Water: A Precious Resource Class 7 Notes Science

Water is a natural resource essential for life on Earth. Though abundant, only a small fraction of water is fit for human consumption. Water exists in three states: solid, liquid, and gas.

Forms of Water

Solid water: glaciers, icebergs, snow, hail, frost, and ice crystals in the clouds.
Liquid water: rain, dew, clouds, lakes, rivers, and oceans.
Gaseous water: water vapor, fog, steam, and clouds.
Water can change its state between 0°C and 100°C and forms the water cycle.
Water Cycle
Continuous circulation of water in nature called the water cycle or hydrological cycle. Water evaporates from the surface, forms clouds, and returns to the surface as precipitation.

Uses of Water

Agricultural use: Nearly 60% of the Indian population depends on agriculture and uses almost 90% of water for irrigation.
Domestic use: Water required for household purposes such as drinking, cooking, bathing, and washing clothes.
Industrial use: Industry uses water at all levels of production as a raw material, solvent, coolant, transport agent, and for generating electricity.
Municipal water treatment plants provide treated water for cities, while rural areas rely on underground sources. Lack of water can lead to reduced yields or crop failure in agriculture and difficulty in meeting daily needs in rural areas.

Sources of Water

Precipitation (rain or snow) is the primary source of freshwater on Earth. Most freshwater returns to the oceans through rivers. A small portion is absorbed by the soil and stored underground. A smaller portion is stored in natural and man-made water bodies. Sources of water can be divided into two categories: surface water and underground water.

Surface Water
Water present on the surface of the Earth is called surface water. It can be classified into three categories based on water purity: rainwater, river and lake water, and sea and ocean water. Rainwater is the purest form of water as it leaves impurities behind during the evaporation process. River and lake water comes from rainfall or melting of snow on mountains. Sea and ocean water is salty and not suitable for domestic or agricultural use
Underground Water
Groundwater is water under the ground where soil is saturated with water. Rainwater seeps through layers of rocks and collects on top of non-porous layers, forming an underground water table. Aquifers are layers of porous rocks that contain saturated underground water. Natural springs or pumping can bring underground water to the surface. Groundwater keeps rivers, streams, and lakes filled with water. Wells can be drilled to extract underground water.

Natural Distribution of Water in India

Dependence on seasonal monsoons. Tamil Nadu receives rain from October to November. Waterless conditions in Thar Desert, rainforest climate in North Eastern States. Northern half of the country sees greater extremes in rainfall. Three major rivers, Indus, Ganga, and Brahmaputra, originate in the Himalayas and drain nearly two-thirds of the land area. Water in these rivers is strongly influenced by the monsoons. Water levels in rivers rise during monsoons and may result in floods. Smaller tributaries and streams generally dry off completely during the dry season. Large dams have been built on a number of rivers to regulate water flow and distribute water more evenly throughout the year.

Dams

An artificial wall built across a river. Water is allowed to collect behind the wall in a reservoir. Artificial canals channelize water from the reservoir to the region downstream for irrigation. Bhakra Dam, built across the Sutlej river, is a part of the larger Bhakra Nangal project.

Groundwater
Mountainous and hilly regions of north and west do not allow adequate seepage. Groundwater is mostly limited to lower-lying areas such as valleys. Peninsular areas do not have large continuous aquifers. Coastal plains are a rich source of groundwater but can be contaminated by saltwater inflow. Gangetic plains are rich in groundwater as the aquifers keep getting recharged and restored.

Scarcity of Water

Scarcity of water is defined as a situation where there is insufficient water to satisfy normal requirements. Although water is a renewable resource, we humans are using it at a faster rate than it is being replenished. Several factors contribute to the depletion of the water table, including increasing population, increasing industries, agricultural activities, and deforestation. In India, uncontrolled use of borewell technology, pollution of freshwater resources, and little done for water conservation are also specific causes of water scarcity.

Effects of Water Scarcity on Plants
Water is crucial for plant growth, and its deficiency weakens the plant. Insufficient water leads to reduced photosynthetic activity, stunted growth, and reduced harvest. Without enough water, plants become more susceptible to diseases, which can eventually lead to the death of the plant. This can result in a lack of food, oxygen, and rainfall, leading to the end of all forms of life on Earth.

Case Study: Water Management in Sandharsi, a Village in Patiala, Punjab
Sandharsi, a village in Patiala, faced a severe drought in 2015, leading to the water level receding to 1000 feet below the ground due to reckless and repeated use of groundwater for irrigation. The villagers built ponds within their farmlands, serving as a reservoir for excess water during monsoons, protecting their crops from floods. During dry spells, the water from these ponds is used for irrigation. This innovative solution has helped in managing water scarcity in the village.

Conservation of Water

The availability of freshwater on Earth is limited, and we are heading towards a water crisis. To avoid further shortages, it is important to conserve water. Here are some water-saving practices that households, communities, and private industries can adopt:

• Fixing Leaks and Dripping Taps
  Promptly fixing leaks and dripping taps is crucial to save water. Even small leaks can result in significant water loss over time. A dripping tap, for instance, can waste up to 72 litres of water every day.
• Recycling of Water
  Water recycling can be started at home. Water used in cleaning utensils can be reused to water plants in the garden instead of being wasted down the drain.
• Controlling Water Loss during Floods
  Constructing dams, reservoirs, and embankments helps to check floods and prevent the loss of water.
• Improving Irrigation Methods
  Farmers can be taught improved irrigation techniques, such as drip irrigation. Drip irrigation involves watering the roots of plants drop by drop, reducing water wastage. This technique has been successfully implemented in water-scarce countries like Israel, resulting in higher yields with the same amount of water. Other countries like Egypt are also exploring water-efficient practices and transitioning away from water-intensive crops.
• Rainwater Harvesting Systems
  Rainwater harvesting allows direct utilization of rainwater. These systems capture and store rainwater during the monsoon season, providing a source of drinking water and irrigation throughout the year. While they may not significantly contribute to India's total freshwater resources, rainwater harvesting systems have a strong local impact.
• Case Study: Rainwater Harvesting at Government School at Untka, Mewat District, Haryana
  The government school at Untka faced a water scarcity issue. Until 2013, the school relied on extracting water from an underground source, which was highly saline. In 2013, the school implemented a Roof Water Harvesting (RWH) system with the support of the Department of Science and Technology (Government of India). The RWH unit was installed on the school's roof area to capture rainwater. Since July 2013, the school has been using the recharged water for drinking purposes. The water is made safe through filtration using a biosand filter, which removes microbes and reduces salinity. The rainwater harvesting method provides a combined storage of 150,000 litres of water, sufficient to meet the school's needs until the next monsoon season when it gets replenished.
  By implementing these conservation practices and adopting rainwater harvesting systems, we can contribute to preserving water, a precious resource for future generations.

Key Words

Activity 

Aim: To demonstrate the water cycle.
Material: A glass jar, water, a small plate, and a lamp.
Method:
Fill the glass jar with water and cover it with the small plate.
Place the lamp above the plate.
Observe the water cycle as the water evaporates, condenses on the plate, and falls back into the jar.
Conversion:
Water is a versatile resource that can be converted into various forms and used for different purposes.
For example, water can be converted into steam and used to generate electricity in thermal power plants.