

It doesn't matter to the existence of the rain shadow if the inhabitants also switch their words (which I agree they probably would, if they find sunrise as interesting as humans do), because that linguistic change doesn't flip the mountains to the other side of the river along with the planet spin or cause the wind to prevail in a different direction. The label is not the terrain.

But this point is really off in the weeds. The practical upshot is that it's merely a way you could save an existing map, assuming you had one already drawn and didn't want to redo it. (Or you scan it into Photoshop and reverse it left-to-right.) Or perhaps you want to save existing text that's chock full of references to this being "east" or "west" of that, and it'd be a chore to edit all those words to match changing planet spins.

Ah, yeah, that's what I meant when I mentioned flipping the map in the OP, and I'd probably do the more labor intensive version if I did it at all.

Also here's very much work-in-progress map of the setting. Hexes are 100 miles. North is at the top, center of the map is 30°S latitude. White means "I am not mapping this because it's far enough away from the center that distortions from projecting a sphere onto a plane are going to get rapidly worse". Gray means "land not otherwise specified". The eastern continent is meant to account for the dryness of the river valley, while the bay in the north-west is mean to provide a way for monsoons to reach the river basin (inspired by the Bay of Bengal).

Finally, I had the thought that if the river basin has a lot of igneous rock and/or clay (edit: or limestone), it could reduce the amount of rain you need to account for the same amount of discharge downriver, because less of the rain ends up as groundwater. Not sure how far that concept really extends, though. Looking at the size and annual precipitation of the Ethiopian highlands, it looks like maybe only 5% of the rain water actually makes it to Aswan. IDK what that percentage would be if much of the river cut through less permeable terrain.

ericthered 12-17-2020 03:25 PM

Re: Can the hydrology of this world be made to work?
 
Quote:

Originally Posted by Michael Thayne (Post 2358618)
Looking at a map of these rivers and their basin, Euphrates does change direction fairly dramatically somewhere around northern Syria, but that's not quite the same thing as having an L-shaped river basin, which is what I'd need here, I think.

I was actually referring to the rivers not flowing strait east.

If you want crazy river shapes, there is always the Niger river, but I don't know that it will actually look right: truth can be stranger than fiction.

Michael Thayne 12-17-2020 05:13 PM

Re: Can the hydrology of this world be made to work?
 
Quote:

Originally Posted by ericthered (Post 2358634)
I was actually referring to the rivers not flowing strait east.

If you want crazy river shapes, there is always the Niger river, but I don't know that it will actually look right: truth can be stranger than fiction.

Oh gosh, the Niger river does offer some interesting possibilities, doesn't it. Thanks for the tip.

Michael Thayne 12-17-2020 06:01 PM

Re: Can the hydrology of this world be made to work?
 
Did a bit more reading on the Niger river and apparently it was once two rivers that got joined together and it just so happened that the original two rivers flowed in directions roughly perpendicular to each other, hence the river that makes a 90° turn. That's amazing and I just may use that. Thank you so much!


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