IB ESS SLTopic 4 — Water SystemsPaper 1 & 2Core skill~11 min read
How Humans Change the Water Cycle
We cannot add water to the planet or take any away. What we can do is change the route it takes — and we do, constantly. Nearly every human impact in this sub-topic comes down to shifting the balance between two flows: how much water soaks into the ground, and how much runs across the top of it.
📚 What you need to know
Human activity mainly alters two flows: infiltration and surface run-off. Change those and everything downstream changes.
Irrigation raises evapotranspiration artificially, can increase rainfall downwind, and if overdone causes run-off carrying fertiliser and pesticide.
Deforestation removes the canopy, so less rain is intercepted: run-off and erosion rise, infiltration and evapotranspiration fall.
Urbanisation replaces permeable ground with impermeable surfaces, which blocks infiltration, speeds run-off and increases flooding.
A water body is in a steady state when total inputs equal total outputs, so the level stays constant.
Sustainable water harvesting means taking water no faster than it is naturally replenished.
If outputs exceed inputs, there is a water deficit and the store shrinks — the cause of aquifer depletion.
Irrigation and agriculture
Irrigation is the artificial supply of water to crops. It directly changes where water is and how much is available in a region, which is the point — but it has knock-on effects.
Evapotranspiration rises artificially, because crops are given far more water than they would naturally receive. More water vapour goes into the air.
That extra atmospheric moisture can produce localised increases in precipitation downwind of irrigated areas, changing the rainfall pattern of a region.
Over-watering causes run-off. If water arrives faster than the soil can absorb it, the excess flows across the surface, carrying sediment, fertilisers and pesticides with it.
That run-off causes water pollution and nutrient imbalances in rivers and lakes downstream.
Deforestation
Forests behave like sponges. They intercept rain on leaves and branches, slow it down, and give it time to soak in. Remove them and that whole buffer disappears.
With no canopy to intercept and slow the rain, more water hits the ground surface directly, so surface run-off rises sharply.
Infiltration falls, so less water reaches the aquifer — groundwater recharge drops and stream flows become less reliable between storms.
Evapotranspiration falls, because there are no trees transpiring, so less moisture is returned to the atmosphere. In large forests this can reduce local rainfall.
Faster run-off over bare soil causes erosion, and the loss of topsoil makes recovery harder.
Deforestation gives you the worst of both worlds: too much water all at once when it rains, and too little in the dry season, because nothing was stored underground in between.
Urbanisation
Urbanisation converts natural landscapes into buildings, roads and drains. Concrete and asphalt are impermeable: water simply cannot get through them.
Impermeable surfaces prevent infiltration, so groundwater recharge is reduced.
Rainfall becomes surface run-off almost immediately, which means more flooding during storms and less water available during dry spells.
Urban drainage systems are designed to remove water fast, which accelerates run-off further and can overload rivers and cause flooding downstream.
The urban heat island effect — caused by concentrated buildings and paved surfaces — raises temperatures, increasing evaporation and altering local rainfall patterns.
The thin red arrow on the left is why a wooded valley rarely floods after a storm. The thick one on the right is why a city with the same rainfall does.
Activity
Infiltration
Surface run-off
Other effects
Irrigation
Falls if water is applied faster than the soil absorbs it
Rises when soil is oversaturated
Higher evapotranspiration, more rain downwind, fertiliser and pesticide carried into rivers
Deforestation
Falls — no canopy to slow the rain
Rises sharply
Less evapotranspiration, more erosion, less groundwater recharge, altered stream flow
Urbanisation
Falls close to zero on paved areas
Rises, and arrives much faster
Flooding, drains overloading rivers, urban heat island raising evaporation
Steady state: inputs versus outputs
To work out whether a lake, river or aquifer is in trouble, you do not need anything clever — you add up what goes in and what comes out.
The rule for every water body
inputs = outputs → steady state • outputs > inputs → water deficit, the store shrinks
Typical inputs are precipitation falling straight onto the water body, surface run-off flowing in over the land, river inflow, and groundwater inflow from underground. Typical outputs are evaporation, river outflow, groundwater outflow into aquifers, and water extracted by people for irrigation, industry or homes.
The 15 units taken for irrigation are sustainable here, because the total still balances. Push that figure much higher without anything else changing and the lake starts to shrink.
Aquifers make this easy to miss, because nothing looks different from the surface. The only visible sign is that wells have to be drilled deeper each year.
Sustainable water harvesting
Sustainable harvesting means taking water from a store at a rate that does not exceed the rate of natural replenishment. Adding up all the inputs and outputs is how you work out what that safe rate actually is, so that extraction does not break the steady state.
For groundwater in particular, extraction has to be balanced against the recharge rate. Recharge is slow — water has to infiltrate and percolate down through soil and rock — which is exactly why deforestation and urbanisation make over-extraction worse. They cut off the recharge at the same time as demand is rising.
Link the two halves of this page. Concrete reduces infiltration, which reduces recharge, which reduces the sustainable extraction rate. The city then pumps more, not less. That connection is worth a mark in almost any extended answer here.
Worked examples
EXAM Q1
A reservoir receives 45 units from rainfall, 60 from river inflow and 15 from run-off. It loses 40 to evaporation, 50 to river outflow and 40 to extraction. Is it in steady state? [3]
Step 1: total the inputs45 + 60 + 15 = 120 unitsStep 2: total the outputs40 + 50 + 40 = 130 unitsStep 3: compare
Outputs exceed inputs by 10 units, so the reservoir is losing water.
Not in steady state — a deficit of 10 units, so the level will fallshow both totals; the marks are for the working, not just the verdict
EXAM Q2
Explain why flooding is more likely in an urban area than in a forested one with the same rainfall. [4]
Point 1: impermeable surfaces
Concrete and asphalt prevent infiltration, so rain cannot soak away.
Point 2: run-off increases
Almost all the rainfall becomes surface run-off instead.
Point 3: it arrives faster
Drains are built to remove water quickly, so a large volume reaches rivers in a short time.
Point 4: contrast with the forest
A canopy intercepts and slows rainfall, and soil absorbs it, so water reaches the river slowly and over a longer period.
Less infiltration plus faster run-off means rivers are overwhelmedthe comparison marks need you to actually describe the forest, not just the city
EXAM Q3
Suggest two ways a city could reduce its impact on the hydrological cycle. [2]
Idea 1: restore infiltration
Use permeable paving, green roofs and parks so rain can soak in and recharge groundwater.
Idea 2: slow the water down
Build retention ponds or wetlands that hold storm water and release it gradually.
Both work by putting back what the concrete removed: infiltration and delay“suggest” wants sensible applied ideas — justify each one briefly
💡 Exam tip
Frame every human impact as a change to a named flow: infiltration down, run-off up, evapotranspiration down. Vague answers about “damaging the environment” score nothing.
For calculation questions, always show both totals and then state the difference and its direction.
Use the word impermeable for urban surfaces. It is the term the mark scheme uses.
Remember irrigation can increase evapotranspiration and rainfall — not every human impact reduces things.
Steady state means inputs equal outputs, not that nothing is moving.
For “sustainable harvesting”, say the rate must not exceed natural replenishment or recharge.
⚠ Common mix-up
Saying humans use up water. Water is not destroyed; it is moved, polluted or stored somewhere less useful.
Thinking more run-off means more water available. The opposite — it leaves fast and does not recharge the groundwater you rely on later.
Assuming a steady state is static. Large flows are moving through it constantly.
Ignoring extraction as an output. In many exam diagrams it is the biggest output of all.
Forgetting the pollution angle of irrigation run-off. Fertilisers and pesticides travel with the water.
Treating deforestation and urbanisation as identical. Both cut infiltration, but urbanisation adds drains and the heat island effect, and deforestation adds erosion and lost transpiration.
Up next: What Decides Who Gets Water — why two places with the same rainfall can have completely different water situations.
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