Environmental Management (0680) | The water cycle

IGCSE Environmental Management (0680) Study Notes

Topic 4.2: The Water Cycle (Hydrological Cycle)

Welcome to the world of water management! This section is crucial because water is the most vital resource on Earth. To manage it effectively, we first need to understand how it moves around our planet—a never-ending journey called the water cycle.

Don't worry if some of the vocabulary seems tricky at first; we will break down each process step-by-step using clear analogies!

1. Quick Context: Where is Earth's Water? (Review 4.1)

The water cycle manages all water on Earth, but remember that most of it isn't readily available for us to drink.

• Most of the Earth’s water (about 97%) is saltwater found in the oceans.
• Only about 3% is fresh water.

The Distribution of Fresh Water

Most fresh water is locked up in places that are hard to reach:

• Ice sheets and glaciers (the largest store)
• Ground water (underground)
• Atmosphere (vapour and clouds)
• Lakes and rivers (the smallest, but most accessible, surface store)

Key Takeaway: The water cycle is responsible for continuously moving the small amount of fresh water we rely on through ecosystems and back to the oceans.

2. Describing and Interpreting the Water Cycle

The water cycle (or hydrological cycle) is the continuous movement of water on, above, and below the surface of the Earth. It is a closed system, meaning no water is added or lost from Earth; it just changes state and location.

The Water Cycle as a Closed System

Think of the water cycle as a massive natural machine powered by the Sun (which drives evaporation) and Gravity (which pulls water down as precipitation and flow).

Main Stores of Water (Reservoirs)

Water is held temporarily in reservoirs like:

• Oceans
• Clouds (Atmosphere)
• Ice Caps
• Lakes and Rivers
• Soil and Groundwater (Aquifers)

3. The Key Processes of the Water Cycle (4.2 Syllabus Focus)

We need to know the definitions and functions of the nine main processes that move water between these stores.

3.1. Processes involving the atmosphere (Changing State)

1. Evaporation

Definition: The process where liquid water turns into a gas (water vapour) due to heating (usually by the Sun).

Example: When a puddle disappears on a hot day, it has evaporated.

2. Condensation

Definition: The process where water vapour (a gas) cools and changes back into liquid water droplets.

Function: This is how clouds form. When enough droplets gather, the clouds become saturated.

3. Precipitation

Definition: Water falling from the atmosphere to the Earth's surface.

Forms: This includes rain, snow, hail, and sleet. Precipitation is the main way water returns to the land and oceans.

4. Transpiration

Definition: The process where moisture is carried through plants from the roots to small pores (stomata) in the leaves, where it changes to vapour and is released into the atmosphere.

Did You Know? A large tree can transpire hundreds of litres of water per day!

Combined Term: Often, evaporation and transpiration are grouped together as Evapotranspiration.

3.2. Processes on or above the surface (Flows and Stops)

5. Interception

Definition: When precipitation (like rain) is temporarily caught and held by the leaves, branches, and stems of vegetation before it reaches the ground.

Importance: Forests and dense vegetation significantly reduce the amount of water hitting the soil directly, slowing the flow and reducing erosion.

6. Surface Run-off (Overland Flow)

Definition: Water flowing across the ground surface, often into rivers, streams, and eventually the sea.

Conditions: This usually happens when the ground is impermeable (like concrete) or saturated (full of water) and cannot soak up any more water. It is a fast process that can cause flooding and soil erosion.

3.3. Processes below the surface (Underground Storage and Flow)

Once water hits the ground, it can either run off or soak in. Water that soaks in moves slowly through the soil and rock.

7. Infiltration

Definition: The vertical (downward) movement of water from the surface into the soil and rock.

Analogy: Think of a sponge soaking up water. The rate of infiltration depends heavily on the type of soil (sandy soil infiltrates quickly; clay soil infiltrates slowly).

Memory Trick: Infiltration means moving Into the ground.

8. Through-flow

Definition: The horizontal movement of water through the soil, often following natural cracks and layers, usually above the water table.

Speed: This flow is faster than ground water flow but slower than surface run-off.

9. Ground Water Flow (Base Flow)

Definition: The very slow, deep horizontal movement of water through saturated permeable rock (called an aquifer).

Importance: Ground water flow supplies rivers and streams with water even during dry periods (this is the river’s base flow). It moves much slower than through-flow, sometimes taking hundreds or thousands of years to travel.

4. Summarising the Cycle's Stages

Understanding the water cycle means seeing how these processes link together:

1. Atmosphere Input: Heat causes Evaporation (from water bodies) and Transpiration (from plants).
2. Cloud Formation: Water vapour cools, leading to Condensation, forming clouds.
3. Precipitation Event: Water returns to Earth via Precipitation.
4. Initial Landing: Some water is caught by trees (Interception).
5. Surface Movement: Water that hits the ground either flows over the top (Surface Run-off) or soaks into the soil (Infiltration).
6. Sub-surface Movement: Water moves horizontally through the shallow soil (Through-flow) or deep underground (Ground Water Flow) until it reaches a river, lake, or the sea, ready to evaporate again.

Key Takeaway: The continuous movement of water is essential for supporting life and for replenishing our fresh water sources like rivers and aquifers.