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Water is a renewable resource, renewed by the water cycle.

If we use water too quickly, water should get evaporated and come down again as precipitation.

So, Why is there water scarcity if water is a renewable resource?

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    $\begingroup$ Basically because is not distributed uniformly. $\endgroup$ – Santiago Mar 20 '18 at 12:50
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    $\begingroup$ In addition to Santiago's comment: if you use water, it does not necessarily evaporate directly. It can drain to become ground water and it can freeze to ice. Moreover, water might be available but be polluted (by nitrate, heavy metals, persistent organic pollutants). $\endgroup$ – daniel.neumann Mar 20 '18 at 13:48
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    $\begingroup$ But, yes, theoretically, we could build pipes and/or desalination facilities so that no human lacks water. Its the same problem like for food. Theoretically, no human on Earth has to die of starvation or thirst. $\endgroup$ – daniel.neumann Mar 20 '18 at 13:50
  • $\begingroup$ It depends what you mean by "use". If "use" is equivalent to dumping it in the ocean, that will not increase precipitation, so your available water is equal to the precipitation. $\endgroup$ – Keith McClary Mar 21 '18 at 3:59
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    $\begingroup$ @KeithMcClary most likely uses of water in our daily lives (e.g. taking a shower; washing the dishes) or in machines (e.g. Washing Machine; Refrigerator; Dishwasher) $\endgroup$ – Eevee Mar 26 '18 at 15:47
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I would summarize the main factors that can lead to water scarcity in the following points:

  1. Increase in demand: As population grows, there is more overall water consumption. The rise is enhanced by the increase in life standards. For example, many countries in Africa live with a hundredth of the US water usage per capita [source].

  2. Concentration of demand: In addition to population growth, there is population densification. Due to preferential migration to large cities or already densely populated areas.

  3. Reduction of natural reservoirs capacity:

    • Snowpack: Snow is a natural seasonal reservoir that stores water in the winter and releases it on the summer. Effectively providing water during the most needed period. Due to overall global warming "Northern Hemisphere spring snow cover has continued to decrease in extent (high confidence)." (from IPCC AR5)

    • Glaciers: Mountain glaciers can store water not only from the winter to release in summer. But from rainy years or decades to release it on the dryer years. As mountain glaciers retreat worldwide, the storage capacity is reduced, making the dry months even dryer.

  4. Reduction of reservoirs levels: Many lakes and specially groundwater reservoirs are been drain at a rate faster than the recharge rate. Therefore, they won't be able to sustain current water usage rates for too long. A classic example, as mentioned by @userLTK is the Aral sea, formerly one of the four largest lakes in the world is now reduced to a 10% of its original size (see figure below)

  5. Depletion of fossil water reservoirs: In the same fashion that a oil well can be used up and it will never "refill". Some groundwater reservoirs are just relics of past climates and do not recharge. Therefore, once used up, the will be extinguished forever. This is the case of the controversial manatiales de Silala in Bolivia or the Ogallala aquifer in the US.

  6. Climate change: Over millenia, population and specially food production have concentrated in areas with reliable water availability (along rivers for example). Climate change can redistribute rainfall and reduce availability in some areas and increase it in others. And due to the fact that is difficult and costly to relocate the population, it is likely that there will some areas with more population than what can be sustained with the available water resources.

  7. Reduced water quality: As @JanDoggen rightly pointed, it is not often the case that people drink the water straight out of rivers. Usually, filtering and cleaning is required. And as the pollution levels increase on rivers and lakes (due to industry, sewage, etc.). It is becoming more expensive and difficult to actually use the available water for agriculture and human consumption.

enter image description here A comparison of the Aral Sea in 1989 (left) and 2014 (right) [Originally from Wikipedia commons here]

Rainwater seem to us an endless resource, in the same way a few hundred years ago the forests or the whales seemed endless. But if we consider that an US citizen uses in average about 1580 $\text{m}^3$ of water a year, and that the total amount of rain that fall on land in the entire world every year is about $10^{14} \text{m}^3$ (100,000 billion $\text{m}^3$). Then, you can easily calculate that all the rain in the world (even if we could use it all), would be enough only to sustain about 70 billion US-type-humans, a number that is not that far off the 11.2 billion we are supposed to be by 2100. And consider that we should leave "some water" for other animals and wild plants too.

Then, recognizing that rainfall is not endless compared with the world's population needs. And acknowledging the fact that we don't have enough storage and/or transport capacity to use much of the rain, as it happens to falls during storms when we don't need it or in faraway places. Then it should become more clear why it might become tricky to supply water to everyone in the future. Specially for:

  • Cities in dry areas and with growing population
  • Or cities that rely in glaciers/snow to get a reliable summer water supply
  • Or cities draining lakes/groundwater faster than it recharge
  • Or cities using fossil water.
  • Or cities currently using all their renewable water resources AND likely to experience dryer climate in the future.

Among others.

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    $\begingroup$ Climate change, global warming, water scarcity, growing population, etc. scares me. I cannot imagine what we have for ourselves in the future. $\endgroup$ – Ram Keswani Mar 27 '18 at 16:18
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What I miss in the other answers is water quality.

Even is there is sufficient water available it is often of insufficient quality for the purpose you want it for:

  • We often don't drink straight out of rivers
  • We don't irrigate with salt water

etc.

So you need to spend effort to prepare it for use. This adds to the experience of shortage.

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  • $\begingroup$ Yes, that's an important point. Good addition! I'll steal it to add it to my answer if you don't mind :-) $\endgroup$ – Camilo Rada Mar 27 '18 at 15:15
  • $\begingroup$ @CamiloRada I mind - that's why I made it a separate answer instead of a comment under yours ;-) $\endgroup$ – Jan Doggen Mar 27 '18 at 17:47
  • $\begingroup$ I see. In that case what do you feel that would be more appropiate: Delete that addition to my answer? Or transform the answer into a community wiki? $\endgroup$ – Camilo Rada Mar 27 '18 at 18:30
  • $\begingroup$ Oh well, leave it. We're not going to argue here ;-) $\endgroup$ – Jan Doggen Mar 27 '18 at 20:17
  • $\begingroup$ Of course, there is no point to argue here. But I'm not sure of how to deal with answer improvements like in this case, so I posted a discussion question on meta, in case you want to have a look: earthscience.meta.stackexchange.com/questions/1637/… $\endgroup$ – Camilo Rada Mar 28 '18 at 0:14
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You have to consider the source of water for each location. Groundwater, for example, is a fairly abundant but non-renewable water source in some locations. It can take a centuries to replenish.

River water fed by mountain glaciers depends on the size of the glacier, as the glaciers shrink, meltwater decreases.

Regions that depend on rainwater or snowmelt can experience water shortages when there's a low-snow winter or drought.

Water shortage depends a lot the location and the source of that regions water.

NYCity for example depends on reservoirs upstate that are fed by rain and snow-melt, and when they started to have some shortages, which weren't uncommon during low snow years in the 80s and 90s, they began (temporarily and only during drought years) drawing water from the Hudson river, which required more filtration but it's abundant. NYC is proud of it's tap water and the Hudson water, even if just 10% of the tap water supply for a summer here or there, wasn't very popular. Could be worse, Los Angeles they drink filtered and purified sewage water. Yum.

Now that the catskill aqueduct is finished, NYC doesn't have to draw from the Hudson during low-snow years. Much of California also depends on enormous aqueducts, which are some of the largest construction projects in the US and perhaps somewhat obvious, they experience water shortages a lot.

With proper infrastructure, water shortages could be largely eliminated, but that's expensive and there's also border issues.

It's also problematic to draw water from lakes because lakes don't necessarily fill up that fast. Lakes are reservoirs, but can be slow to refill. Drawing water from the great lakes, the largest fresh water source in the world outside of the large glaciers isn't a long term solution because the Great lakes don't refill very fast. Also, if you were to draw enough water from the great lakes, that would reduce Niagara Falls' electrical generation. (which might be secondary to water needs but just throwing that out there.)

Fresh water demand has largely emptied the Aral sea. The water that kept it full has been diverted for soviet irrigation projects and the sea, since it's no longer refilling, is now nearly gone.

Bottom line is that there's a finite amount of rain water and snow runoff which is the vast majority of the renewable water supply and there's a lot of demand for it, when you consider all it's used for both naturally and by man, including filling rivers and lakes, supplying the forests and natural areas, farming demands and tap water needs. There's a huge demand. The entire picture is complicated, but because ocean water is difficult and costly to purify and because lakes and underground sources can be depleted, there is a finite amount of readily usable water which varies year to year and that hits some regions harder than others.

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    $\begingroup$ Re "with proper infrastructure", water shortages could also be eliminated if large numbers of people didn't insist on living where there isn't enough local water to supply their wants. $\endgroup$ – jamesqf Mar 27 '18 at 4:52
  • $\begingroup$ @jamesqf That's true too, but if an entire nation has a drought, which has happened, they can't up and move across the border, or settle across the border permanently. Much of the southwest was planned on higher rain and snowfall than they currently get. The 50s-70s were a wetter than normal period. They didn't know what normal was. It's not always easy to know the long-term forecast. There's also climate change. Should 1/3 of the people in the city of phoenix be asked to move or should they find other solutions. These aren't easy questions. $\endgroup$ – userLTK Mar 27 '18 at 11:21
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Due to the following reasons; our world is facing water scarcity:

  1. Climate change
  2. Unequal distribution of water
  3. population Growth
  4. Rapid urbanization
  5. Industrialization In many countries due to the climate change the snow fall changed to the rain and flood so the water goes away instead of recharging the ground water
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    $\begingroup$ This answer hits some key points but lacks explanation or references. Please give a little more detail about how each reason leads us to a world where there's a global abundance of water and yet a lot of apparent scarcities of water $\endgroup$ – cr0 Mar 26 '18 at 13:31
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    $\begingroup$ one link per point in this answer had realy elevated the quality of this answer,even a couple of links to more information had been nice. $\endgroup$ – trond hansen Mar 26 '18 at 14:37
  • $\begingroup$ @cr0 I believe the answer adds on to userLTK's answer, which provides a 10-paragraph explanation. $\endgroup$ – Eevee Mar 26 '18 at 15:42
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Most of the underlying causes of water shortages have been noted by others, but there are a few issues that need to be highlighted: the primary use of water is Agriculture. As populations advance and develop tastes for Western food, much greater pressure is put on the land to produce more. Certain crops like maize and cotton are water hungry. Even the best quality soils have only a certain water retaining capacity. Artificial fertilizers are added,usually in a greater concentration than is necessary for plant growth. Gradually the soil becomes burnt out or salinated. Any water applied to such soils will flow off the land and contaminate lakes (Lake Erie for example) or rivers. Thus large amounts of water become useless for other uses (Industry or Drinking)

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1)You have to consider the source of water for each location. Groundwater, for example, is a fairly abundant but non-renewable water source in some locations. It can take a centuries to replenish. 2)River water fed by mountain glaciers depends on the size of the glacier, as the glaciers shrink, meltwater decreases. 3)Regions that depend on rainwater or snowmelt can experience water shortages when there's a low-snow winter or drought. Even is there is sufficient water available it is often of insufficient quality for the purpose you want it for We often don't drink straight out of rivers We don't irrigate with salt water This adds to the experience of shortage.

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