GATE Civil Engineering (CE) 2027 Test: Introduction to Hydrology Free Online

The percentage of Earth covered by oceans is approximately:

• The oceans cover about 71% of the Earth's surface.

• This extensive coverage plays a crucial role in regulating climate and supporting diverse ecosystems.

• Oceans are vital for marine life, providing habitat and resources.

• They also influence weather patterns and are essential for human activities such as fishing and transportation.

Approximately 97% of the world's water is saline.

This saline water is primarily found in the oceans, which cover a vast portion of the Earth's surface. Here are some key points regarding saline water:

• Saline water has a high concentration of salt, typically around 35 grams per litre.
• Only about 3% of the Earth's water is freshwater, which is essential for drinking and agriculture.
• Freshwater is found in rivers, lakes, glaciers, and underground aquifers.
• The distribution of freshwater is uneven, making access a global challenge.

Understanding the prevalence of saline water highlights the importance of conserving and protecting our freshwater resources.

The percentage of total quantity of fresh water in the world available in the liquid form is about 3%.

The vast majority of the Earth's water is in the form of oceans, which are saltwater and not suitable for drinking or irrigation. Freshwater, which is water that is free of salt and other minerals and is suitable for drinking and irrigation, makes up a very small percentage of the Earth's total water supply. According to the United Nations, about 2.5% of the Earth's water is freshwater, and of that, only about 0.3% is available as surface water in lakes, rivers, and streams, while about 2.2% is found in groundwater reserves. In other words, about 3% of the total quantity of water on Earth is available in the liquid form as freshwater.

The average stream flow

Annual  run off

Whenever precipitation occurs, a part of rainfall is infiltrated into the soil and other part is surface runoff meeting in the rivers. 
Moreover, not all the water evaporated from the sea is transported towards land and most of it fall back as precipitation in oceans itself.
Therefore, it is found that about 9% more, water evaporates from the ocean than what falls back them as precipitation.

In the hydrological cycle, the residence time of water varies significantly across different reservoirs. Here are key points regarding the average residence times:

• The average residence time of water in the atmosphere is generally longer than that found in global rivers.
• In contrast, the residence time in the oceans is shorter compared to that of global groundwater.
• When comparing rivers and groundwater, the residence time in rivers is typically shorter than in groundwater.
• Overall, the oceans hold water for a longer period than groundwater.

This information highlights the dynamics of water storage in different parts of the hydrological cycle.

Most of the earth’s water sources such as rivers, fakes, oceans, ground water, etc, get their supplies 
from rain, while the rain water in itself is derived from the evaporation from these sources. 
Water is intact lost to the atmosphere as vapour from the earth, which is then precipitated back in the form of rain, snow, hail, dew, sleet or frost, etc. This evaporation and precipitation continues forever and thereby, a balance is maintained between the two. This process is known as hydrologic cycle.

The hydrologic cycle operates on the principle of conservation of mass, which states that the difference between total inflow and outflow of water in a system equals the change in water storage over time. This is expressed as:

Mass Inflow − Mass Outflow = ΔMass Storage

This equation accounts for variations in storage (e.g., groundwater, soil moisture, snowpack) and applies to both short-term and long-term hydrological processes.

Why Other Options Are Incorrect:

• A: Assumes a fixed constant difference, ignoring dynamic changes in storage.

• B: Focuses only on subsurface flow, neglecting surface and atmospheric components.

• C: Implies a steady-state balance (no storage change), which rarely occurs in natural systems.

D correctly captures the fundamental principle of the hydrologic cycle.