Thinking in systems

A system is a set of interrelated components working together. Geographers describe them using a shared vocabulary:

• Inputs and outputs — matter or energy entering and leaving the system.
• Stores (or stocks) — where matter is held; fluxes / flows — the rate of transfer between stores.
• Open system — exchanges both energy and matter with surroundings (a drainage basin). Closed system — exchanges energy but not matter (the planet's water as a whole).
• Dynamic equilibrium — inputs and outputs are balanced over time, so stores stay broadly stable even as flows continue.

Feedback: negative feedback dampens change and restores balance; positive feedback amplifies change and drives the system further from its original state (e.g. melting ice lowers albedo, causing more warming and more melting).

Open, closed or feedback?

The drainage basin as a local open system

At the local scale the hydrological cycle operates in the drainage basin — the area drained by a river and its tributaries, bounded by a watershed. It is an open system:

• Input: precipitation (the only true input).
• Stores: interception, surface storage, soil moisture, groundwater / aquifer, channel storage.
• Flows / transfers: infiltration, throughflow, overland (surface) flow, percolation, baseflow, stemflow.
• Outputs: evapotranspiration, channel discharge (river flow to the sea), and groundwater flow out of the basin.

Store or flow?

The water balance equation

The water balance accounts for inputs, outputs and changes in storage over a period (usually a year):

Rearranged to find runoff: Q = P − E ∓ ΔS. If storage increases (water fills soils and aquifers, ΔS positive) then less water is available as runoff.

Seasonality: in a wet season inputs exceed outputs and storage recharges (positive ΔS); in a dry season stores are drawn down (negative ΔS) and baseflow sustains the river.

Your turn — solve for runoff

River discharge

Discharge is the volume of water passing a point per second, measured in cumecs (m³/s). It is the product of the water's velocity and the channel's cross-sectional area:

Storm response: discharge peaks after the rain (the lag time). Impermeable geology, steep slopes, saturated soils and urbanisation all shorten lag time and raise the peak — a "flashy" hydrograph.

Your turn — calculate discharge

Global water stores & residence times

At the global scale water is a closed system — a fixed quantity cycling between stores. The distribution is strikingly uneven:

• Oceans hold about 97% of all Earth's water — by far the largest store.
• The small remaining freshwater is mostly locked in ice caps and glaciers, then groundwater; lakes, rivers, soil and the atmosphere hold only tiny fractions.

The residence time is the average time a water molecule spends in a store. It is very long in oceans, ice sheets and deep groundwater (thousands of years), but only days in the atmosphere and rivers.

Fluxes that move water between stores: evaporation, transpiration, condensation, precipitation, cryospheric exchange (freezing/melting) and runoff.

The biggest reservoir

Store, flow or output?

Tap a drainage-basin component, then tap the category it belongs to.

The carbon cycle

Carbon is held in stores of very different sizes and cycles between them via biological, chemical and physical fluxes:

• Stores (largest first): the lithosphere — sedimentary rocks such as limestone, plus fossil fuels — is overwhelmingly the largest; then the oceans (mostly as dissolved carbon), soils, the atmosphere, and the biosphere (living things).
• Fluxes: photosynthesis (draws down CO₂), respiration and decomposition (release CO₂), combustion, weathering of rock, and ocean–atmosphere exchange.

Fast vs slow: the biosphere, atmosphere and surface ocean form a fast carbon cycle (years to centuries); the lithosphere is the slow carbon cycle, exchanging carbon over millions of years via weathering, sedimentation and volcanism.

The largest carbon store

The ocean carbon pumps

The oceans move carbon from the surface to the deep, keeping atmospheric CO₂ lower than it would otherwise be. Three "pumps" work together:

• Biological pump — phytoplankton fix CO₂ by photosynthesis; when they die, carbon-rich material sinks to the deep ocean and sea floor.
• Carbonate pump — marine organisms build shells of calcium carbonate; these sink and accumulate as sediment (eventually limestone).
• Physical / solubility pump — CO₂ dissolves more readily in cold water; cold, dense, carbon-rich water sinks at high latitudes (thermohaline circulation) and carries carbon downwards.

Net primary production at the ocean surface links to the biological pump — the more carbon phytoplankton fix, the more can be exported downward.

Which pump?

Gross and net primary production

Plants fix carbon by photosynthesis (gross primary production, GPP) but respire some of it back to the atmosphere. What remains as new plant growth is the net primary production (NPP):

NPP measures how much carbon an ecosystem locks into biomass each year — the food and habitat base of the whole ecosystem, and a key part of the terrestrial carbon store.

Your turn — net primary production

Contrasting biomes

The water and carbon cycles operate very differently in two contrasting biomes:

• Tropical rainforest — warm and wet all year. Rapid photosynthesis and dense biomass give high NPP; intense recycling of water through evapotranspiration; fast decomposition returns carbon quickly. Most carbon is held in the vegetation, not the soil.
• Arctic tundra — cold, with a short growing season and permafrost. Low NPP; waterlogged, frozen ground slows decomposition, so large amounts of carbon accumulate in frozen soils and peat.

Why it matters: warming can thaw tundra permafrost, releasing stored carbon (including methane) — a positive feedback that further amplifies warming.

Match each process to its carbon flux

Tap a description on the left, then its matching flux on the right.

Disturbing & managing the cycles

Human activity shifts carbon from long-term stores into the fast cycle and alters water pathways:

• Fossil-fuel combustion transfers carbon from the lithosphere to the atmosphere, raising CO₂.
• Deforestation removes a carbon sink, releases carbon when trees are burned or decay, and reduces interception and evapotranspiration.

Management responses aim to restore balance:

• Afforestation and forest protection to rebuild the biosphere store.
• Wetland and peatland restoration to keep carbon locked in waterlogged soils.
• International agreements (such as the Paris Agreement) to coordinate cuts in emissions.

Managing the carbon store

The big ideas to know

You've covered the systems, the water cycle, the carbon cycle and their management. Press Finish to see your score.