A Closed System

Think of the hydrological cycle like a sealed terrarium sitting on a windowsill. Nothing new water ever gets added to it, and no water ever truly leaves it — it just keeps changing form and location, over and over, forever. That's exactly what "closed system" means here: the total amount of water on Earth stays the same. It just gets constantly recycled between the atmosphere, the land surface, underground, and the oceans.

Because nothing enters or leaves the system, geographers say the hydrological cycle only has two kinds of components: stores (places where water sits still for a while) and transfers (the processes that move water from one store to another). There are no "inputs" or "outputs" — that terminology is reserved for open systems, like the drainage basin you'll meet in Topic 2.

Stores of the Hydrological Cycle

A store is simply anywhere water is held for a period of time before it moves on again — it could be held for seconds (a raindrop on a leaf) or thousands of years (deep groundwater in an aquifer). The main stores are:

• Atmosphere — water sits here as invisible water vapour or as visible water droplets in clouds.
• Surface stores — puddles, lakes, rivers, reservoirs.
• Aquifers — permeable rocks like limestone and sandstone that can soak up and hold huge volumes of water underground, like a giant natural sponge.
• Ice and snow — frozen water, often locked away for very long periods (glaciers, polar ice).
• Seas and oceans — by far the largest store on Earth.
• Vegetation (interception store) — when rain lands on leaves or branches, it's temporarily "caught" before it reaches the ground. Some evaporates straight back into the atmosphere; the rest trickles down the leaves, branches, and trunk to the ground — this trickling-down process is called stemflow.

Transfers of Water Within the Cycle

A transfer is a process — a verb, essentially — that physically moves water from one store to another. Here's the full set, grouped by where the water is heading:

Upward transfers (surface/plants → atmosphere):

• Evaporation — heat from the sun turns liquid water into water vapour, lifting it from the surface into the atmosphere. Higher temperatures and stronger winds both speed this up.
• Transpiration — plants release water vapour from tiny pores in their leaves (like plants "breathing out" moisture).
• Evapotranspiration — this is just the combined term for evaporation + transpiration happening together across a landscape; you'll see it a lot in exam answers because it's hard to measure the two separately in the field.

Downward transfer (atmosphere → surface):

• Condensation — water vapour cools and turns back into tiny liquid droplets, which is what forms clouds.
• Precipitation — water falls from the atmosphere to the Earth's surface as rain, hail, sleet, or snow.

Surface and underground transfers (once water has landed):

• Overland flow — water flowing across the top of the ground (surface runoff).
• Infiltration — water soaking into the soil from the surface.
• Throughflow — water moving sideways through the soil, between the surface and the water table.
• Percolation — water seeping downward from the soil into the rock below.
• Groundwater flow — water moving slowly through rock (much slower than throughflow, because rock is denser than soil).

Advection

Advection is the odd one out in this list because it doesn't move water up or down — it moves water sideways, horizontally, through the air. It's simply the wind carrying water vapour and water droplets from one place to another.

Picture warm, moist air sitting over the ocean. If the wind picks that air up and blows it inland, the moisture travels with it — this is how coastal regions can "export" humidity, clouds, and eventually rain to areas further inland. Advection matters because it's one of the main ways heat and moisture get redistributed around the planet, which helps shape regional weather patterns.

What Is a Drainage Basin?

A drainage basin (also called a catchment area) is the whole area of land that drains into one particular river and its tributaries. Picture it like a giant funnel carved into the landscape — every drop of rain that lands anywhere inside that funnel's boundary eventually makes its way, one way or another, into the same river system.

Unlike the hydrological cycle, a drainage basin is an open system. This is one of the most commonly tested distinctions in this chapter, so it's worth being precise: an open system has water genuinely entering and leaving it, not just recycling internally.

• Input — precipitation falling into the basin.
• Outputs — water leaving the basin permanently via evaporation, transpiration, or by flowing out to the sea/lake at the river's mouth.
• It still has stores and transfers too, just like the hydrological cycle — but the presence of inputs/outputs is what makes it "open."

When rain falls into a drainage basin, it can travel to the river channel via four different routes:

• Direct channel precipitation — rain falls straight into the river itself.
• Overland flow — rain runs across the surface because the ground is impermeable (can't be absorbed).
• Throughflow — water travels sideways through the soil.
• Groundwater flow — water travels sideways through the rock.

Every drainage basin is unique — no two look alike — because each one differs in shape, size, rock type, relief (steepness), and land use, and these differences directly change how fast water reaches the river.

Drainage Basin Features

Learn this vocabulary set as a labelled diagram — it comes up constantly as short "define this term" or "label this diagram" exam questions.

Channel Network & Drainage Density

The channel network is simply the main river plus every tributary that feeds into it — the entire "family tree" of streams within the basin. How dense or sparse that network is tells you a lot about the ground beneath it.

Drainage density = the number of tributaries (length of channel) per unit area of the drainage basin. It's controlled almost entirely by whether the underlying rock/soil is permeable or impermeable:

• Impermeable rock/soil → water can't infiltrate → it's forced to flow over the surface, carving lots of small surface tributaries → high drainage density.
• Permeable rock/soil → water infiltrates easily and disappears underground instead of forming surface streams → low drainage density.

River Regimes

Discharge is the amount of water passing a fixed point on a river at a given moment — it's measured in cumecs (cubic metres per second). Discharge is never constant; it rises and falls depending on rainfall, temperature, and season.

A river regime is simply a record of how a river's discharge changes across an entire year. A river fed mostly by snowmelt, for example, will show a huge spike in discharge every spring as the snow melts, then taper off. A river in a temperate climate might show gentle peaks in wet winter months and dips in dry summer months.

Many factors shape a river's regime: climate, vegetation, land use, geology, soil, human activity, drainage basin size/shape, drainage density, and relief. You'll see most of these again below, because they affect storm hydrographs in exactly the same directions.

Storm Hydrographs

While a river regime tracks a whole year, a storm hydrograph zooms right in — it tracks how discharge responds to a single rainfall event, usually over a 24-hour window. It's one of the most graph-heavy, calculation-heavy parts of this topic, so let's break down every labelled feature carefully.

Factors Affecting River Regimes & Hydrograph Shape

Here's the pattern that ties this whole section together: anything that increases overland flow (surface runoff) instead of infiltration will shorten lag time, steepen the rising limb, and increase peak/overall discharge — raising flood risk. Everything below is really just a different way of pushing water toward the surface instead of into the ground.

• Climate — snow/ice melt raises discharge (typically spring); high temperatures increase evaporation and lower discharge; heavy rainfall (autumn/winter) raises discharge; convectional rainfall in hot, moist climates raises discharge.
• Vegetation — more vegetation → more interception + infiltration → less overland flow → lower discharge. Deciduous trees lose this protective effect in winter when their leaves fall, so overland flow and discharge rise.
• Land use — concrete and tarmac in urban areas are impermeable, forcing high overland flow which drains rapidly (often via drains/gutters) straight into rivers, sharply increasing discharge.
• Geology — permeable rock increases infiltration/percolation, reducing overland flow and discharge; impermeable rock does the opposite.
• Soils — compacted or frozen soil can't absorb water, so infiltration drops and overland flow/discharge rise.
• Abstraction — water removed for irrigation or domestic use reduces river discharge.
• Dams — can both increase and decrease discharge depending on how they're managed; their reservoirs also lose water to evaporation, which can lower discharge.
• Relief — steep slopes speed up overland flow (less time for infiltration), increasing discharge.
• Drainage density — higher density means more channels ready to rapidly collect and funnel water, increasing discharge especially after rainfall.