CBSE Class 7 Science Chapter 16: Water - A Precious Resource NCERT Solutions

(a) T - Groundwater holds a significantly larger volume of freshwater compared to the surface water found in rivers and lakes globally.

(b) F - Water shortage affects both rural and urban populations due to increasing demand and mismanagement of resources.

(c) F - While rivers are a major source, irrigation also relies on groundwater, canals, and other water bodies.

(d) T - Rain is the primary source that replenishes all other water bodies on Earth, making it the ultimate source.

Groundwater is recharged through a natural process where rainwater and water from surface sources like rivers and ponds gradually seep through the soil and rock layers. This water fills up the empty spaces and cracks deep beneath the Earth's surface. The movement of water downwards through the soil is called infiltration. This continuous infiltration process replenishes the underground water reserves, thereby recharging the groundwater.

Having ten tube-wells for fifty houses suggests a high rate of groundwater extraction. If the rate at which water is pumped out exceeds the rate at which groundwater is naturally replenished (recharged), the water table will progressively decrease over the long term. This excessive withdrawal can lead to a significant drop in the water level, potentially causing water scarcity for the community and impacting the availability of groundwater for future use.

To minimize water usage in maintaining a garden, several strategies can be employed:

• Efficient Irrigation Methods: Utilize drip irrigation systems or sprinklers. Drip irrigation delivers water directly to the plant roots, minimizing evaporation loss. Sprinklers can also be more efficient than manual watering if used correctly.
• Timing of Watering: Water the plants during the cooler parts of the day, such as early morning or late evening. This reduces water loss due to evaporation from the soil surface and plant leaves.
• Mulching: Applying a layer of mulch around plants helps retain soil moisture by reducing evaporation and suppressing weed growth, which competes for water.
• Choosing Appropriate Plants: Select native or drought-resistant plant species that require less water.

Several factors contribute to the depletion of the water table:

• Increasing Demand for Water: The rising population leads to a greater demand for water for domestic use, agriculture (irrigation), and industrial activities. This increased extraction rate often surpasses the natural replenishment rate.
• Deforestation: Forests play a crucial role in the water cycle. Trees help in the infiltration of rainwater into the ground. Removing forests reduces the surface area available for water to seep into the soil, thus hindering groundwater recharge.
• Urbanization and Construction: Paved surfaces in urban areas prevent rainwater from seeping into the ground. Large-scale construction projects can also alter natural drainage patterns.
• Over-extraction for Agriculture: In many agricultural regions, groundwater is heavily pumped for irrigation, leading to a significant drop in the water table over time.

The mismatch between the increasing demand for water and the reduced rate of groundwater recharging is the primary reason for the depletion of the water table.

(a) People obtain groundwater through wells and tubewells.

(b) Three forms of water are solid, liquid and gas.

(c) The water bearing layer of the earth is water table.

(d) The process of water seepage into the ground is called infiltration.

The correct option is (iii) Heavy rainfall. Heavy rainfall actually helps to replenish water sources and alleviate water shortage. The other options—rapid industrial growth, increasing population, and mismanagement of water resources—all contribute to increased demand and/or reduced availability, leading to water scarcity.

The correct option is (iv) of the world remains constant. While water moves between different reservoirs (oceans, atmosphere, land) through the water cycle, the total amount of water on Earth remains relatively constant. The water in lakes, rivers, and underground can fluctuate significantly, but the overall global water volume is conserved.