4.1A Hydrological cycle Notes

Movements of Water in the Hydrosphere

1. Water is constantly circulating within the hydrosphere, which includes all water found on, under, and above the Earth’s surface.
2. These continuous movements are driven by solar energy and gravity, two fundamental forces maintaining the global water cycle.

Solar Radiation as a Driving Force

1. Solar energy provides the heat that drives evaporation and transpiration.
2. During evaporation, water molecules gain energy and transform from a liquid to a vapour state, entering the atmosphere.
3. Transpiration occurs when plants lose water vapour through small openings (stomata) in their leaves.
4. Together, these two processes are known as evapotranspiration, responsible for most of the water vapour in the atmosphere.

Gravity: The Force of Downward Flow

1. Gravity causes precipitation to fall to the Earth’s surface and directs surface runoff, infiltration, percolation, and streamflow.
2. Water at high altitudes (e.g., mountains) possesses gravitational potential energy that converts to kinetic energy as it flows downhill.
3. This gravitational movement returns water from land back to the oceans, completing the global water cycle.

The Global Hydrological Cycle as a System

1. The global hydrological cycle operates as a closed system, meaning that water is neither added to nor removed from Earth.
2. It only moves and changes state within the system.
3. However, it remains open in terms of energy because solar radiation and gravity drive these processes.
4. It is composed of stores (reservoirs of water) and flows (movements between them).
5. Flows are the transfers (movement without state change) and transformations (state change, e.g. evaporation) that move water between stores.

Main Components of the Global Water Cycle

1. Inputs: Solar radiation (energy source).
2. Stores: Oceans, glaciers, groundwater, rivers, lakes, soil moisture, and the atmosphere.
3. Flows (Transfers): Precipitation, infiltration, runoff, and groundwater flow.
4. Transformations: Evaporation, condensation, and freezing or melting.

Interactions Within the System

1. The hydrological cycle connects the atmosphere, biosphere, hydrosphere, and lithosphere.
2. It transfers energy through processes such as evaporation (absorbs heat) and condensation (releases heat).
3. It transfers matter (water and dissolved nutrients) through precipitation, runoff, and groundwater movement.
4. These interactions influence climate patterns, weather events, and ecosystem productivity.

Major Stores in the Water Cycle

1. Earth’s water exists in six main reservoirs: oceans, ice and snow, groundwater, surface water, the atmosphere, and living organisms.
2. Each store differs in volume, state (solid, liquid, vapour), and residence time (the average duration water remains there).

1. Oceans

1. Oceans contain approximately 96.5% of Earth’s total water, making them the largest water reservoir.
2. This water is saline and unsuitable for direct human consumption.
3. Energy from the Sun causes evaporation, transferring vast amounts of water vapour to the atmosphere.
4. Ocean currents also play a major role in redistributing heat energy globally.

2. Glaciers and Ice Caps

1. Glaciers and polar ice caps store around 1.7% of Earth’s water, nearly all of which is freshwater in solid form.
2. Found mainly in Antarctica, Greenland, and high mountain ranges, these frozen stores represent long-term water reservoirs.
3. When melting occurs, freshwater enters oceans, influencing sea levels and climate regulation.

3. Groundwater

1. Groundwater resides in aquifers, which are permeable rock layers capable of storing and transmitting water.
2. It makes up around 1.7% of total water but represents the largest accessible source of liquid freshwater.
3. Groundwater is replenished through infiltration and percolation from precipitation, though recharge rates vary with soil type and climate.
4. Over-extraction for agriculture and urban use can lead to aquifer depletion and land subsidence.

4. Surface Water

1. Includes rivers, lakes, wetlands, and streams, accounting for only 0.02% of Earth’s total water.
2. Although small in volume, it is the most accessible freshwater source for human use.
3. Surface water responds quickly to climatic events, such as precipitation or drought, making it vital for ecosystems and agriculture.

5. Atmospheric Water

1. The atmosphere holds a small yet crucial amount of water (≈0.001% of global total) in the form of vapour and clouds.
2. Water vapour absorbs and retains heat, making it an important greenhouse gas that regulates Earth’s temperature.
3. Average residence time is only about 8–10 days, reflecting rapid turnover through precipitation and evaporation cycles.

6. Water in Living Organisms

1. A minuscule fraction of total water (0.0001%) exists within plants, animals, and microbes.
2. Despite its small amount, it is essential for biochemical processes such as photosynthesis, digestion, and nutrient transport.
3. Organisms continually exchange water with their surroundings through respiration, transpiration, and excretion.

Flows in the Hydrological Cycle: Transfers and Transformations

1. The hydrological cycle consists of flows that transfer water between different stores through physical and atmospheric processes.
2. These flows include phase changes (transformations) and movement of water across Earth's surface and subsurface (transfers).

Transfers in the Hydrological Cycle

1. Precipitation

1. The process by which condensed water vapour in the atmosphere falls to Earth as rain, snow, sleet, or hail.
2. Returns atmospheric water to the lithosphere and hydrosphere.
3. Precipitation distribution varies greatly with latitude, altitude, and wind patterns.
4. Plays a key role in recharging groundwater and maintaining stream flow.

2. Advection

1. The horizontal movement of water vapour, clouds, or precipitation through the atmosphere due to wind.
2. Responsible for redistributing moisture across the globe.
3. Without advection, regional climates would become highly unbalanced.

2. Surface Runoff and Streamflow:

1. Surface runoff occurs when rainfall exceeds the soil’s infiltration capacity, causing excess water to flow overland into rivers and lakes.
2. Streamflow is the movement of water within river channels, ultimately transporting it to the oceans.
3. Both processes are driven by gravity and are key components in shaping landscapes.

3. Infiltration

1. The downward movement of water through soil pores.
2. Influenced by soil texture, vegetation cover, and land use.
3. Allows water to recharge aquifers and sustain base flow in rivers during dry periods.

4. Percolation

1. The deeper movement of infiltrated water through rock layers to the groundwater table.
2. Slow process, creating long-term water storage.

5. Throughflow and Groundwater Flow

1. Throughflow moves laterally through soil, contributing to stream discharge.
2. Groundwater flow represents the slowest transfer, occurring deep below the surface, often taking years or centuries to reach oceans.

Transformations in the Hydrological Cycle

1. Evaporation

1. Conversion of liquid water into water vapour due to solar energy.
2. Major energy transfer process linking the hydrosphere and atmosphere.
3. Influenced by temperature, wind speed, humidity, and surface area.

2. Condensation

1. Cooling of water vapour into liquid droplets, forming clouds and dew.
2. Releases latent heat energy, which drives atmospheric circulation and storms.

3. Freezing and Melting

1. Changes between liquid and solid states store or release latent heat energy.
2. Important in regulating polar climates and seasonal water availability.

4. Sublimation and Deposition