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The Dead Sea, rivers in California and the Aral Sea are said to be shrinking due to water usage, e.g. for agriculture. Yet the water must go somewhere.

After the fields are irrigated, the water either soaks into the ground or evaporates. In the first case, the groundwater is replenished. In the second case, the water ends up as rain, presumably within a few hundred kilometers of the evaporation point. Either way, the water gets back in circulation.

So what is happening here? How can water be "used up"?

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    $\begingroup$ In addition the what has been stated in the answers, water gets exported out of a region as moisture in produce. Water melon, cucumber or any fruit or vegetable, even meat from livestock is sent to consumers in other parts of the country & around the world via agricultural trade. $\endgroup$ – Fred Jan 8 at 13:25
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    $\begingroup$ @Fred I think you're talking about virtual water? This is the idea that agricultural irrigation is lost from the local system. One example I recall is that the cotton required for a single pair of jeans requires about 1000 $m^3$ of irrigation! However, this water isn't physically removed by exporting the goods. Rather, it is lost by evaporation during the life-cycle of the crop plant and is circulated away by the atmosphere. As others have mentioned, it doesn't precipitate back down anywhere near. $\endgroup$ – Oscar Bravo Jan 8 at 13:57
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    $\begingroup$ @OscarBravo: The water is no longer in the resulting pair of jeans. But the fruits and meat still contain water, which does get transported. You are correct that even more water is used to make the fruits and meat (just like for cotton), but Fred was focusing on the water that is still in the fruit and meat at the time it is exported. $\endgroup$ – Flater Jan 8 at 14:27
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    $\begingroup$ @OscarBravo actually even dry cotton contains some water - or rather the cellulose that makes up the cotton is made from water+CO2 $\endgroup$ – Chris H Jan 8 at 15:43
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    $\begingroup$ Southern California is a huge non-agriculture consumer of water. One of the reasons rivers in California are 'shrinking' is due to diverting them for household use. $\endgroup$ – CramerTV Jan 8 at 17:23
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You're making a mistake, at least for the second case:

In the second case, the water ends up as rain, presumably within a few hundred kilometers of the evaporation point.

You cannot model a dry region (or indeed any region on earth) as a closed system for hydrological purposes. When water evaporates in a dry climate, it transports much farther than a few hundred kilometres. General circulation can transport airmasses for thousands of kilometres. In all likelihood, when the water finally precipitates it will do so in a different catchment area and/or far upstream, often in an area that already has plenty of precipitation. From there it may flow thousands of kms to different climate areas yet. This is why in particular hydro lakes in hot climates have such a large impact on ecology: a hydro lake is far larger evaporation than a river, due to its much larger surface area and other factors. When water from a dry region is gone, it can, for all practical purposes, be counted as a loss.

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This is a really complex problem and would require a really detailed explanation about atmospheric circulation, meteorology and hydrology.

The short answer to your question is that water is going somewhere else. If you look at the studies about moisture recycling, among the others van der Ent et al 2014 or 2010, you can see that the precipitation that generates from locally generated moisture is just a minor fraction, and usually is higher in very wet areas. So, your assumption of the "few hundreds kilometers" is generally wrong. Considering a very wet area like the Amazon forest, you can see that the moisture locally generated is the major source (about 70% in quantity) of the precipitations in South-Eastern Brazil. This is due to the atmospheric circulation and the topography of the continent.

enter image description here

Source van der Ent et al 2010.

To fully answer your question, both in case of precipitation and in the case of underground percolation, the water used locally in agriculture or other uses, would go to contribute to other drainage basins (both in the cases of surface water and underground aquifers).

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    $\begingroup$ In the case of one California-adjacent river, the Colorado River, the Colorado River Aqueduct diverts a volume of river water 242 miles to Los Angeles and other Southern California destinations -- across a desert and through a mountain range, to an area where used water would flow into the ocean, if anywhere. This and other diversions out of the watershed reduce the river's flow to a relative trickle once it reaches the ocean. In the case of Southern California's Salton Sea, that briny lake is shrinking because there's not enough leftover farmwater draining into it. $\endgroup$ – jeffronicus Jan 8 at 16:07
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    $\begingroup$ @jeffronicus, As the question did not refer to this specific issue, I did not take at all into consideration water transfers or other infrastructural diversions of water flow. If I well understand, the question just refers to local irrigation practices (surrounding the river or the water body, I guess). $\endgroup$ – Nemesi Jan 8 at 17:04
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    $\begingroup$ In the case of Colorado, there are many inter-basin transfers that contribute to dry up the dowstram part of the river. See here for example riversimulator.org/Resources/Graphs/… $\endgroup$ – Nemesi Jan 8 at 17:11
  • $\begingroup$ @jeffronicus, ok, water is diverted. But the water does not disappear, it is not "used up." Every molecule continues to exist. But I guess you could say that LA gets wetter as the watershed of the Colorado river gets drier. $\endgroup$ – Joshua Fox Jan 8 at 19:41
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    $\begingroup$ @JoshuaFox Yes, if you consider the entire planet, then the hydrological cycle can be practically considered as a closed system, at least on short (sub-billion year) time scales. $\endgroup$ – gerrit Jan 9 at 10:39
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Most of the world's land is more than one hundred kilometers from the nearest ocean, yet still gets rain. There are portions of the Rockies that are about a thousand kilometers from the nearest ocean. And since clearly a large portion of the precipitation that lands in the Rockies flows to the ocean, there must be some mechanism taking water back to the Rockies from the ocean.

Furthermore, "several hundred kilometers" is a rather large distance. The Dead Sea is only about 50 km long and 15 km wide with an area of 605 km^2; if you draw a circle of radius 100 km, the Dead Sea would make up about 2% of that region. 100 km from the Dead Sea gets you to the Mediterranean, and 200 km gets you to Egypt, Saudi Arabia, Syria, or Lebanon. So even if water were to stay within a few hundred kilometers, it would be unlikely to make its way back to the Dead Sea.

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    $\begingroup$ Also note that much of the land between the Rockies and the Sierra Nevada/Cascade ranges is desert or semi-desert. And a good bit of what falls on the west side of the range winds up in the Great Basin or the closed basin in Wyoming. $\endgroup$ – jamesqf Jan 8 at 19:10
  • $\begingroup$ I don't think the way you approach the problem is accurate. It depends on the hydrology of the area. If you consider the Nile, for instance, the majority of the water that arrives at its delta comes from the precipitation in the Ethiopian highlands, another portion from Lake Victoria, even further away. Moreover, although oceans are the main supplier of air moisture, they are not the only ones and you cannot neglect the role of the forested areas that often have the function of "water pumps". $\endgroup$ – Nemesi Jan 11 at 9:15
  • $\begingroup$ The problem tackled in the question here, is related to the dispersion of water from a basin. Assume for instance, to have river flowing parallel to the Nile, but in the opposite direction, southwards. let's call it South-Nile. Now, if you irrigate a plot that is on the edge of the two hydrological basins, even assuming that all the water used for irrigation would flow back to a river (so, no dispersion), you would have some of the runoff to flow on the Nile, and some to the South-Nile. This would have consequences on the final amount of water that would flow on the Mediterranean sea. $\endgroup$ – Nemesi Jan 11 at 9:19
  • $\begingroup$ @Nemesi I'm not clear on what your point is. $\endgroup$ – Acccumulation Jan 11 at 16:27

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