Samacheer Class 8 Social Science - Hydrologic Cycle

1. The degree of water vapour present in the atmosphere is known as humidity. Humidity is an important meteorological parameter that affects weather patterns, comfort levels, and various atmospheric processes. 2. There are three phases in the water cycle. The water cycle consists of evaporation, condensation, and precipitation as its three main phases, though some classifications include additional processes like transpiration, infiltration, and runoff. 3. The falling of water towards the earth surface from the atmosphere in any form is known as precipitation. Precipitation includes rain, snow, sleet, and hail, and is a crucial component of the hydrological cycle that returns water from the atmosphere to the earth's surface. 4. The precipitation with a rain drop size of less than 0.5mm in diameter is known as drizzle. Drizzle consists of very fine water droplets that fall slowly and are often associated with overcast conditions and low clouds. 5. Mist is denser than clouds. Mist forms closer to the ground and has higher water droplet density compared to clouds, which form at higher altitudes and have lower droplet concentrations.

1. Water boils at 212°F temperature but it begins to evaporate at 32°F - True. This statement correctly illustrates that evaporation is a process distinct from boiling. While boiling occurs at a specific temperature when water reaches its boiling point, evaporation can occur at any temperature, even at freezing point (32°F or 0°C). Evaporation is the process by which water molecules gain enough energy to escape from the liquid phase into the gaseous phase, and this can happen gradually over time even at lower temperatures. 2. Mist is not the tiny droplets of water hanging in the air - False. Mist is actually composed of tiny water droplets suspended in the air near the ground surface. It forms when water vapor condenses into liquid droplets due to cooling, creating a visible haze that reduces visibility. 3. The sub-surface runoff is usually referred to as interflow - True. Interflow, also called lateral flow or throughflow, refers to water that infiltrates the soil and moves laterally through the soil layers before eventually reaching streams and rivers, representing an important component of the hydrological cycle.

An aquifer is an underground layer of water-bearing rock.

The hydrological cycle is a global sun-driven process where water is transported from oceans to the atmosphere, from the atmosphere to land, and from land back to oceans

Dew is a water droplet formed by the condensation of water vapour on a relatively cold surface of an object. It forms when the temperature of an object drops below the dew point temperature.

Surface Runoff is the portion of rainfall, which enters the stream immediately after the rainfall. It occurs when the rainfall is longer, heavier, and exceeds the rate of infiltration. In this condition, the excess water makes a head over the ground surface, which tends to move from one place to another following land gradient and is known as overland flow. When the overland flow joins the streams, channels or oceans, it is termed as surface runoff or surface flow.

1. Infiltration of water is low in the region of non-porous soil - True. Because percolation moves the infiltrated water through the soil profile and rock layers to form groundwater, non-porous soils have low infiltration rates as they do not allow water to easily pass through them. Porous soils with larger spaces between particles allow water to infiltrate more readily and percolate downward to recharge groundwater reserves. 2. Freshwater is less on the earth - True. Most of the water on the earth is saline and is found in seas and oceans, which constitutes about 97.2% of all water on Earth. Only approximately 2.8% of Earth's water is freshwater, and of this freshwater, much is locked in ice caps and glaciers, leaving only a small percentage available for human use in the form of groundwater, rivers, and lakes. 3. Snowfall is common in the polar region and mountainous regions - True. Often water vapour in a cloud is converted directly into snow pieces due to lowering of temperature, and so it is common in the polar and mountainous regions. This process is called deposition, where water vapor transforms directly into ice crystals without passing through the liquid phase, occurring in areas where temperatures remain consistently below freezing.

The three important phases of the hydrologic cycle are:
Evapotranspiration,
Precipitation and
Runoff.
1. Evapotranspiration:
It is defined as the total loss of water from the earth through evaporation from the surface water bodies and the transpiration from vegetation. In cropped areas, it is difficult to determine the evaporation and transpiration separately. Therefore it is collectively called evapotranspiration.
2. Precipitation:
It refers to all forms of water that fall from clouds and reaches the earth’s surface. For the occurrence of precipitation, cloud droplets or ice crystals must grow heavy enough to fall through the air. When the droplets grow large in size, they tend to all. While moving down, by collecting some small droplets, they become heavy enough to fall out of the cloud as raindrops.
3. Runoff:
Runoff is the water that is pulled by gravity across the land’s surface. It replenishes groundwater and surface water as it percolates into an aquifer (it is an underground layer of water-bearing rock) or moves into a river, stream, or watershed. It comes from unabsorbed water from rain, snowmelt, irrigation or other sources, comprising a significant element in the water cycle as well as the water supply when it drains into a watershed. Runoff is also a major contributor to erosion which carves out canyons, gorges, and related landforms.
2. Distinguish between evaporation and transpiration.
Water boils at 100°C temperature but, it actually begins to evaporate at 0°c. Much of the water taken up by the plants is released through transpiration.
Evaporation
Transpiration
1. The process in which the liquid form of water changes into gaseous form.
The process by which the water content in the plant are released into the atmosphere in the form of water vapour.
3. The amount of water gets evaporated is about the same as the amount of water delivered to earth as precipitaion.
The soil water content and the ability of the soil to conduct water the roots and the nature of the plant parts also determine the transpiration rate.
4. The rate of evaporation is low during the periods of calm winds than during windy times.
The rate of transpiration is affected by the temperature, wind and humidity.
3. Give a detailed explanation on different forms of precipitation.
The form of precipitation in a region depends on the kind of weather or the climate of the region. The Precipitation in the warmer parts of the world is always in the form of rain or drizzle.
In colder region, precipitation may fall as snow or ice.
Common types of precipitation include rain, sleet, freezing rain, hail & snow.
Rain:
The most common kind of precipitation is rain.
The precipitation in the form of water droplet is rain.
Below 0.5 mm in diameter is known as drizzle and above 0.5 mm in diameter is known as rain.
Sleet:
The precipitation which takes place in the form of a mixture of water droplets and tiny particles of ice is known as sleet.
Freezing Rain:
Raindrops falling through cold air near the ground do not freeze in the air.
Instead, the raindrops freeze when they touch a cold surface.
This is called freezing rain.
Hail:
The precipitation which consists of round pellets of ice which are larger than 5mm in diameter is called as hail or hailstones.
Snow:
The precipitation in form of a powdery mass of ice is known as snowfall which is common in the polar and high mountainous regions.4. Explain the run-off and its types.
Runoff is the water that is pulled by gravity across the land’s surface. It replenishes groundwater and surface water as it percolates into an aquifer (it is an underground layer of water-bearing rock) or moves into a river, stream, or watershed.
Types of Runoff:
Based on the time interval between the instance of rainfall and generation of runoff, the runoff may be classified into the following three types
1. Surface Runoff:
It is the portion of rainfall, which enters the stream immediately after the rainfall. It occurs when the rainfall is longer, heavier, and exceeds the rate of infiltration. In this condition, the excess water makes a head over the ground surface, which tends to move from one place to another following a land gradient and is known as overland flow. When the overland flow joins the streams, channels, or oceans, it is termed as surface runoff or surface flow.
2. Sub – Surface Runoff:
The water that has entered the subsoil and moves laterally without joining the water-table to the streams, rivers or oceans is known as sub-surface runoff. The subsurface runoff is usually referred as interflow.
3. Base Flow:
It is a flow of underground water from a saturated groundwater zone to a water channel. It usually appears at a downstream location where the channel elevation is lower than the groundwater table. Groundwater provides the streamflow during dry periods of small or no precipitation.

1. Water in India is available in three sources - True. Water in India is available from three main sources: surface water from rivers and lakes, groundwater from underground aquifers and wells, and precipitation in the form of rainfall, which is the primary source that replenishes both surface and groundwater resources. 2. Rain is the tiny droplets of water hanging in the air - False. Rain consists of larger water droplets that fall from clouds to the earth's surface due to gravity. Tiny droplets of water hanging in the air are called mist or fog, not rain. Rain forms when water droplets in clouds combine and become heavy enough to fall as precipitation. 3. Saturation occurs when the temperature increases - False. Saturation occurs when the air becomes completely filled with water vapor and cannot hold any more moisture at a given temperature. In fact, saturation is more likely to occur when temperature decreases, as cooler air has a lower capacity to hold water vapor. 4. Evaporation is the process in which the liquid form of water changes into a gaseous form - True. Evaporation is a fundamental process in the water cycle where liquid water is converted into water vapor through the addition of heat energy, allowing water molecules to escape from the liquid surface into the atmosphere.