Understanding the Water Cycle

To understand ground water, we need to understand the water cycle. Both ground water and surface water are connected and so what happens to one can affect the other.

Water never leaves the Earth. It is constantly being cycled through the atmosphere, oceans and lands in liquid, gas or solid forms. This process, known as the water cycle, is driven by energy from the sun. The water cycle is crucial to the existence of life on our planet.

Water cycle is an endless process of movement of water around our planet

The Water Cycle

The water cycle is also called the hydrologic cycle. In the water cycle, water from oceans, lakes, swamps, rivers, plants and even from people and animals, is converted into water vapours and released into the atmosphere. Water vapours condense into millions of tiny droplets that form clouds. Clouds lose their water as rain or snow, that either infiltrates into the ground or runs off into rivers and lakes or escapes into the atmosphere. The water that infiltrates into the ground is either taken up by plants or moves deeper below the ground eventually replenishing the ground water. Plants suck up moisture from the soil and lose water from their leaves in the form of water vapours, a process called transpiration, that transfers water back into the atmosphere. Some of the water that runs off into rivers, flows into ponds, lakes, or oceans also evaporates back into the atmosphere. The cycle continues!

Water that enters and percolates through the soil is very important as it recharges the ground water. We know that ground water resources in India are under serious threat from over-use and thus it is very important to replenish the ground water through recharge. Some of the ground water is discharged into streams and that is why we continue to see flows in streams and rivers long after the rains (base-flow). Ground water moves through rocks and sand from one region to another. Therefore, when one draws water from a well, the water from adjoining areas moves into maintain the water level. This also means that ground water pumping draws ground water from long distances from beneath someone else’s land.

Water storage in ice and snow : Some of the precipitation is stored in the form of ice and snow, such as in Antarctica where about 90% of the total ice on Earth is currently stored. Most of the remaining 10% is in the Greenland ice cap. 

A smaller amount is present at high elevations, e.g. in mountain ranges such as the Himalayas, the Andes and the Rocky Mountains. Accumulation of snow leads to compaction and formation of ice, which gradually becomes so heavy that it starts to move slowly. This mass of moving ice is called a glacier. The Greenland ice cap contributes to the global water cycle (estimated to be 517 cubic km every year) as some of the Greenland glaciers reach to the sea and icebergs break off the glacier, move with currents in the ocean, and melt. Climate change is expected to cause faster melting of glaciers and consequently a rise in sea levels.

Water storage underground : Most of the available freshwater is stored underground as ground water. Water can remain stored underground for a long time, which means the ground water can serve as a long-term resource as well as a buffer against major fluctuations in the water cycle (e.g. drought). However, for its sustainability we need to make sure we do not over-exploit it. The following table gives an idea of the time scales associated with the residence time in different water storages. 

We can store water underground for hundreds of years.

Know the key terms of the water cycle

Condensation: Water vapours condense on tiny dust, salt or smoke particles and form droplets. In this way, water vapours form clouds - this is called condensation.

Precipitation: Following condensation, the droplets grow in size. When the water droplets in the clouds get too heavy, they fall back on to the Earth - called precipitation. This includes rain, snow and hail, but most precipitation is in the form of rain.

Evaporation: Heat energy from the sun causes water in puddles, streams, rivers, seas or lakes to change from a liquid to a water vapour form called evaporation. The vapours rise into the air and gathers in clouds.

Transpiration: Transpiration is the process by which plants lose water through their stomata present in leaves. Stomata are tiny pores found in the epidermis of leaves and stems. Surrounded by a pair of guard cells they open and close depending on plant’s need for gas exchange. Transpiration gives evaporation a bit of a hand in getting the water vapour back up into the air. It also helps plants to grow, absorb carbon dioxide and to release oxygen.

Infiltration and percolation: The process where water on the surface enters into the soil (infiltration) and moves deeper into subsurface (percolation) and in time to the ground water.

Runoff: When rain falls on the land, some of the water infiltrates into the ground, while most of the remaining water runs off on the land surface and into nearby streams or rivers. This water is called runoff. Sometimes large volumes of runoff water during heavy rains results in a flood.

The water cycle can be altered by way of a change in the land use due to urbanisation, mining and clearing of forests. The rise in temperature due to greenhouse gases (climate change) can significantly influence the water cycle. For example, in recent years many areas have experienced greater incidences of droughts and floods due to changes in rainfall patterns.

How to measure rainfall?

Rainfall data are very useful for making a lot of decisions. It helps farmers deciding which crops to grow and engineers in designing dams and bridges. Good rainfall data from local areas are often hard to find or are unreliable. Therefore, it is a good idea to measure and record your own rainfall daily. This way you can also keep track of rainfall with time.

Instruments

The standard instrument for the measurement of rainfall is the 203 mm (8 inch) rain gauge. The rain gauge is made of a circular funnel with a diameter of 203 mm which collects the rain into a graduated and calibrated cylinder. This can measure up to 25 mm of precipitation at a time.

In modern automatic weather stations a Tipping Bucket Rain Gauge is employed. There are two advantages of this type of rain gauge. Firstly, it never needs to be emptied, and secondly the amount of rainfall (and even the rate at which the rain is falling) can be recorded automatically. An electronic pulse is generated each time the volume of water collected in one of the small buckets causes the bucket to tip. This is equivalent to 0.2 mm of precipitation. The time and number of tips are then recorded electronically.

Where and how to install a rain gauge?

Gauges sited near buildings, fences and trees do not give accurate measurements. The distance of the gauge from buildings, trees or other objects should be at least twice the height of the obstruction (h), and preferably four times the height. For example, the gauge should be installed more than 10 m away from a 5 m high building.

The top surface of the gauge should be horizontal and chest high; the grass and vegetation around it should be less than a knee high.

Fasten it securely to a post or something solid so that it does not blow over in strong winds and storms.

How to read a rain gauge?

Read every day at the same time, as near to 9 am as possible. During heavy rains it may be necessary to read and empty the gauge frequently to prevent it from overflowing. Add this amount to next 9 am reading.

To read the contents of the rain gauge, first ensure that the gauge is vertical. Bring your eye level with the surface of the liquid in the gauge and read from the scale the position of the liquid surface.

Make sure you read the bottom of the liquid surface and not the meniscus, which is the slightly higher lip formed where the water surface meets the cylinder wall.

How to record rainfall?

Keeping a proper record of rainfall is as important as the appropriate installation of a rain gauge and regular reading of rainfall. Rainfall is an important indicator of water availability in a village or a town. Long-term records help in developing a water budget for a village. Meaning how much water is available, how much can be harvested in check dams and then to recharge ground water. It helps in calculating the size of recharge structures needed and the resulting ground water recharge. Rainfall amounts should be carefully recorded in a notebook and an example of a Table is shown in the Activities section of this module.

Human influence on the water cycle

Since water is central to life, virtually all human activities have some impact on the water cycle. Whether it is to do with using land to build homes or to grow food or undertake industrial activities to manufacture goods for our daily use, we affect the water cycle through all of these activities. Some key human activities that have major impact on the water cycle are discussed next.

By 2050, there will be more than 9.8 billion people living on Earth

Population growth and deforestation

Human population on the Earth has grown dramatically in the last 100 years, nowhere more so than in India. The world population increased from about I.5 billion 100 years ago to more than 7 billion people in 2016. This unprecedented growth of population resulted in clearing of many forest areas to release land for agriculture, industry and habitation. It is estimated that every year more than 10 million hectares of forests are either cleared or destroyed by fire. As trees transpire water from land into the atmosphere, the large-scale deforestation means there is less total water that is being sent back into the atmosphere and more water running off or infiltrating, thus affecting the water cycle. 

Urbanisation and industrialisation

The world’s population is not only growing rapidly but also becoming rapidly urbanised, as people migrate from rural to urban centres. Small towns are becoming like cities and need facilities like safe water supply and sewerage systems. As urban centres rapidly encroach on land that previously supported natural vegetation that contributed to transpiration, thus the urbanisation influences the water cycle. Cities increase demand on water resources and generate wastewater that needs to be managed. For example, the water falling on roofs, roads and hard surfaces (pavements) runs off rapidly without infiltrating into the soil and recharging ground water. 

Many industrial processes require high volumes of water and release polluting chemicals. The industrial process of making the cement that we use for construction of buildings and infrastructures (a basic need for urbanisation) is a simple example of a carbon dioxide (CO₂) emitting activity. Let us consider the impact of these gases on our water cycle. 

Release of greenhouse gases and climate change

Human activities such as power generation using fossil fuels (e.g. coal, petroleum, natural gas), deforestation, industrial activities and agriculture release certain gases into the atmosphere such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). These gases trap heat in the atmosphere and are therefore called "greenhouse gases". Many other industrial activities release greenhouse gases such as hydrofluorocarbons, perfluorocarbons, sulphur hexafluoride, and nitrogen trifluoride. These gases are typically emitted in much smaller quantities than CO₂, but they are more powerful in terms of global warming.

Climate Change has a major effect on the water cycle

The build-up of greenhouse gases in the atmosphere has resulted in “climate change”, that is the Earth is progressively becoming warmer. It is predicted that at least a 2^oC change in Earth’s average temperature is almost unavoidable now. “Global Warming” has a major effect on the water cycle in terms of distribution and amounts of rainfall.

With increased temperatures, the glaciers are melting away and thus affecting the distribution of water on land. In addition, as temperature increases more water is evaporated in the form of water vapours. Water vapours also add to further warming, through a similar effect as greenhouse gases. The effect of climate change on the water cycle may lead to uneven distribution of rains resulting in extreme weather events such as draughts, floods and cyclones.

Cleaner source of energy such as solar and wind are needed to minimise greenhouse gases

CO₂ makes the bulk of the total greenhouse gas emissions and therefore there is a global effort to reduce its emission. India has set a goal to reduce its carbon emission (CO₂) by 35% by 2030 (compared to 2005). Use of fossil fuel for energy production is a major contributor to the carbon emission and therefore alternative (cleaner) sources of energy production, such as solar and wind power are needed. India has initiated major solar power programs to generate cleaner electricity in coming years. Plants absorb CO₂ from atmosphere and store (sequester) it on land and therefore planting trees helps mitigate climate change.

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Source : Jaldoot Resource Book