Welcome to the Water Cycle!

Hello Geographers! This chapter is all about the incredible movement of water across our planet, a process fundamental to climate, landscapes, and life itself. We are going to examine the water cycle (or hydrological cycle) using a systems approach—thinking about inputs, outputs, stores, and flows. Understanding this cycle is crucial because water availability is central to human welfare and global environmental security.

Don't worry if the number of terms seems overwhelming at first. We will break down the cycle into its global components and then zoom in on the specific processes that happen in a local area, known as a drainage basin.

1. Water and Carbon Cycles as Natural Systems (The Core Idea)

Before diving into water, remember that in physical geography, we often study systems. A system is a set of interacting components.

The Water Cycle: A Global Closed System

Globally, the hydrological cycle is a closed system.

  • The total amount of water on Earth remains constant.
  • Energy (from the sun) can enter and leave, but the matter (water) stays within the system.

The Drainage Basin: A Local Open System

When we look at a local area, like a river valley, it becomes an open system.

  • Both energy and matter (water) can cross the boundaries.
  • For example, precipitation (an input) falls into the basin, and river discharge (an output) leaves the basin.

Quick Review: The system concept relies on:

  • Inputs (what enters the system, e.g., precipitation).
  • Outputs (what leaves, e.g., evaporation, runoff to the sea).
  • Stores/Components (where water is held, e.g., lakes, soil).
  • Flows/Transfers (how water moves between stores, e.g., infiltration).

Key Takeaway: The water cycle is a continuous flow powered by solar energy and gravity, maintaining a constant global supply, though local distribution changes constantly.

2. Global Distribution and Size of Major Water Stores

Water is stored in different parts of the Earth, known as reservoirs. The magnitude (size) of these stores dictates how much water is available and for how long.

The Four Major Global Stores (LHC-A)

Remember these four main reservoirs:

1. Hydrosphere (Oceans, Seas, Lakes, Rivers)
  • Description: Liquid water found on the Earth's surface.
  • Size and Significance: Contains over 97% of all global water (mostly in the oceans). This is the largest store, but it is largely saline (salty) and therefore inaccessible for most human use without treatment.
2. Cryosphere (Ice Sheets, Glaciers, Snow)
  • Description: Water locked up as solid ice.
  • Size and Significance: Holds nearly 69% of the world's fresh water (about 2.1% of all water). The cryosphere is crucial for regulating sea levels and freshwater supply, particularly in the tropics (mountain glaciers).
3. Lithosphere (Groundwater and Soil Water)
  • Description: Water stored in rocks and soil beneath the surface.
  • Size and Significance: The second largest fresh water store. Groundwater held in permeable rocks (aquifers) is a vital, but often non-renewable, resource for human consumption and irrigation.
4. Atmosphere (Vapour, Clouds)
  • Description: Water vapour, liquid droplets, and ice crystals held in the air.
  • Size and Significance: The smallest store (less than 0.001%). However, it has the fastest transfer rate, moving water thousands of kilometers in days. It is key to weather and climate.

Did you know? If all the water vapour in the atmosphere condensed at once, it would only cover the Earth to a depth of about 2.5 cm! It is a tiny store, but incredibly active.

Key Takeaway: Most water is salty (hydrosphere). Most available fresh water is frozen (cryosphere) or hidden underground (lithosphere).

3. Processes Driving Change: Flows and Transfers

These processes move water between the major stores over varying timescales—from minutes (a rain shower) to thousands of years (deep groundwater).

A. Atmospheric Transfers

1. Evaporation
  • Process: Liquid water turning into water vapour (a gas) when heated by solar radiation.
  • Factors: Heat energy, wind speed, relative humidity, and water body size.
2. Condensation
  • Process: Water vapour cooling down and turning back into liquid water droplets.
  • Mechanism: This typically happens when moist air rises, expands, and cools (adiabatic cooling). The water vapour condenses around tiny airborne particles called condensation nuclei (dust, salt, pollution) to form clouds.
3. Precipitation
  • Process: Water (in any form: rain, snow, sleet, hail) falling from clouds to the Earth's surface.
  • Causes of Precipitation: Precipitation occurs when cloud droplets grow large enough (through collision and coalescence) that they can no longer be supported by updrafts, or via the Bergeron Process in cold clouds where ice crystals grow at the expense of supercooled water droplets.

B. Cryospheric Processes

These involve changes related to ice and snow, particularly important at hill slope, drainage basin, and global scales.

  • Ablation: The general term for the loss of mass from a glacier or ice sheet (e.g., melting, sublimation).
  • Accumulation: The gain of mass, usually from snowfall.
  • Sublimation: The process where ice turns directly into water vapour without first becoming liquid water (e.g., dry ice, or snow disappearing on a windy, cold day).
  • Storage Time: Water held in the cryosphere can be locked up for thousands of years, meaning climate changes affecting the cryosphere have very long-term impacts on the global water budget.

Common Mistake: Remember that Evapotranspiration is often treated as a single process in the water cycle model, combining evaporation from open water/soil AND transpiration from plants.

Key Takeaway: Heat drives upward movement (Evaporation), cooling drives cloud formation (Condensation), and gravity drives downward movement (Precipitation and Runoff).

4. The Drainage Basin as an Open System

A drainage basin (or catchment area) is the area of land drained by a river and its tributaries. It is defined by a watershed (the high ground separating it from adjacent basins).

Inputs and Outputs

Input: Precipitation (P)

This is the only significant input into the basin system (the water falling onto the land).

Outputs: Evapotranspiration (E) and Runoff (Q)
  • Evapotranspiration (E): Water lost to the atmosphere from evaporation and transpiration.
  • Runoff (Q): Water flowing out of the basin, mainly via the main river channel, eventually reaching the sea or a lake.

Stores and Flows within the Basin

Once precipitation hits the basin, it moves through several stores and flows.

Stores/Components (Where water is held)
  1. Interception: Water caught and stored on the leaves and branches of vegetation (temporary store).
  2. Surface Storage: Water held in puddles, ditches, lakes, and reservoirs.
  3. Soil Water: Water held within the pores of the soil layer (available to plants).
  4. Groundwater Storage: Water stored in the deeper rock layers (aquifers).
  5. Channel Storage: Water contained within the river channel itself.
Flows/Transfers (How water moves)
  1. Stemflow: Water running down the trunks and stems of plants and trees.
  2. Infiltration: Water soaking vertically downwards from the surface into the soil. (The Infiltration Capacity is the maximum rate at which soil can absorb water.)
  3. Percolation: Deeper, slower vertical movement of water from soil down into the bedrock.
  4. Overland Flow (Surface Runoff): Water flowing horizontally over the ground surface (common when precipitation intensity exceeds the infiltration capacity or when the ground is saturated/impermeable).
  5. Throughflow: Water flowing horizontally through the soil layer, often guided by natural pipes or layers.
  6. Groundwater Flow (Baseflow): The very slow, deep movement of water through the rock, feeding the river channels.
  7. Channel Flow (Discharge): The movement of water within the river or stream channel itself.

Analogy: Imagine your kitchen sink (the drainage basin). The water flowing from the tap is Precipitation (Input). Water splashing onto the dishes is Interception. Water pooling in the basin is Surface Storage. Water going down the drain is Channel Flow (Output).

5. Water Balance and Runoff Variation

The Concept of Water Balance

The water balance describes the relationship between the inputs and outputs in a drainage basin over time. It helps determine if an area is experiencing surplus (wet) or deficit (dry).

The basic equation (based on the system structure) is:

$$P = Q + E \pm S$$

Where:

  • \(P\) = Precipitation (Input)
  • \(Q\) = Runoff / Discharge (Output)
  • \(E\) = Evapotranspiration (Output)
  • \(\pm S\) = Change in Storage (The difference between inputs and outputs must equal the change in stored water)

The Flood Hydrograph and Runoff Variation

A flood hydrograph shows how the discharge (volume of water flowing in the river) changes over time in response to a rainfall event (storm). Analysing the hydrograph is key to understanding flood risk.

Key Hydrograph Components:
  • Peak Rainfall: The time of maximum precipitation during the storm event.
  • Peak Discharge: The time when the river reaches its highest flow level.
  • Lag Time: The time delay between peak rainfall and peak discharge. A short lag time suggests a rapid response and higher flood risk.
  • Rising Limb: The segment of the graph where river discharge is increasing rapidly.
  • Recession Limb (Falling Limb): The segment where discharge decreases as stores empty.
Factors Affecting Runoff Variation (and Lag Time):
  • Physical Factors: Size and shape of the basin, relief (steep slopes reduce lag time), drainage density (a dense network of streams speeds up runoff), and soil saturation (saturated soil increases overland flow).
  • Human Factors: Land use (urbanisation creates impermeable surfaces, dramatically reducing lag time), deforestation, and agricultural practices.

Key Takeaway: The water balance helps us calculate changes in storage, while the hydrograph is a practical tool for studying how quickly water moves through a basin, which is essential for flood management.

6. Changes in the Water Cycle Over Time

The water cycle is not static. It varies naturally over different timescales and is increasingly influenced by human activity.

A. Natural Variations (Temporal Scales)

1. Seasonal Changes
  • In temperate zones, summer often means higher Evapotranspiration (due to heat and plant growth), reducing soil moisture and runoff.
  • In winter, low temperatures reduce Evapotranspiration, leading to saturated soils and increased runoff, even with moderate rainfall.
  • In glaciated regions, spring melt causes high runoff as the cryospheric store transfers its water to the hydrosphere.
2. Storm Events

Extreme events, such as tropical storms or prolonged depressions, involve very high precipitation intensity over a short period. This rapid input often exceeds the soil's infiltration capacity, leading to dramatic increases in overland flow and flash flooding (a short lag time on the hydrograph).

Did you know? Climate change is predicted to increase the frequency and intensity of short-duration, high-intensity storm events, leading to more volatile runoff patterns globally.

B. Human Impact on the Water Cycle

Human activities interfere with the natural stores and flows, often changing the balance dramatically.

1. Land Use Change (Urbanisation and Deforestation)
  • Impact on Interception/Infiltration: When forests are cleared, interception (water caught by trees) and infiltration capacity drop significantly.
  • Impact on Runoff: Urban areas use impermeable materials (concrete, tarmac). This prevents infiltration, accelerates overland flow, and reduces groundwater recharge, leading to a much higher peak discharge and a shorter lag time on the hydrograph.
2. Farming Practices
  • Ploughing: Ploughing parallel to slopes can create channels that encourage rapid overland flow and soil erosion.
  • Crops: Different crops have different evapotranspiration rates. For example, large-scale irrigation in dry areas heavily depletes local surface and groundwater stores.
3. Water Abstraction (Extraction)
  • Process: Humans remove water from surface stores (rivers, lakes) or sub-surface stores (groundwater/aquifers) for drinking, industry, and agriculture.
  • Impact: Excessive abstraction reduces the channel storage and groundwater stores. If the rate of extraction exceeds the rate of recharge, aquifers can become depleted, leading to water stress and potentially land subsidence.

Key Takeaway: Human actions, especially changing land use and intensive abstraction, disrupt the natural water balance, generally leading to faster runoff, reduced infiltration, and depleted sub-surface stores.