Water: the cycle and the tap
By the end of this lesson you will be able to
- Outline why carbon, nitrogen and water are the three substances every living thing must obtain, and why each moves in a cycle rather than being used up.
- Describe the water cycle — evaporation, transpiration, condensation, precipitation, infiltration and run-off — and explain each step in terms of particles and energy.
- Explain where Botswana’s water comes from: dams, the North–South Carrier, boreholes into aquifers, and the Okavango, and why rationing happens.
- Describe the stages of water treatment — screening, coagulation, sedimentation, filtration and chlorination — and explain the chemistry of coagulation and disinfection.
- Compare water recycling and desalination as ways of making new supply, using energy, cost and consequences for people.
The puzzle
In 2015 Gaborone Dam — the reservoir built to supply the capital — fell to about one per cent of its capacity and effectively ran dry. Taps in the city were rationed; water arrived in tankers and through a 360 km pipeline from the north. Yet the amount of water on the planet had not changed by a single litre: the same water that was missing from the dam was somewhere — in the air, in the sea, in the rock under the Kalahari, in a cloud over Angola. If water is never destroyed, how can a whole city run out of it?
A litre of water is poured onto hot Kalahari sand at midday. Predict, in order, where the water is after one hour, one week and one year — and which of those places a borehole could reach.
Compare your answer
After an hour most has soaked into the sand (infiltration) and some has already evaporated from the hot surface into the air as water vapour. After a week, the water near the surface has all evaporated or been drawn up by roots and released from leaves (transpiration); the vapour has risen, cooled, and may have condensed into a cloud that rained hundreds of kilometres away. After a year, a small fraction has crept downwards through sand and rock into an aquifer — the only part a borehole can reach — while the rest has been round the cycle several times. Water is never used up; it moves. The problem for a city is not the amount of water on Earth but the amount that is fresh, in the right place, at the right time.
The science — structured notes
1.1 Three things every living thing must have — and none of them is ever used up
Strip a living cell down to what it is made of and three substances dominate. Water makes up about two thirds of its mass and is the solvent in which every reaction of life happens. Carbon is the backbone of every biological molecule — sugars, fats, proteins, DNA — because carbon atoms form four strong bonds and chain together. Nitrogen is in every amino acid, so in every protein and enzyme, and in every base of DNA. Plants take carbon from the air as CO₂, nitrogen from the soil as nitrate, and water from the soil through their roots; animals take all three from what they eat and drink.
The Earth has had essentially the same stock of these three since it formed. What keeps life going is that each of them cycles: it is taken up, built into living things, released again by respiration, decomposition or excretion, and taken up once more. A cycle is a set of transformations in balance — as much leaving each store as entering it, over a year or a century. This unit is about the three cycles, what happens when human activity tips one out of balance, and what that does to the climate that all of them depend on.
| Cycle | Main stores | Key transformations | How humans have changed it |
|---|---|---|---|
| Water | Oceans (97 %), ice, groundwater, lakes and rivers, air | Evaporation, condensation, precipitation, infiltration | Dams, boreholes, irrigation, pollution; a warmer air holds more vapour |
| Carbon | Rocks and fossil fuels, oceans, soil, living things, air | Photosynthesis, respiration, combustion, decomposition | Burning fossil fuels and clearing forests: air CO₂ up from 280 to over 420 ppm |
| Nitrogen | Air (78 % N₂), soil, living things | Fixation, nitrification, assimilation, decomposition, denitrification | The Haber process now fixes roughly as much nitrogen each year as all the natural processes on land |
1.2 Why water is a strange substance — and why that matters
A water molecule, H₂O, is small and bent, and its oxygen end carries a slight negative charge while the two hydrogen ends are slightly positive. That makes water polar, and polar molecules attract each other strongly — a link called a hydrogen bond. Almost every unusual property of water follows from that one fact.
| Property | Because… | Consequence for the cycle and for life |
|---|---|---|
| Liquid at room temperature; boils at 100 °C (methane, a similar-sized molecule, boils at −162 °C) | Hydrogen bonds hold the molecules together; a lot of energy is needed to separate them | Water exists as liquid, vapour and ice at Earth temperatures, so it can cycle between them |
| Very high specific heat capacity (4200 J per kg per °C) | Energy goes into loosening hydrogen bonds before the molecules move faster | Oceans warm and cool slowly; coastal climates are mild; your body temperature is stable |
| Large energy needed to evaporate (2.3 MJ per kg) | Every molecule leaving the surface must break free of its neighbours | Evaporation cools — sweat, a wet cloth on a water pot, a dam losing 2 m of depth a year to the sun |
| Dissolves more substances than any other liquid | Polar water molecules pull ions and polar molecules apart and surround them | Carries nutrients to roots and through blood; also carries salts, nitrate and pollutants wherever it goes |
| Ice floats | Hydrogen bonds lock the molecules into an open lattice that is less dense than the liquid | Lakes freeze from the top and life survives beneath; sea ice reflects sunlight (Lesson 8.2) |
1.3 The water cycle, step by step
The cycle is driven by the Sun, which supplies the energy to lift water into the air, and by gravity, which brings it back down. Each step is a change of state or of place, and each has a particle explanation.
The Okavango River brings about 10 cubic kilometres of water a year from the Angolan highlands into the Delta in north-west Botswana. Almost none of it leaves. Explain where it goes.
The Delta spreads the water over some 15 000 km² of shallow channels, lagoons and floodplain in one of the hottest, driest climates on Earth. A huge surface area in dry, warm, windy air means very rapid evaporation, and the papyrus and reeds transpire enormous volumes — together about 95 % of the inflow returns to the air.
Around 2 % soaks into the Kalahari sand and recharges groundwater; the remaining trickle reaches the Boteti River and, in wet years, the Makgadikgadi pans, where it evaporates too.
The salts the river carried in do not evaporate. They are left behind, concentrated in the soil of islands, which is why the Delta’s islands have salt crusts at their centres and why the water in the channels stays fresh: the plants and the islands act as a salt sink. Evaporation transports water and leaves everything dissolved in it behind.
A clay water pot kept in the shade stays cooler than the air around it. Explain, using particles, why the water inside is cooled — and why the pot must be porous for it to work.
Reveal answer
Water seeps through the pores of the unglazed clay to its outer surface and evaporates there. Only the fastest-moving molecules have enough energy to break free from their neighbours, so the ones that leave take more than the average share of energy with them and the average energy of the molecules left behind falls — the water cools. The pot must be porous so that the surface is always damp and evaporation is continuous; a glazed pot has a dry surface, nothing evaporates, and the water simply comes to air temperature. The same physics cools the Okavango and your skin when you sweat.
1.4 Where Botswana’s water comes from
Botswana receives about 450 mm of rain a year on average — over 600 mm in the north-east, under 250 mm in the south-west — almost all of it in summer thunderstorms between November and March. Potential evaporation is around 2000 mm a year, more than four times the rainfall, so most rain returns to the air within days and only a small fraction reaches a river or an aquifer. There are no permanent rivers in the south and east; the Limpopo and its tributaries flow for a few weeks a year. The country’s water comes from four sources.
| Source | Where | What it provides | Limit |
|---|---|---|---|
| Dams on seasonal rivers | Gaborone Dam (capacity about 141 million m³), Letsibogo, Dikgatlhong (about 400 million m³, the largest), Shashe, Ntimbale, Lotsane, Thune | Most of the water for Gaborone and the eastern towns | They fill only in good summers and lose about 2 m a year to evaporation; a run of dry years empties them |
| North–South Carrier (NSC) | A 360 km pipeline from the northern dams to Mmamashia treatment works near Gaborone, opened in 2000; a second pipeline has been under construction to double it | Moves water from the wetter north-east to the capital | Pumping costs energy; a pipe can carry only what the northern dams hold |
| Boreholes into aquifers | Thousands across the country; whole districts and the Kalahari cattle posts depend on them | Roughly half of the water used nationally; almost all rural supply | Recharge is slow — many aquifers are being pumped faster than rain refills them, and some are salty |
| Shared rivers | The Chobe and Zambezi in the north; the Okavango; the Limpopo | Maun, Kasane and irrigation; potential future transfers | Shared with Angola, Namibia, Zambia, Zimbabwe and South Africa, so use is negotiated |
Gaborone Dam holds about 141 million m³ when full. Suppose the greater Gaborone area draws 150 000 m³ a day from it and the dam loses a further 60 000 m³ a day to evaporation, with no inflow. Estimate how long a full dam lasts.
Total loss per day = 150 000 + 60 000 = 210 000 m³.
Days = 141 000 000 m³210 000 m³ per day ≈ 670 days, a little under two years.
So two failed rainy seasons in a row can empty the dam — which is what happened in 2014–2015 — and about 30 % of the loss is evaporation that no one drinks. That is why Gaborone was joined to the northern dams and why deep dams with a small surface are better than shallow wide ones.
During the 2015–2016 rationing, Gaborone’s supply was cut on set days and residents were told to store water. Using the water cycle and the table above, give one reason why the northern dams still had water when Gaborone Dam was empty, and one reason why boreholes near the city could not simply replace the dam.
Reveal answer
The north-east receives more rain (over 600 mm a year against about 540 mm at Gaborone) and its rivers, such as the Shashe, have larger catchments feeding larger dams like Dikgatlhong, so the same drought left them with a smaller fraction lost. Boreholes draw on groundwater that is recharged only by the small fraction of rain that infiltrates past the root zone — a few millimetres a year in this climate — so pumping enough for a city of a quarter of a million people would empty the aquifer in years and could not be sustained; and some of the groundwater near Gaborone is too salty to drink without treatment.
1.5 From dam to tap: treating water
Dam water carries mud, algae, dissolved organic matter and microorganisms, some of which cause disease. A treatment works turns it into water that is clear, safe and pleasant to drink. The stages are the same at Mmamashia, which treats the NSC water for Gaborone, and at every large works in the world.
| Stage | What happens | The chemistry or physics |
|---|---|---|
| 1. Screening | Water passes through metal grids and fine screens | Removes leaves, sticks, plastic and fish — anything large enough to be caught physically |
| 2. Coagulation and flocculation | A coagulant, usually aluminium sulfate (alum), Al₂(SO₄)₃, is stirred in rapidly, then the water is stirred slowly | Fine clay and bacteria stay suspended because their surfaces carry negative charges and repel each other. Al³⁺ ions neutralise the charges and react with water to form a sticky precipitate of aluminium hydroxide, Al(OH)₃; slow stirring lets the particles collide and grow into visible clumps called flocs |
| 3. Sedimentation | Water flows very slowly through wide tanks for a few hours | The flocs are denser than water and settle to the floor as sludge, taking most of the mud and microbes with them |
| 4. Filtration | Water trickles down through a bed of sand about a metre deep over gravel | Remaining small flocs are trapped in the spaces between grains; a film of microbes on the sand also digests organic matter. Filters are cleaned by backwashing |
| 5. Disinfection (chlorination) | Chlorine gas or sodium hypochlorite solution is dosed so that about 0.5 mg per litre remains | Cl₂ + H₂O → HCl + HOCl. Hypochlorous acid passes through the cell membranes of bacteria and viruses and destroys their enzymes. The residual keeps the water safe in the pipes to your tap |
| 6. Adjustment | Lime may be added; fluoride is added in some countries | Lime raises the pH back to about 7–8 so that the water does not corrode pipes; fluoride at 1 mg per litre strengthens tooth enamel |
A small village treatment plant runs out of alum and sends water straight from sedimentation to chlorination. The water is chlorinated to the normal dose. Explain two reasons why the water may still be unsafe.
Reveal answer
Without coagulation the fine clay and the bacteria attached to it stay suspended, so the water is cloudy (turbid). First, microbes hidden inside clay particles are shielded from the hypochlorous acid, so the normal dose does not reach them all. Second, chlorine reacts with the organic matter that should have been removed, using up the dose so that little residual is left to protect the water in the pipes — and forming unwanted chlorinated by-products. Turbidity is measured at every works precisely because it predicts whether disinfection will work.
1.6 Making water twice: recycling and desalination
Once a dam is empty and the aquifers are falling, a country has two ways to make new supply. Both are chemistry and physics; both cost energy; both have consequences.
| Recycling treated wastewater | Desalinating sea or brackish water | |
|---|---|---|
| Source | A city’s own sewage — grows with the city and does not depend on rain | The sea (Botswana has no coast) or salty boreholes |
| Energy per m³ | About 0.5–1 kWh for irrigation quality; 1–2 kWh for drinking quality | 3–4 kWh by reverse osmosis; more by distillation |
| Waste produced | Sludge, which can be digested for biogas and used as fertiliser | Hot, salty brine that harms sea life if returned carelessly; for inland plants, brine with nowhere to go |
| Public reaction | Many people are uneasy about drinking “recycled sewage” even when it is cleaner than dam water | Generally accepted; the water tastes flat unless minerals are added back |
| Best use in Botswana | Irrigation and industry now; drinking water where Windhoek’s model is followed | Small plants for salty village boreholes |
“Water that has been through a sewage works and then a reverse-osmosis membrane can be cleaner than the water in Gaborone Dam.”
Reveal answer
True. A reverse-osmosis membrane rejects almost everything except water molecules — salts, bacteria, viruses, most dissolved organic molecules — so the product has fewer dissolved substances than dam water, which carries salts, algae and everything the catchment washed in. What makes people hesitate is the history of the water, not its chemistry; every molecule of water on Earth has been through countless living things already. The scientific question is whether the treatment is monitored well enough that a failure is caught before the water reaches a tap.
A student writes: “The water cycle is evaporation, then clouds, then rain. Water evaporates from the sea because the Sun heats it to 100 °C. In Botswana the water cycle does not really work because there are no rivers.”
Reveal answer
Three errors. Evaporation does not need boiling: at any temperature the fastest molecules at the surface have enough energy to escape, which is why washing dries on a cold day and dams evaporate at 25 °C. The cycle includes transpiration, infiltration, groundwater and run-off, not just the three steps named. And Botswana’s cycle works vigorously — the problem is that evaporation returns almost all the rain to the air before it can collect in rivers; the missing rivers are the evidence of a working cycle in a hot, dry climate, not of a broken one.
Try it — what evaporation leaves behind
At Sua Pan in the Makgadikgadi, the country’s largest salt works pumps brine from beneath the pan and lets the sun evaporate it in shallow ponds until sodium chloride and then sodium carbonate crystallise out. The cycle moves the water and abandons the salt — and how much salt water can hold depends on its temperature. Measure the solubility of three solutes at 10–80 °C: which one does a hot day help most, and which hardly cares?
Suppose your sodium chloride results were 36.0, 36.3, 36.9, 37.5 and 38.1 g per 100 g at 10, 20, 40, 60 and 80 °C, and potassium nitrate gave 23, 36, 68, 110 and 162 g.
(a) Compare the two relationships in one sentence each, quoting values.
(b) A brine pond at Sua Pan holds water that is nearly saturated with sodium chloride. Explain, using the cycle and your data, why crystals form mainly because water evaporates rather than because the pond cools at night.
(c) Explain why a dam that loses 2 m of depth a year to evaporation slowly becomes saltier, and what that means for the groundwater that leaks from it.
Reveal answer
(a) Sodium chloride solubility rises only slightly with temperature, from 36.0 g at 10 °C to 38.1 g at 80 °C — about 6 % over 70 °C. Potassium nitrate rises steeply and by more each step, from 23 g at 10 °C to 162 g at 80 °C, seven times as much; neither is proportional to temperature (the line does not pass through the origin).
(b) Cooling from a hot day to a cool night changes the solubility of sodium chloride by less than 1 g per 100 g, so cooling can drop out only a trickle of crystals. Evaporation removes the water itself: when half the water has left, the remaining solution holds twice the salt per 100 g of water, far above 36 g, so most of the salt must crystallise. That is why the ponds are shallow and wide — maximum surface for the Sun — and why the season with the driest, warmest air is the productive one.
(c) Only water molecules evaporate; the dissolved ions stay behind, so each year the same salt is dissolved in less water and the concentration rises. In a dam that is regularly flushed by inflow the effect is small, but in a drought the water becomes noticeably more mineralised — and the water that seeps from the dam floor into the aquifer carries that raised concentration with it, slowly making the groundwater saltier too. The same process, over centuries, made the salt pans.
Common mistakes to avoid
- “Water evaporates when it boils” — evaporation happens at every temperature from the surface; boiling happens throughout the liquid at 100 °C.
- Leaving out transpiration and infiltration — over land, plants return more water to the air than open water does, and infiltration is the only route to a borehole.
- “Chlorine cleans the water” — it disinfects; coagulation, sedimentation and filtration remove the solids.
- “Alum filters the water” — alum coagulates: Al³⁺ neutralises the charge on particles and forms Al(OH)₃ flocs that settle. Filtration is the sand bed.
- “Rain is pure water” — evaporation leaves salts behind, but rain dissolves CO₂ and other gases on the way down, so it is slightly acidic (pH about 5.6).
- “Botswana has a water shortage because it has no water” — it has rain and aquifers; it has a shortage because evaporation exceeds rainfall fourfold and demand is concentrated in a few towns.
How we found out — rain gauges on the Seine and a pump handle in Soho
For two thousand years scholars believed rain could not possibly supply the rivers. Aristotle argued that rivers were fed by underground caverns where air turned into water; the idea was still taught in 1670. Then a French civil servant, Pierre Perrault, did what nobody had bothered to do: he measured. For three years he recorded the rainfall over the upper catchment of the Seine and compared it with the flow he measured in the river. In 1674 he published the result: the rain that fell was about six times the water the river carried away. Rain was more than enough — the puzzle was where the rest went, and his colleague Edmé Mariotte showed that it soaked into the ground and reappeared in springs. A few years later the English astronomer Edmond Halley measured the rate of evaporation from a pan of salt water at the temperature of the Mediterranean summer and calculated that the sea lost more to the air each day than all its rivers brought in. Evaporation, rain, infiltration and rivers were shown, with numbers, to be one closed loop. The water cycle was not discovered; it was measured into existence.
The tap end of the story turned on a single map. In the summer of 1854, cholera killed about 500 people in a few streets of Soho, London, in ten days, and 616 in all. The accepted explanation was “miasma” — bad air. The physician John Snow did not believe it. He walked the streets, marked every death on a map, and found them clustered around the public water pump in Broad Street; the brewery next door, whose workers drank beer, had no cases, and a woman who had the pump water carried to her house miles away had died. He persuaded the parish to remove the pump handle. It was later found that a cesspit was leaking into the well. Snow’s map is the founding document of epidemiology, and it made the case that water carried disease three decades before the cholera bacterium was seen under a microscope. Continuous chlorination of a public supply followed in 1908, in Jersey City, after a court case in which the water company’s chemist, John Leal, proved that a few parts per million of chlorine cut deaths from typhoid — a treatment that had to be argued for before it could be trusted, and that now protects every tap in Gaborone.
Perrault found six times more rain than river flow, and Halley found more evaporation than river inflow. Explain why both results were needed to establish the water cycle, and what each ruled out.
Reveal answer
Perrault’s measurement showed that rain alone could supply the river with plenty to spare, which ruled out the need for Aristotle’s underground caverns — the source of rivers was the sky. But it left a gap: if rain was six times the river, where did five-sixths go? Halley’s measurement supplied the other half of the loop: the sea loses more water to the air than rivers return, so the air must be carrying water back over the land as rain. Together they closed the circuit — sea to air to rain to ground and river to sea — with no store gaining or losing over a year. One measurement showed the supply was sufficient; the other showed the return was real; the balance between them is what makes it a cycle.
Where it is used now
- Water Utilities Corporation. Every large town in Botswana is supplied through the stages in section 1.5; Mmamashia treats the NSC water for Gaborone, and the residual chlorine you can sometimes smell is the last stage working.
- Boreholes and cattle posts. Rural Botswana runs on groundwater; the Department of Water and Sanitation logs borehole levels because falling levels mean an aquifer is being mined faster than infiltration refills it. Solar pumps have replaced many diesel engines.
- Glen Valley and Windhoek. Gaborone’s treated wastewater irrigates vegetables; Windhoek drinks its own recycled water. Both are studied by cities from Cape Town to Los Angeles facing the same arithmetic.
- Satellites watching groundwater. The GRACE-FO satellite pair measures tiny changes in the Earth’s gravity as water moves, and maps the drawdown of aquifers across southern Africa month by month — the water cycle audited from orbit.
- Sua Pan. Botswana Ash evaporates brine to produce soda ash and salt for the whole region — an industry that is the water cycle turned into a factory.
- Cape Town 2018. A city of four million came within weeks of “Day Zero”, when taps would have been shut; rationing to 50 litres a person a day, and then rain, averted it. Gaborone had been there three years earlier.
Science now — and the scientist you need to be
What is happening in 2026
The water cycle is speeding up. A warmer atmosphere holds about 7 % more water vapour for every degree of warming, so evaporation is faster, downpours are heavier and droughts bite harder — the same cycle with more energy in it, which is why southern Africa now swings between failed seasons and floods. Water utilities have responded with data: smart meters that flag leaks by the hour, satellite and drone surveys of dam levels, and machine-learning models that forecast a reservoir’s level months ahead from sea-surface temperatures in the Pacific. Reverse-osmosis membranes have become cheaper and thinner, so the energy to desalinate a cubic metre has halved since 2000, and solar-powered desalination of brackish borehole water is being trialled in villages that previously trucked water in. Direct potable reuse — Windhoek’s idea — is being adopted from Texas to Singapore. And Gaborone’s second pipeline from the north, together with plans to draw on the Chobe and Zambezi, shows the direction of travel: connect the wet places to the dry ones, waste less, and reuse everything. None of it changes the amount of water on the planet; all of it changes where it is when you turn on the tap.
The mindshift
What you need to be able to do
Criterion A questions on this lesson ask you to explain — evaporation at particle level, coagulation with the ions named, why chlorine is dosed to leave a residual — not to list stages. Every explanation should reach a particle, an ion or an energy transfer. Criterion D questions in this unit will hand you a water decision (a new dam, a recycling plant, a desalination scheme) and expect the consequences for named groups across several categories and a justified judgement — the Think task in Lesson 8.4 and the unit test rehearse it.
See an example
Meter readings 7 days apart: 2 847 and 2 866 m³ — 19 m³ in a week for five people, which is 19 000 ÷ 7 ÷ 5 ≈ 540 litres a person a day, about ten times the Cape Town ration; the garden tap and a leaking cistern accounted for most of it. The AI’s first answer said “filters and chlorine remove bacteria”; asked to separate the steps, it explained that coagulation and settling remove most microbes with the solids, sand filtration traps more, and chlorine kills the remainder and protects the pipes — the version that earns marks.
Key terms
| Cycle | A set of transformations in which a substance moves between stores and returns, with the total unchanged; in balance when inflows equal outflows. |
| Evaporation | The escape of the fastest-moving particles from the surface of a liquid at any temperature; it cools the liquid left behind. |
| Transpiration | The loss of water vapour from plant leaves through stomata; with evaporation, evapotranspiration. |
| Condensation | Vapour turning back to liquid as it cools, releasing the energy that evaporation absorbed. |
| Precipitation | Water falling from clouds as rain, hail or snow. |
| Infiltration | Rain soaking into the ground; the part that passes below the roots recharges groundwater. |
| Aquifer | A body of porous or cracked rock holding groundwater, reached by boreholes. |
| Coagulation | Adding Al³⁺ (alum) so that charged fine particles stop repelling and clump into flocs of Al(OH)₃. |
| Sedimentation | Letting flocs settle out of slow-moving water. |
| Chlorination | Adding chlorine, which forms hypochlorous acid, HOCl, to kill microorganisms and leave a protective residual. |
| Reverse osmosis | Forcing water through a membrane under high pressure so that water passes but dissolved ions do not; used in desalination and water recycling. |
| Residence time | The average time a water molecule spends in a store — days in the air, years in a dam, millennia in a deep aquifer. |
Think like a scientist
Criterion A A village in the Kgalagadi District draws water from a borehole 90 m deep. The water is clear but slightly salty and is chlorinated in a tank before distribution. During a drought the borehole level falls by 4 m in a year, and a proposal is made to pipe water instead from a new small dam on a seasonal river 40 km away, which would need a full treatment works. Answer using the science of this lesson.
(a) Explain, in terms of the water cycle, why the borehole level falls during a drought even though the village pumps the same volume each year. [3]
(b) Explain why borehole water needs only chlorination while dam water needs coagulation, sedimentation and filtration before chlorination. [4]
(c) Describe what happens, at the level of ions and particles, when aluminium sulfate is added to the dam water, and state why the water is stirred slowly afterwards. [3]
(d) Explain why the borehole water is salty while the dam water is not, using evaporation and dissolving. [2]
(e) Compare the dam and the borehole as long-term sources for the village, using evaporation and recharge, in two or three sentences. [3]
Model answer
(a) The aquifer is refilled only by the small fraction of rain that infiltrates below the root zone — in the Kgalagadi, perhaps a few millimetres out of 300. In a drought, rainfall falls and evaporation from the hot surface removes almost all of it before it can infiltrate, so recharge drops close to zero. Pumping continues at the same rate, so outflow now exceeds inflow and the stored water — and the level in the borehole — falls; the aquifer is being mined rather than used in balance.
(b) Borehole water has already been filtered by passing slowly through tens of metres of sand and rock, which trap suspended particles and most microorganisms, and it has never been exposed to sunlight, so it carries no algae; it is clear, so only disinfection is needed to kill any remaining bacteria and protect the pipes. Dam water is open to the catchment and the air: run-off carries mud, animal dung and bacteria into it, sunlight grows algae, and these fine particles stay suspended because their charged surfaces repel each other. Chlorine cannot remove solids, and microbes hidden in particles are shielded from it, so the solids must be coagulated, settled and filtered out first — otherwise the water is cloudy, the chlorine is used up by organic matter, and disinfection fails.
(c) Aluminium sulfate dissolves to give Al³⁺ ions; the positive ions are attracted to the negatively charged surfaces of clay particles and bacteria and neutralise their charge, so the particles no longer repel one another. At the same time Al³⁺ reacts with water to form a sticky, insoluble precipitate of aluminium hydroxide, Al(OH)₃, which traps particles as it forms. Slow stirring makes the neutralised particles and precipitate collide gently and grow into large flocs heavy enough to settle; fast stirring would break the fragile flocs apart.
(d) Water infiltrating through the Kalahari sand dissolves salts from the sand and rock over centuries, and where groundwater is close enough to the surface for some to evaporate, only water molecules leave and the salts remain, concentrating the solution. Dam water is recent rain, which is nearly pure because evaporation left the salts behind in the sea and soil; it has had only weeks to dissolve anything from the catchment.
(e) The dam is recharged in every good summer but loses around 2 m of depth a year to evaporation and can empty in two dry seasons, so it delivers a large volume in wet years and nothing in a long drought; the borehole loses nothing to evaporation and gives a steady supply, but its recharge is so slow that pumping above the recharge rate slowly exhausts it. The most resilient arrangement uses the dam when it has water and rests the borehole so that the aquifer recovers, keeping the borehole as the drought reserve.
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
Explain, in terms of particles, why a wet school shirt dries faster on a windy day than on a still day at the same temperature.
Reveal answer
Water molecules at the surface of the wet cloth that have enough energy escape into the air as vapour [1]; on a still day the air next to the cloth fills with vapour and some molecules return to the liquid, slowing net evaporation [1]; wind sweeps the vapour away and replaces it with drier air, so more molecules leave than return and the cloth dries faster [1].
State two differences between a dam and an aquifer as stores of water.
Reveal answer
Any two [1 each]: a dam is open to the air and loses water by evaporation, an aquifer does not; a dam refills in one wet season, an aquifer recharges slowly over years; dam water carries suspended solids and microbes and needs full treatment, aquifer water is naturally filtered and usually needs only disinfection; groundwater is often saltier.
A student writes: “At the treatment works the water is filtered through sand to remove the bacteria, then chlorine is added to remove the mud, then alum makes it taste better.” Identify three errors.
Reveal answer
Sand filtration removes small flocs and particles, not all bacteria — chlorination is what kills the microorganisms [1]; chlorine removes nothing solid, it disinfects [1]; alum is the coagulant added before sedimentation to make particles clump into flocs, not a flavouring, and the correct order is coagulation, sedimentation, filtration, chlorination [1].
Write the equation for chlorine reacting with water, name the substance that kills bacteria, and explain why a residual of about 0.5 mg per litre is left in the water leaving the works.
Reveal answer
Cl₂ + H₂O → HCl + HOCl [1]; hypochlorous acid, HOCl, which passes through microbial cell membranes and destroys their enzymes [1]; the residual continues to kill any bacteria that enter the pipes through leaks or repairs between the works and the tap, so the water is still safe when it arrives [1].
True or false, with a reason: “Desalination by reverse osmosis is a good solution to Gaborone’s water shortage.”
Reveal answer
False, or at least not on its own [1]: Gaborone is over 1000 km from the sea, so the only water to desalinate is brackish groundwater, and reverse osmosis needs 3–4 kWh per cubic metre and leaves a brine with nowhere to go inland; recycling wastewater, reducing leaks and the northern pipeline supply more water for less energy [1].
A warmer climate increases both rainfall and evaporation over Botswana. Explain why the effect on dam storage could still be negative, and name one design change that reduces the loss.
Reveal answer
Storage depends on inflow minus evaporation, not on rainfall alone [1]; evaporation from an open dam rises with temperature every day of the year, while extra rain arrives as heavier storms that mostly run off or fall in the wrong place, so the loss can grow faster than the gain [1]; building deeper, narrower dams (less surface per cubic metre), or storing water underground by recharging aquifers, reduces evaporation [1].
Summary
- Carbon, nitrogen and water are never used up; they cycle through stores by transformations that are in balance until something tips them.
- The water cycle is driven by the Sun and gravity: evaporation and transpiration lift water, condensation and precipitation return it, infiltration feeds aquifers and run-off feeds rivers.
- Botswana’s potential evaporation is about four times its rainfall, so most rain returns to the air within days; dams, the North–South Carrier, boreholes and shared rivers supply the small fraction that is left.
- Treatment is a sequence — screen, coagulate with Al³⁺, settle, filter, chlorinate to leave HOCl — each stage removing what the previous one could not.
- Recycling and desalination make new supply at an energy cost; purity is a matter of chemistry and monitoring, not of where the water has been.
Next time → Carbon: the cycle and the greenhouse — how CO₂ moves between air, life, sea and rock, and why a gas at 0.04 % of the air sets the temperature of the planet.