4.1 Water systems

Sunlight supplies energy for evaporation; condensation releases latent heat; gravity drives drainage, runoff and flow downhill.

Separate phase change from movement. Solar energy changes liquid to vapour, while gravity moves precipitation through soil, rivers and groundwater toward lower elevation.

Sun warms a lake until water evaporates; after rain, gravity pulls runoff into a stream.

After precipitation, gravity drives drainage, run-off, streamflow and groundwater flow toward lower elevation; solar energy supplied the earlier phase-change energy.

Do not assign one driver to the whole cycle—different arrows have different causes.

Draw stores as boxes and flows as labelled arrows from source to destination, then state the system boundary.

At global scale, water matter is approximately closed: it changes state and location. Energy crosses the boundary as solar input and heat loss, so the system is open to energy.

A diagram can show ocean → evaporation → atmosphere → precipitation → river → ocean, with each arrow named.

The boundary determines which inputs and outputs count; a catchment is open even when the global cycle is nearly closed.

A store is an amount, not a process; flows need direction and often a rate.

Oceans hold about 96.5% of Earth’s water; ice and groundwater are the next large stores, while rivers, lakes, air and organisms are tiny fractions.

Use the order of magnitude to interpret access: global abundance does not mean freshwater is easy to reach, clean or renew quickly.

A lake may be vital locally but still contain only a minute share of global water compared with the ocean.

Most water is saline or locked in slow stores; accessible freshwater is a small, uneven fraction.

A percentage of global water is not a measure of local availability or sustainable supply.

Transformations change water state; transfers move water, and infiltration is entry into soil while percolation is movement through it.

Evaporation, transpiration, condensation, freezing, melting and sublimation involve a change of state or release of water vapour. Advection moves vapour, liquid droplets or ice crystals horizontally; precipitation, surface run-off, streamflow and groundwater flow transfer water between locations.

Rain infiltrates the surface, then percolates through porous soil toward groundwater.

Advection is wind-driven horizontal transfer of water in the atmosphere; condensation changes vapour into liquid and precipitation transfers water from atmosphere to surface.

Infiltration and percolation are sequential but not synonyms.

Land use changes interception, evapotranspiration, infiltration and drainage, which can alter peak flow, recharge and flood risk.

Deforestation can reduce canopy storage; compaction and urban surfaces reduce infiltration and speed runoff. Irrigation may raise evapotranspiration or runoff depending on soil, rate and drainage.

Replacing permeable ground with roads shortens the time to peak flow and can increase flash flooding after the same rainfall.

Impermeable surfaces reduce infiltration and groundwater recharge while faster surface run-off shortens lag time and can raise peak discharge and flash-flood risk.

One land-use change can affect several flows; do not assume every deforestation site has the same direction or size of effect.

For a defined water body and time period, storage changes according to its inputs, natural outputs and harvesting.

ΔS is change in storage, I is total input, O is natural output and H is harvested water. All quantities must use the same volume-per-time unit and the same system boundary.

Example: a lake receives 180 million m³ yr⁻¹ and loses 150 million m³ yr⁻¹ naturally. At steady state, H = 180 − 150 = 30 million m³ yr⁻¹. This is an arithmetic maximum; a sustainable quota may be lower to protect ecological flows and allow for drought and uncertain recharge.

A balanced annual budget does not guarantee sustainability in every season: state the boundary, time period, units and ecological allowance.

Water’s polarity, heat capacity, transparency, density behaviour and gas solubility create conditions for transport and life.

Polarity supports cohesion, adhesion and solvent action; transparency lets light enter; high specific heat capacity buffers temperature. Freshwater is densest near 4°C. At the same pressure, cooler water generally holds more oxygen and carbon dioxide, while at the same temperature a higher gas partial pressure increases equilibrium solubility.

Ice floats and insulates liquid water below, allowing aquatic life to persist through winter.

Cohesion holds water molecules together, while adhesion attracts water to other substances; together they support capillary movement.

A property matters through a mechanism; do not list ‘polarity’ without saying what it enables.

CO₂ enters the ocean when atmospheric concentration exceeds surface-water concentration, but uptake is a changing flux, not a fixed saturated container.

Mixing and biology move carbon away from the surface and allow exchange; warming lowers gas solubility and changing chemistry can reduce the fraction absorbed. A sink can weaken while remaining a sink.

If emissions rise faster than ocean uptake, ocean carbon still increases even while the ocean absorbs less of each additional tonne.

Ocean uptake can weaken when warming lowers gas solubility or when the air–sea concentration gradient and carbonate chemistry change; a sink need not stop completely to lose capacity.

Sink does not mean unlimited uptake or zero outgassing; compare both directions and rates.

Ocean carbon can move into dissolved chemistry, biomass, carbonate or buried sediment, and each pathway has a different residence time.

Dissolved CO₂ can lower pH quickly; biology transfers carbon through food webs, but most biomass is respired or decomposed. A small fraction reaches the seabed as organic matter or inorganic carbonate, may be buried in sediment and, over millions of years, can contribute to fossil-fuel formation.

Phytoplankton fix CO₂ this season, but most carbon is respired or decomposed; a small sinking fraction may be buried for much longer.

Long-term sequestration requires carbon to reach and remain in buried seabed sediment; temporary biological uptake alone does not provide the same residence time.

‘Biological’ does not mean permanent; distinguish flux, pool and residence time.

In a commonly stratified water body, warmer, less-dense water lies above colder, denser water and the density contrast restricts mixing. Freshwater adds an important exception because it is densest near 4°C.

Ice is less dense and floats, slowing heat loss. This creates a surface-down freeze and leaves liquid water below for life.

A lake surface cools from 8°C to 4°C and sinks; cooling to 1°C afterward leaves that water nearer the surface rather than driving it below 4°C water.

Ice floats and insulates; maximum density occurs at 4°C, not 0°C.

Do not apply seawater density behaviour to freshwater without checking salinity.

A thermocline is a depth zone of rapid temperature change; the density contrast separates a mixed surface layer from deeper water.

Surface water often gains oxygen from air and photosynthesis, while sinking organic matter decomposes below, releasing nutrients and consuming oxygen. Seasonal wind and productivity can change the profile.

A strong thermocline can leave deep nutrients below the lighted surface, limiting phytoplankton until mixing occurs.

Below a strong thermocline, decomposition releases mineral nutrients and consumes oxygen, while the density contrast restricts oxygen resupply from the surface.

A thermocline is a gradient, not a permanent sharp wall; depth and strength vary with season and circulation.

Warmer or fresher surface water is less dense, so a stronger density contrast can suppress vertical mixing.

Observed stratification increases are strongest in the upper 200 m. Surface warming lowers density globally, while Antarctic ice melt can freshen surface water and lower its density further; reduced mixing can limit nutrient supply upward and oxygen renewal at depth.

Meltwater freshens a polar surface layer; if it becomes lighter than deep water, sinking weakens and deep oxygen renewal may fall.

Evidence for stronger stratification combines a larger density contrast with reduced vertical mixing or nutrient and oxygen exchange; warming alone is not the complete test.

Temperature is not the only control of seawater density; salinity can reinforce or reverse the effect.

When winds move surface water away, deeper water rises to replace it; this upwelling often brings nutrients into the lighted zone.

Deep water can be nutrient-rich because decomposition regenerates nitrate and phosphate. In oceans, wind-driven displacement can bring this water upward and ENSO can weaken or strengthen the pattern. In stratified lakes, seasonal cooling and wind can erode the density barrier and produce seasonal upwelling or turnover.

Coastal upwelling raises cold nutrient-rich water, supporting phytoplankton and fisheries; El Niño often weakens this delivery in the eastern Pacific.

Upwelling raises productivity only when nutrient-rich water reaches sufficient light and producers are not limited by another factor.

Upwelling is not automatically a fish increase; trace nutrients, light, food-web response and timing.

Thermohaline circulation arises from density differences caused by temperature and salinity: cold, salty water can sink and drive deep flow.

North Atlantic surface water loses heat and may become saline enough to sink; deep currents connect basins while compensating upper flows redistribute heat. Freshwater input can inhibit sinking, and winds also drive surface currents.

Meltwater freshens a sinking region, lowers surface density and weakens the downward branch even if cooling continues.

Salinity matters too; warm salty water can be denser than fresher cold water, so use both variables.

Thermohaline circulation is a network, not one conveyor belt with a fixed speed or route.