Chapter 5: Water

Water Cycle

Water is a dynamic substance that is constantly in motion. The process by which water continuously changes its form and circulates between the Earth's oceans, atmosphere, and land is known as the water cycle. Our planet functions like a giant terrarium, a closed system where the total amount of water remains the same. The very water that exists today has been on Earth for centuries, cycling through different forms and locations.

The water cycle involves four main stages:

1. Evaporation: The sun's heat warms the water in oceans, rivers, and lakes, causing it to turn into water vapour and rise into the air.
2. Condensation: As the warm, moist air rises, it cools down. The water vapour then condenses into tiny water droplets or ice crystals, which come together to form clouds.
3. Precipitation: When these water droplets in the clouds become too heavy to remain suspended in the air, they fall back to Earth in the form of rain, snow, sleet, or hail.
4. Collection: The water that falls back to Earth is collected in oceans, lakes, and rivers (a process called runoff), or it seeps into the ground to become groundwater. This water then evaporates again, continuing the cycle.

Distribution Of Water Bodies

Although about three-fourths (71%) of the Earth's surface is covered by water, most of it is not usable for drinking or agriculture. This paradox of a water-rich planet facing water scarcity is due to the distribution between saline and fresh water.

• Saline Water: The vast majority of Earth's water (over 97%) is found in oceans and seas. This water is salty, or saline, because it contains large amounts of dissolved salts, primarily sodium chloride (common table salt).
• Fresh Water: Only a small fraction (less than 3%) of the Earth's water is fresh water. The major sources of fresh water are rivers, ponds, springs, and glaciers.

Salinity is the measure of the amount of salt (in grams) dissolved in 1000 grams (1 kg) of water. The average salinity of the oceans is about 35 parts per thousand. The Dead Sea in Israel is an example of extreme salinity (340 parts per thousand), which makes the water so dense that people can easily float in it.

The distribution of Earth's water highlights how precious and limited our freshwater resources are:

This distribution shows that most of the Earth's fresh water is locked up in ice caps and glaciers, making it inaccessible. This makes the conservation of usable fresh water critically important. A significant day for promoting this cause is World Water Day, celebrated annually on March 22.

Ocean Circulation

Unlike the still waters of a pond, ocean water is in constant motion. The movements of ocean water can be broadly classified into three categories: waves, tides, and currents.

Waves

Waves are the rhythmic, alternating rise and fall of water on the surface of the ocean. They are primarily created when winds scrape across the ocean surface. The stronger the wind, the larger and more powerful the wave becomes.

While most waves are caused by wind, massive waves can be generated by underwater disturbances. An earthquake, a volcanic eruption, or an underwater landslide can displace a huge amount of ocean water, resulting in a series of extremely large and destructive waves called a tsunami (a Japanese word meaning "harbour wave"). Tsunamis can travel at speeds of over 700 km per hour and cause catastrophic damage to coastal areas.

Tides

Tides are the predictable, rhythmic rise and fall of ocean water that typically occurs twice a day. High tide is when the water level rises to its highest point, covering much of the shore. Low tide is when the water recedes to its lowest level.

Tides are caused by the strong gravitational pull exerted by the moon and the sun on Earth's oceans. The moon, being much closer to Earth, has a stronger influence than the sun.

• Spring Tides: These are the highest tides, occurring during the full moon and new moon phases. At these times, the sun, moon, and Earth are aligned in a straight line, and their combined gravitational pull creates stronger tides.
• Neap Tides: These are weaker tides with the least difference between high and low tide. They occur when the moon is in its first and last quarter. During these phases, the gravitational pull of the sun and moon are at right angles to each other, partially cancelling each other out.

Tides are very important for navigation, as high tides allow large ships to enter shallow harbours. They also help in fishing by bringing more fish closer to the shore.

Ocean Currents

Ocean currents are large streams of water that flow continuously in definite paths on the ocean surface. They act like massive rivers within the ocean and play a crucial role in regulating global climate by distributing heat around the planet.

Ocean currents can be classified as warm or cold:

• Warm Currents: Generally, these currents originate near the equator and flow towards the poles. They bring warm water into colder regions, raising the temperature of coastal areas. The Gulf Stream is a well-known warm current.
• Cold Currents: These currents carry cold water from polar or higher latitudes towards the equator. They cool the coastal areas they flow past. The Labrador Current is an example of a cold current.

The meeting point of warm and cold currents creates some of the world's best fishing grounds, as the mixing of waters brings up nutrients that support a rich marine ecosystem. The seas around Japan and the eastern coast of North America are prime examples. However, these areas also tend to experience foggy weather, which can be hazardous for navigation.