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I'm worldbuilding a continent that's mostly desertic (still trying to decide if sand or rocky desert, but definitely a hot desert). All the rain would fall on the edges of it, through a chain of mountain-ranges on all sides. I want said continent to have pockets of population, so I need to know how to distribute the water underground.

How can I draw a realistic network of underground aquifers (or where can I learn how to do it)? How will that translate in oases or locations where people might dig up wells?

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    $\begingroup$ This might help: when the IBM RISC System/6000 came out, I worked on porting several scientific models to AIX, including MODFLOW . Look over that to see what it needs as input. Or, just use it yourself on your fantasy maps to get the “real” answers! $\endgroup$ – JDługosz Dec 5 '16 at 6:11
  • $\begingroup$ @JDługosz: I'll look into MODFLOW, thank you. $\endgroup$ – Pedro Gabriel Dec 5 '16 at 19:59
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    $\begingroup$ @AndreiROM If people answer questions about quantum physics and crazy orbital patterns I'm sure we'll find someone who knows about aquifiers $\endgroup$ – Zxyrra Dec 6 '16 at 13:06
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The key word you're looking for is Aquifer.

This is water present in underground rock layers. Generally you get an oasis or springs when the water present in layers of rock that water can pass through are funneled to the surface by layers of rock that water cannot pass through. This will usually be in lowland areas relative to nearby hills or mountains, as the water underground still has to flow down with gravity.

You can find a number of maps of these structures online by searching for aquifer maps:

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The location of these aquifers depends a great deal on the local geology in the areas; depending on how in depth you want to world build you could construct a detailed local geology or just follow some existing topographical features. For example on the U.S. Aquifer map it is relatively easy to find aquifer contours running adjacent to mountain ranges, plateaus, and coastlines.

The USGS has a large amount of information on the basics and some detailed maps in the US Groundwater Atlas

On a related note, digging wells isn't the only way to get at underground water, in Earth's desert regions a common un-natural feature for moving water to facilitate human settlement is the qanat. It is basically a tunnel used as an underground irrigation canal, dug to let water flow horizontally out of a hillside aquifer.

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  • $\begingroup$ Thank you for your comment. One doubt: You said that the location depends a great deal on the local geology of the areas. What kind of geology would you recomend to maximize the number of oases on a flat desert, especially far away from the mountains? $\endgroup$ – Pedro Gabriel Dec 5 '16 at 20:02
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I don't know how to draw a realistic network of aquifers, but I do know that such things exist under the Sahara, and are being tapped to produce farms in the desert. So perhaps that may help? You'd probably need to investigate the geology behind what formed such aquifers (or perhaps rather why the Sahara formed over them).

http://www.bbc.co.uk/news/science-environment-17775211 http://www.bgs.ac.uk/research/groundwater/international/africanGroundwater/maps.html

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  • $\begingroup$ Thank you for your answer. I'll read this with more detail later, but I think it will be most helpfull. I'm surprised by the amount of groundwater in the Sahara desert... reality is indeed stranger than fiction. $\endgroup$ – Pedro Gabriel May 23 '16 at 21:09
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Large fossil water deposits depend on the climate having been different in the geologically recent past. The significant dryland aquifers in the world are not that old. There are broadly two types.

The first type is like the Nubian Sandstone aquifer across the North Sahara, primarily in Libya. The water in this aquifer comes from the Neolithic Subpluvial, also called the 'Green Sahara'. This was a time period from about 7000 BC to 5000 BC. At this time, the Sahara was much smaller and the North Sahara area got signficant winter rainfall, much like the climate farther north in Tunisia or Greece. Since the land is relatively flat, it did not rapidly flow away, but tended to form chains of lakes. These lakes allowed much groundwater to seep into the bedrock. Eventually it pooled underground where the rock layers stopped it and is still here to the present...at least until thirsty humans pump it all out.

The second type is like the Ogalla reservoir in the High Plains. This isn't strictly a desert, but it is semi-arid and doesn't get much rain. The massive reservoir is deposited from water seeping from the many rivers coming off the Rockies. The Missouri, Plattes, Canadian, Arkansas and more rivers come down from Montana to Colorado and flow across the plains to the Mississippi. Their flow rate isn't that high, but it is enough to recharge the aquifer. So in this case, the water level is actively recharging, unlike the Nubian Sandstone.

If you are talking about a very dry desert ringed with mountains, then one of the best examples would be the Tarim basin in China. It too has a large aquifer underneath it, of the Ogalla type. Ringed by high mountains like the Kunlun and Tien Shan, spring snow melts brings water into the basin every year. The water doesn't get very far, but can still be used for irrigation at famous Silk Road outposts like Kashgar or Khotan. What does make it to the desert sands disspaears, partly through evaporation, and partly through seepage into this aquifer. On the other hand, it is worth noting the fact that this particular aquifer is only being found today because it is at depth where where you need an oil rig to get it out. So it wasn't exactly usable to the ancients, who needed to depend on meltwater.

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  • $\begingroup$ Thank you for your comment. Could underground rivers be able to extend an Ogalla type reservoir into the desert, far away from the mountains where the water is collected? $\endgroup$ – Pedro Gabriel Dec 5 '16 at 20:05
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    $\begingroup$ @PedroGabriel An Ogalla type reservoir would still exist in the desert. The rivers would just have to run into the desert it and evaporate/dissapear into the ground due to seepage. The aquifer could then spread by possibly hundreds of miles underground. This is the situation with many rivers running from Ethiopia into Somalia, or from Central African Republic north into Ouaddai and Darfur in the Sahara. I don't know if those places have aquifers, but they could. $\endgroup$ – kingledion Dec 5 '16 at 20:09
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Here are two mechanisms that can produce oasis (is oases the proper plural?):

Underground water forced to the surface at the edge of a mountain range. For example the "Palm Canyon Oasis" in southern california (tried to link, URL length seems to be an issue. You might try feeding that phrase into images.google.com or similar. A beautiful hike, if nothing else.) So, one might have a rain-shadowing mountain range, and along the base of the lee of that range, there could well be a series of precious, potable springs.

Faults can also direct groundwater to the surface (and faults can usefully change, to create/destroy scarce water sources, as quickly as plot requires.

Hot springs?? (Not all of them smell vile. Those associated with carbonate minerals {vs. sulphides} are usually quite pleasant and sought after.)

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