Hydrological Cycle

  • The water cycle, also known as the hydrologic cycle or the H2O cycle, describes the continuous movement of water on, above and below the surface of the Earth.

  • Water goes through three different states in the water cycle. It can be a liquid (water), a gas (water vapor) or a solid (ice). These three states are interchangeable, as water can freeze into ice or evaporate into water vapor, water vapor can condense as water, and ice can melt into water. The water cycle consists of a number of steps which sees water go through each of these states.

    Evaporation: Water is found in lakes, oceans, swamps, and soil, as well as in all living creatures and plants. When heat is applied from the sun, through exertion, or by artificial means, the water molecules become excited and spread out. The loss of density is called ‘evaporation’, and it sees the water rise into the air forming clouds of water vapor. Normally, the evaporation of water occurs when the water hits boiling point, around one hundred degrees centigrade.

    Condensation: The water vapor that has risen into the sky cools significantly when it comes into contact with the cooler air found up high. The vapor becomes a cloud, which is pushed around the world by moving air currents and winds. If the water vapor cools to anything above zero degrees centigrade, it will condense as water. Essentially, the water vapor will start to condense on the surface of tiny particles of dust and dirt that rose with the vapor during the process of evaporation. These tiny droplets will start to fall into one another and merge, producing a larger droplet. When a droplet is large enough, gravity will pull it down at a rate that exceeds the updraft in the cloud, leading to the droplet falling out of the cloud and onto the ground below. This process is called ‘precipitation’, or – more commonly – rainfall.

    Precipitation: The water that has fallen as rain is absorbed into the ground through a process known as ‘infiltration’. Soil and other porous materials can absorb great deals of water this way, while rocks and other harder substances will only retain a small amount of water. When the water infiltrates soil, it will move in all directions until it either seeps into nearby streams or else sink deeper into what is known as ‘groundwater storage’.

    Runoff: After the water has fallen and the soil has become saturated, or the snow has melted, the water follows gravity and falls down any hills, mountains, or other inclines to form or join rivers. This process is known as ‘runoff’, and it is how water comes to rest in lakes and returns to the ocean. The water falls according to the incline of the place from which it is falling, and when several threads of water meet they form a stream. The direction in which the water moves is known as ‘streamflow’, and it is central to the concept of the currents within rivers and streams. These streams and rivers will run off eventually to either form lakes or rejoin the ocean, depending on their proximity to the ocean.