Water cycle

What would we see if we followed a drop of water for a day, a year, or a decade?

How many phase transitions would happen?

How much time would it spend at the same sphere?

Which natural processes would it take part in?

To answer these questions, let’s see the water cycle in this post.

Water cycle

Source: https://www.sciencelearn.org.nz/resources/721-the-water-cycle

The Earth is a closed hydrological system which means that there’s a constant amount of water on the planet for millions of years. However, water is always in motion. It moves in different phases between the 3 large reservoirs of the Earth: atmosphere, lithosphere, and hydrosphere. This one-way periodic circulation is the system of water cycle. The circulation is generated and sustained by the energy from the Sun and it’s based on the principle of minimum energy.

In the 3 spheres water is present in different states for different periods of time. The least water, less than 0.01% of the total amount, is stored in the atmosphere mostly for 8-10 days. Water can be found here in the form of vapour or in the clouds as ice crystals, snowflakes, or water drops which can be colder than 0°C due to the law pressure at high altitudes.

Water also exists in the lithosphere, typically in three forms: as groundwater moving by gravity, as water bound to the walls of the pores, and as water of crystallization. Groundwater means only 0.61% of the total amount. The residence time here is various due to different groundwater flow systems and different rocks, sediments. It can be only a week but water can stay in the lithosphere for more than 10,000 years or more as well.

The most significant sphere in the water cycle is the hydrosphere. It contains every surface area covered by water, snow, or ice. This means more than 99% of the water on Earth. As a remarkable part of hydrosphere, seas and oceans store about 97% of the total amount with an average residence time of 4000 years. We can also mention here ice caps and glaciers where water can stay 10-10,000 years long. Besides that, surface freshwaters also belong to the hydrosphere but they store only 0.05% water of the planet for 1 week – 10 years.

To describe the amount of stored water and its change in different spheres, hydrological basins, or even countries, the hydrological equation is used. It refers to a given volume and period of time and based on the principle of mass conservation. The equation is the following:

Inflow = Outflow +/- Change in storage

Counting inflow and outflow can help to develop sustainable water management and to understand hydrological conditions. But how can water move between the spheres? Read our next post to find it out!