Water is a Renewable Resource because Air Exists!

The Earth's atmosphere is key to the Earth's water cycle. Without the atmosphere, Nature's  engine of delivery of a constant supply of water safe for drinking and irrigation all over the world, could not exist or function as it does and has for a very very long time.
The Atmosphere

Water Exists in Atmosphere
Source - op.gfz-potsdam.edu

All the water in the atmosphere is concentrated in the atmosphere near to and touching the ground.
It is from this atmosphere that water precipitates in its many forms - rain, snow, hail etc - down to the ground, with assistance from a combination of forces that include gravity, temperature variations, variations in the content of particles and bacteria in the atmosphere, etc.
Without the atmosphere, water would not move around as smoothly as it does today. And, maybe, it's renewable ability would be at risk if the atmosphere disappears or reduces in volume.
How much atmosphere is there?

Total volume of atmosphere
and all Water (safe and unsafe)
Source - Science Photo Library

Actually, not that much more than water!
If we represent the total volumes of water (1.4 billion cubic kilometers) and air as spheres:
- The water sphere would be 1,390 kilometers in diameter. Of course the safe water part of this sphere is miniscule.
- The air sphere would be 1,999 kilometers across.
Half of the air lies in the first 5 kilometers above ground level which contains 90% of the miniscule safe water in the atmosphere.
Running out of safe water
The key question that arises on running out of safe water is: What is the probability that we might run out of air?

Lot's of Freshwater, But NOT WHERE It's Needed

Precipitation, commonly called rain, the predominant source of freshwater, is more than enough for a 10+ billion human population plus the other species and nature's ecosystems that must have freshwater.
Rainfall

Source - countryofsb.org

From the very beginning, rain has supplied the freshwater needs of human, other species and nature's ecosystems in two ways: 
- Either, by dropping manna-from-the-heavens directly onto crops and where the freshwater was needed and/or 
- Replenishing lakes and other bodies tapped by humans, other freshwater species and nature's freshwater ecosystems.
Total Rainfall on land

Source - sciencemag.org

A total of 98,500 cubic kilometers of rain is estimated to fall on land every year.
This liquid precipitation is in addition to the 12,500 cubic kilometers that fall as ice and snow on land.
Ignoring for a minute that this rainfall in not geographically uniform and that it has extreme spacial and temporal variations, the 98,500 cubic kilometers/year
= 270 cubic kilometers/day
As 1 cubic kilometer = 264 billion US gallons,
the average rainfall of 270 cubic kilometers/day
= 71,280 billion US gallons/day
For a 10 billion human population (the projected human population in 2050), this rainfall
= 7,128 US gallons per day.
Why Freshwater Scarcity?
Ignoring freshwater accessed over long distances from surface bodies (lakes, rivers, etc) and underground aquifers, with so much rainfall available per person per day, why do we have scarcity anywhere in the world?
The answer: Scarcity is the result of rainfall not being uniform all over Earth's land mass and the unpredictable spacial and temporal variations of rainfall.

Every Water Is Not The Same!

We all are quite familiar with the salty taste of ocean water compared to water deposited by rain and water streaming out of a faucet. Another difference may help us tap on-demand, the oldest freshwater source on Earth.
The Ocean in the Atmosphere
The hydrologic cycle maintains and continuously replenishes a vast ocean of freshwater in the air closest to the ground. This ocean is the source of all rain and snow, the dew we find deposited on our cars, dew deposited on the ground early in the morning and the moisture that appears on the outside of a cold glass. 
Isotope Hydrology

Change in isotope content during the hydrologic cycle
Source - ANTSO.gov.au

It is an established fact that water from different sources contains different number and types of isotopes.(see post dated Oct 15, 2011).
Isotope Hydrology is the new science that uses existence of isotopes to estimate age and origin of water in each stage of the hydrologic cycle.
What makes Isotope Hydrology most interesting is that it can map a "local" hydrologic condition to include an identification of the the origin and content of water in the local atmosphere.

Predicting & Catalyzing Precipitation
The primary cause of freshwater scarcity is continued lack of rain where and when we humans want and need it. Precipitation (mostly rain) directly waters our food crops, adds runoff to increase river flows and lake levels, and replenishes ground water reservoirs.
We could soon produce rain on-demand using Isotope Hydrology that can accurately measure the isotopic content of local air, compare it to conditions that produce rain and identify what changes are necessary to produce rain on demand anywhere on Earth!

Water Resources impacted by Climate Change?

Implications of climate change on the Earth's Hydrologic Cycle are slowly coming into focus but the underlying phenomenon are still not well understood. Any assessment of climate change impact on water supplies are, thus, only our attempt to err on the 'safe' side for humanity and nature's ecosystems.


The Greenhouse Effect

Source - ucar.edu

If the greenhouse effect can be attributed to a single event, this event happened in 1988. Scientists, seeking to explain a series of unusually warm years, were able to link for the first time in 1988, the increase in atmospheric temperature to the retention of long-wave radiation (received in sunlight) in the atmosphere. Further research linked this increase in retention to an increase in greenhouse gasses that include Carbon dioxide, water vapor, nitrous oxide, methane and chlorofluorocarbons.
(As addition of carbon dioxide fuels increase in all the greenhouse gases, we are developing ways to reduce human carbon dioxide emissions)
Predicting Climate
We all know too well how difficult it still is for us to predict tomorrow's weather. 
Our difficulty is simply due to our lack of understanding on how the vast number of processes and their feedback cycles, come together to make the weather we experience.
Climate Change
As we don't really know how to predict the impacts of climate change, we have developed the approach of trying to understand (and model) historical data on temperature rises and the amount of greenhouse gases in the atmosphere. So, using the historical record as a 'given' we are measuring the increase in amount of greenhouse gases to assess what climate we should expect
Pinpointing Impacts of Climate Change
Climate change is expected to cause rapid changes in temperature, precipitation and concentrations of greenhouse gases (water and Carbon Dioxide). To predict the direction of these changes (i.e. will the result be a flood or a drought?) we make models to represent nature's complex ecosystems. The conclusions we reach are only as good (i.e. as closely representing the functioning of the ecosystem) as the models we can make.
Keeping in mind that our conclusions on the impact of climate change on water resources is only as good as the models we make, 

Rainfall is THE Concern

Source - loadstorm.com

Numerous computer model-based assessments have raised concerns of BIG changes in the amounts of rainfall.

• In high latitudes areas (in equatorial Africa and Asia, in Southeast Asia) annual rainfall is projected to dramatically increase, while in mid-latitudes and in most subtropical regions rainfall is expected to decrease. 
• A general reduction in precipitation of snow is expected to reduce the amount and duration of snow cover

Additional implications of reduced or unanticipated rainfall can also be postulated:

• If we pollute more and more water, less rainfall implies that the concentration of pollutants in our water supply will remain higher longer
• As we use an enormous volume of water to generate electricity and cool thermal power plants, any reduction in rainfall will only increase pressure on water supplies
• Current levels of irrigated food production will have inadequate amounts of irrigation water (for example, in the dry US West) leading to reduction in food supplies - some studies predict a drop in USfood production by 30% for a 3 Degree Centigrate increase in air temperature.
• And ..........

Maybe, the solution is to develop ways to extract both water vapor and carbon dioxide from the atmosphere!!

Water Defines Climate Everwhere

Water in the troposphere, the atmosphere closest to the surface, conditions the Earth. In some locations water runs the local heating system while, at the same time, it acts as the cooling system in other locations. Water has an inordinately strong effect on climate. 


What is climate?

Climate Map
Source - blueplanetbiomes.org

Climate is weather averaged over a long period of time, usually, 30 years.
In this averaging, weather is defined by many variables. The predominant variables are temperature, precipitation and wind movement.


Temperature

Air Temperature Variations
Source - griffin.uga.edu

Temperature is a measure of the 'heat' in any location.
This 'heat' is the sum of the amount of energy deposited on a place by sunlight and the amount of energy existing on Earth at that place.
Water in the atmosphere and in surface bodies acts as the controller of heat distribution around the planet.
Water in the wind moves heat from one location to another.
Ice sheets and clouds cool the Earth's surface by reflecting the heat energy back into space.
Ocean and river currents transport heat to locations far and near.


Precipitation

Source - valdosta.edu

Precipitation IS water - in every form that water exists!









Water's Venetian Blind effect

Source - b4i.ie

Water in the air (with other greenhouse gasses like carbon dioxide) acts in much the same way as slats in a Venetian blind. 
The angle of each slat influences how much of the suns' heat and light enters a room. We can move the slats to form a 'closed' surface to, in effect, stop most light and heat from entering a room. We can fully 'open' the blinds to allow entry of the most amount of light and heat into a room.
In a similar way, nature manipulates the water content of air at individual locations (in conjunction with other parameters) to influence (raise and lower) in a very large way the temperature at a particular location.