Water on the planet

Most of the planet's water is stored in the oceans. In them, which constitute the central and most abundant element of the hydrosphere, resides more than 97% of the global water component, about 1,348 million km3.
In addition to being the receptacle for most of the water, the oceans contain large volumes of dissolved salts (3.5% of the total).
Another considerable part of the water (2.01%) is frozen in the form of ice in the inlandsis, glaciers and cliffs. Its volume is around 27.8 million km3. These ice masses remain relatively constant, although they decrease below 2% of total water during the austral summer (when the Antarctic ice caps partially melt), and amount to just over 2% in the southern winter. In recent decades, there has been a thinning of the icy layers of the Arctic and Antarctic ice sheets and the inlands of Antarctica and Greenland as a result of the greenhouse effect.
More than 0.5% of the global water is contained in the geological formations in the form of groundwater. These constitute approximately 8 million km3. An important part of these liquids are relatively immobilized, "fossilized", and therefore, they do not circulate. Another part is recharged and discharged into surface systems.
Groundwater has varied saline contents. There are "sweets", with low salt content, there are brackish (with intermediate salt content), salt (similar to the sea) and true brine (close to the saturation point).
Just over half a thousandth (0.06%) of the total water on the planet occurs as surface water, or about 225,000 km3. As more than half of this is brackish or salty, only 0.02%, equivalent to 100,000 km3 cubic kilometers, can be classified as "freshwater".
Of all surface waters, 95% are stored in lakes, by which watercourses, rivers and streams, only contain 0.001% of the total, that is, "only" 10,000 km3
Although this proportion seems small, when measured in m3 the volume is still considerable. The total surface fresh water, without counting the ice, reaches one hundred trillion (1013) of cubic meters, equivalent to one hundred thousand trillion liters (1016). If we distributed this amount among all the inhabitants of the planet, it would correspond to each 18,000 cubic meters, that is, 18 million liters per person!
To this we must add freshwater aquifers, which are 30 times more abundant. If we include groundwater, the total per person would amount to 600,000 m3.
In abstract terms, this volume seems to be more than enough to satisfy all current needs and the near future.
However, the available quantities are much lower. In them you can not consider all the water stored, because its current indiscriminate use, would limit its future use. The water available from the environmental point of view is only renewable water.

The water equation
The water cycle on the surface of the earth is usually defined by a formula usually called the conservation law.
According to it the main water parameters would be the following:
P = precipitation
E = evaporation
Qs = superficial flow
Qg = underground flow (geological)
R = reserves
U = use in the biological cycle

For practical purposes Qs and Qg are integrated into a single term: Q and it is assumed that R and U are considered constants.
The simplified formula would be:
P = E + Q
That is, the precipitations are equal to the evaporation plus the runoff.
In other words, Q = P- E
That is to say that the runoff is equal to the rainfall minus the evaporation.
In this formula it is assumed that the infiltrated water will reappear sooner or later to join Q or to evaporate.
In fact, the integration of surface and underground water, Qs and Qg, in a single term and the elimination of the infiltration of the formula, can make lose sight of some aspects of the water exchange between aquifers and surface water bodies.
Once the waters infiltrate their underground itineraries can be very complex, and difficult to determine. Therefore, it may be convenient to include the underground flow as a separate term.
On the other hand, U is not constant. The dynamics of natural systems, and more importantly, the human inflluencias on them, leads to U change incessantly. Due to multiple factors, the vegetation cover in many basins (especially the densely populated ones) is in permanent evolution. For that reason, the term U suffers constant modifications that make the evaluations difficult.
In the same way, R (reserves) may experience decreases or increases affecting the accuracy of hydrographic calculations.

Renewable water
Each year, 496 thousand cubic kilometers of water fall on the planet, four times the total volume contained in lakes and rivers. If the rainfall were evenly distributed, its annual height would be 973 millimeters.
Only 25% of this total falls on the continents. Even with average rainfall of only 696 mm per year, Asia receives the largest share (28%) of the fallen water. South America, with less than half of the Asian area, collects 25%, due to its average higher precipitation (1,464 mm per year). The African average is similar to that of Asia and the American average is slightly lower (645 mm per year). Assuming that the water stored in the aquifers remained stable, it can be estimated that evaporation from the continents reaches 84% of rainfall in Africa, 67% in Australia and 62% in North America. In Asia and South America evaporation losses represent 60% of the water fall; in Europe, 57%. Only in Antarctica is the rate considerably lower (17%).
Limiting our calculations exclusively to the continental precipitations (and subtracting the evaporated volume that is approximately 60%) there would be more than 80 thousand cubic meters of renewable annual water available for the consumption of each person on the planet.
Per capita needs vary from place to place, but are generally less than 1 cubic meter per day per person, or about 200-350 cubic meters per year.
The foregoing shows that the availability of water for human consumption is not related to its volume, even with its renewability. Rather, as we will explain below, it depends on other factors, some of a natural nature, others of a social nature.

Distribuion and access
Despite the enormous volume of fresh water circulating through the continents annually, enough to meet the needs of humanity for centuries, many people in various parts of the world have no access to this vital liquid.
There are several reasons for this to happen. In the first place, because usable fresh water exists only in large quantities in small circumscribed areas (low river courses, large lakes and high-flow aquifers). Secondly, because the available waters are not always suitable for human consumption, sometimes due to natural causes, but more frequently as a result of anthropogenic degradation. Third, because not all water resources are renewed at a sufficiently appropriate rate for long-term use. Finally, the demand for water is concentrated in a few densely populated areas that do not necessarily coincide with the places of greatest availability.
In summary, waters of good quality and in sufficient quantity to be used to meet the needs of the population and production, are not easily found. Water availability is one of the main limiting factors for demographic and economic growth.
From the book "Drought in a world of water", D.Antón, Piriguazú Ediciones