The title of this article paraphrases the famous sentence of French chemist Antoine Lavoisier in his “Law of Conservation of Mass.” He sustained that mass is neither created nor destroyed in chemical reactions. And something similar happens with water. This process of transformation is known as the Hydrological Cycle.

Hydrological Cycle

Water is a substance whose molecule contains two hydrogen and one oxygen (H2O) atoms. It has three stages: solid, liquid and gaseous. Water is essential for the origin and survival of most life forms on our planet. During this process it experiences numerous transformations and is constantly changing.

Solar energy makes water evaporate. Once it is condensed, gravity leads to precipitation whether it is in the form of rain, snow or hail. This creates a continuous movement: The Hydrological Cycle.

Storage and Flows

The Hydrological Cycle has two main processes: the first one is water storage. The second one is flowing from one storage to another. Some examples of storage in liquid form are oceans, lakes, aquifers, clouds—yes, clouds! Although most people think they’re made of water vapor, they actually contain many microdroplets in the liquid state. In the solid form, glaciers and seasonal snow collect water. Finally, in the gaseous form, the atmosphere stores water.

The following percentages represent the distribution of water in the different storage forms:

We know there is a large amount of water on the planet. However, only a small percentage of water supports the vast majority of species and provides biodiversity.


Water distribution

On the other hand, flows represent how water moves from one storage cycle to another. Some important examples are:

  • Condensation: when water vapor rises to the atmosphere it cools down and becomes liquid through condensation.
  • Precipitation: through condensation, the water vapor turns into microdroplets and these, in turn, become clouds. From there, water can precipitate in liquid or solid form.
  • Evaporation and evapotranspiration: liquid water stored on large surfaces tends to become vapor by the action of solar energy. Living beings produce water vapor when breathing, a phenomenon called evapotranspiration.
  • Infiltration: the water that falls on earth tends to infiltrate the soil. Plants absorb a portion, another portion is evaporated, and another portion goes to the aquifers due to percolation.
  • Runoff: once the ground is saturated, the water tends to slide through the ground due to the effect of gravity, forming rivers.
  • Melting: the melting of seasonal snow produces a flow of water.

The average time that water spends in these storage cycles is called residence time. It varies from a few days to hundreds of years.


Residence Times:

Only a small percentage of water supports the vast majority of species

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Water Balance

Although around the world the system is conventional, depending on the scale of the subsystems, they can yield or capture water. In general, the water balance in a given moment takes into consideration the previous state, and the input amount, minus the yield in the system during the period. It is similar to what happens with money: in order to know the balance of a given account, at a specific moment; you have to know the previous balance and the expenses and income that took place during that period of time.

Water Quality

According to this scenario, how is it possible to have water shortage problems? Part of the answer is that only a small percentage of the available water is fit to support life. During the flow processes, it transports dissolved substances or in the form of suspended solids, which may cause it to be unsuitable for consumption.

Furthermore, water quality depends in part on natural processes. These are produced, for example, by the erosion of minerals in soil, the transport of organic matter, the evapotranspiration of living beings, the physical, chemical and biological processes in the aquatic environment, etc.

The quality of this scarce resource also depends largely on human behavior. For this reason, we must protect it. It is everyone’s responsibility.