How does urbanisation typically impact the local water cycle?

Urbanisation increases impermeable surfaces (like concrete), which reduces infiltration and increases surface runoff. This leads to higher peak discharges in rivers and a higher risk of flash flooding.

What is the primary difference between an open system and a closed system in hydrology?

An open system, like a drainage basin, allows both energy and matter (water) to cross its boundaries. A closed system, like the global hydrological cycle, allows energy to enter and leave but keeps the total amount of matter constant within the system.

How do porosity and permeability differ in their influence on groundwater?

Porosity refers to the total volume of empty spaces (pores) within a rock or soil that can hold water. Permeability is the measure of how easily those pores are connected, allowing water to actually flow through the material.

What is the distinction between infiltration and percolation?

Infiltration is the initial entry of water from the surface into the upper soil layer. Percolation is the subsequent downward movement of that water through the soil layers and into the underlying rock or groundwater stores.

What is a common error when identifying 'Groundwater' in a systems diagram?

A common mistake is labeling 'Groundwater' as a flow or transfer. In a systems approach, groundwater is a **store** (a component where water is held), whereas 'Groundwater Flow' is the actual movement or transfer of that water.

Why is it incorrect to assume that all precipitation becomes surface runoff?

This ignores the processes of interception by vegetation, infiltration into the soil, and evapotranspiration. Runoff only occurs when the rainfall intensity exceeds the soil's infiltration capacity or when the soil is already fully saturated.

What mistake is made when treating the global water cycle as an open system?

Treating the global cycle as open suggests that water is being lost to space or gained from external sources. In reality, the Earth's gravity and atmospheric conditions keep the total mass of water constant, making it a closed system for matter.

Define the term 'Aquifer'.

An aquifer is a body of permeable rock, such as sandstone or limestone, that can contain or transmit groundwater. It acts as a significant underground store that can be tapped for human use.

What is 'Sublimation' in the context of the water cycle?

Sublimation is the phase change where water moves directly from a solid state (ice or snow) to a gaseous state (water vapor) without passing through the liquid phase. This often occurs in cold, dry conditions with high solar radiation.

What does the term 'Cryosphere' encompass?

The cryosphere includes all the frozen water parts of the Earth system, such as glaciers, ice sheets, sea ice, snow cover, and permafrost. It is a major freshwater store that influences global sea levels.

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Water & Carbon Cycles

Key Terms: The Water Cycle

Summary

The water cycle, or hydrological cycle, is a complex system of stores and flows that describes the continuous movement of water on, above, and below the Earth's surface. It operates as a closed system at the global scale, driven by solar energy and gravity, while functioning as an open system at the local drainage basin level.

1. Definition & Core Concepts

Hydrological Cycle: The continuous, closed-loop movement of water through the Earth's atmosphere, land, and oceans. It involves the transition of water between solid, liquid, and gaseous states.
Systems Approach: A method of studying the water cycle by identifying inputs (e.g., precipitation), outputs (e.g., runoff), stores (e.g., aquifers), and flows or transfers (e.g., infiltration).
Closed vs. Open Systems: At a global scale, the water cycle is a closed system because matter (water) does not enter or leave the Earth's system, though energy (solar radiation) does. At a local scale, such as a drainage basin, it is an open system where both matter and energy cross system boundaries.

2. Underlying Principles

Energy Drivers: Solar radiation is the primary energy source that drives evaporation and transpiration, while gravity facilitates the downward movement of water through precipitation, infiltration, and runoff.
Mass Balance: The principle that the total amount of water in the global cycle remains constant over time. Changes in one store (e.g., melting glaciers) must be balanced by changes in another (e.g., rising sea levels).
Dynamic Equilibrium: A state where the inputs and outputs of a system are balanced over time, maintaining a stable overall condition despite constant internal fluctuations.

A conceptual diagram of the hydrological cycle showing precipitation as an input, evaporation as an output, and infiltration as a transfer to groundwater stores.

3. The Global Water Cycle: Major Stores

4. The Drainage Basin System: Local Processes

5. Key Distinctions

6. Exam Strategy & Tips