Water Supply Factors

Borrowed, not consumed

Food is eaten and fuel is burned. Water is neither. Almost every litre a household, a farm or a factory uses goes back into the system afterwards, which means water is borrowed rather than consumed, and the questions that decide whether a place has enough are not about how much is taken but about what condition it is returned in and how fast the store refills. That one idea organises the whole topic. Over-abstraction draws the store down faster than rain and rivers can put it back, so the level falls; pollution is returning it in a state nobody can use, so the store is there and unavailable. They look like two separate items on a list of factors and they are the same problem seen twice: a failure of the return. Something like four fifths of the wastewater produced worldwide is returned untreated or barely treated, and around 2 billion people drink from a source contaminated with human waste. The second thing to hold on to is that the constraint is timing and place rather than total. About 97 per cent of the world's water is saline and about two thirds of the fresh remainder is locked in ice, so the accessible share is small to begin with, and it is distributed nothing like the population. A country with heavy annual rainfall can still be insecure if it arrives in four months and there is nowhere to hold it, which is why an annual average is close to useless on its own. Geographers use thresholds for this: below 1,700 cubic metres per person per year a country counts as water stressed, and below 1,000 as water scarce.

Six terms, and two of them are one idea

Terms three and four are the ones this module is built on, because each of them breaks the same loop at a different point.

How a lake disappears

Six stages describe what happened to the Aral Sea after its feeder rivers were diverted from the 1960s. Put them in order.

• The two rivers feeding the lake are diverted to irrigate cotton across the surrounding plains
• Inflow falls far below the rate at which water evaporates from the lake surface
• The shoreline retreats year after year and the lake splits into separate basins
• The remaining water becomes saltier as it shrinks, and the fishery collapses
• Exposed lake bed dries into a salt and chemical dust that the wind carries over nearby farmland and towns
• By 2014 the eastern basin has dried out completely, leaving ships stranded on sand

Why group pollution with over-abstraction?

The specification lists pollution and over-abstraction as separate factors affecting water supply. Which reasoning treats them most usefully?

• That the two of them interrupt one cycle in different places. Water is borrowed and sent back, so taking it faster than it is replenished shrinks the store, while sending it back contaminated leaves the store in place and unusable. Treating them as one mechanism explains why a place beside a large river can still be short of water.
• They are effectively the same thing, since both reduce the amount of water in a river.
• Pollution is the more serious of the two and should be dealt with first.
• Both are human factors rather than physical ones, so they belong together.

What limits supply, and what shortage does

Factors sit in the first two columns and consequences in the third, and that third column is where a top answer lives, because the consequences escalate rather than sitting side by side.

Match each situation to what it explains

Follow the water back

The strongest habit on this topic is to follow the water back to the store it came from and ask what happens to it afterwards. Almost every factor on the specification list is an answer to one of those two questions, and organising an answer that way turns a list into an explanation. It also stops the commonest error, which is treating an annual rainfall total as a measure of water security. It is not. A place that receives most of its rain in a few months and has no aquifer beneath it and no reservoir above it can be dry for half the year at any total you like, and a place with modest rainfall over porous rock can be comfortable. Timing, storage and geology decide it, and saying so is worth more than any figure. When you write the impacts, show them escalating rather than listing them. Unsafe water makes people ill first, which is where the deaths are; the shortfall then reaches irrigation, which is the largest single use of fresh water anywhere; then industry, which quietly needs a great deal of it; and on a shared river it finally becomes a matter between governments. One chain written properly beats four impacts named. Two cautions. Use water stress and water scarcity as the measured thresholds they are, not as synonyms for dry. And keep the register honest: nobody is short of water through carelessness, and an answer that explains the circumstance rather than blaming the people in it is both fairer and more accurate.

Complete the water supply figures

About _____ per cent of the world's water is saline, and roughly two thirds of what remains is locked up in ice, so the accessible share is small before distribution is considered at all. A country holding less than _____ cubic metres per person each year is classed as water stressed, and below 1,000 as water scarce. The condition water comes back in matters as much as the amount taken, and roughly _____ per cent of the world's wastewater is returned without adequate treatment, which is part of why around _____ billion people draw their drinking water from a source contaminated with human waste. Diversion can empty a lake altogether: the eastern basin of the Aral Sea had dried out completely by _____.

Spot the three sound water supply statements

Select the THREE statements that could stand as they are in an answer.

• Taking water faster than it is replenished and returning it contaminated are the same failure seen twice, since one removes the store and the other removes its usability.
• An annual rainfall total says little on its own, because rain concentrated into a few months over impermeable rock leaves a long dry season that only storage can bridge.
• Water stress and scarcity are measured thresholds, at roughly 1,700 and 1,000 cubic metres per person a year, so population growth alone can move a country across a line.
• Countries with high annual rainfall are water secure, and countries with low rainfall are not.
• Water used by industry and agriculture is consumed and permanently lost to the system.
• Water shortages in poorer countries are largely the result of people wasting what they have.

A 6-mark factors answer, marked

Explain the factors that can reduce the water available to a region. [6 marks] The first factor is physical and it is about timing rather than totals. A region can receive a large amount of rain each year and still run short if most of it falls within a few months, because the dry season then has to be bridged by storage. Whether that storage exists is partly a question of geology: porous rock such as chalk or sandstone holds the wet season underground and releases it slowly, while impermeable rock sends identical rainfall straight off the surface into rivers and out to sea. Two regions with the same rainfall figure can therefore be in completely different positions, which is why an annual average on its own explains very little. The second factor is over-abstraction, which is taking water out of a river or aquifer faster than it is replenished. This is the quiet one, because nothing appears to be wrong until a well that has served a farm for generations comes up dry. The High Plains aquifer under the American Midwest has been pumped for irrigation since the 1940s and parts of it are down by tens of metres, and the Colorado, over-allocated by a compact signed as long ago as 1922, has only rarely reached the sea since the 1960s. In each case the water is not being used so much as the store is being spent. The third is pollution, and it is worth seeing as the mirror image of the second rather than as an unrelated item. Sewage, industrial discharge and fertiliser run-off return water to the system in a condition nobody can use, so the volume in the river is unchanged and the supply is gone. Around four fifths of the wastewater produced worldwide is returned untreated or barely treated, which is why a town beside a large river can still have nothing safe to drink. What earned the marks: three genuinely separate factors instead of one idea repeated; a mechanism traced for each rather than a name given; figures and dates attached; the annual-total misconception addressed directly; and pollution and over-abstraction connected as two forms of the same failure rather than listed apart.

Find the two that would cost marks

Five sentences from answers about water supply. Two would lose credit for the argument they make rather than for being factually wrong. Tap those TWO.

• This region receives plenty of rain each year, so it does not have a water supply problem.
• Pumping has run ahead of replenishment for decades, so the aquifer is being spent rather than used.
• People in these areas do not look after the water they have, which is why they run short of it.
• The river still carries the same volume, but what returns to it is contaminated, so the supply has gone even though the water has not.
• Below about 1,700 cubic metres per person a year a country is classed as water stressed, which is a measured threshold rather than a description.

Build the mechanism sentence

Assemble the sentence that explains why two factors are really one.

Four shortages, four explanations

Each situation is a real water problem. Work out which factor is operating before saying anything about solutions.

• A district records high annual rainfall and runs short of water every year from February to May.
• Wells across an irrigated plain have been deepened three times in forty years, and rainfall has not changed.
• A settlement on the bank of a major river has high rates of diarrhoeal illness among children.
• A country downstream on a shared river sees the flow it receives change after a dam is built upstream in another state.

Explain the factors

Explain the factors affecting water supply, and the impacts of water insecurity. [9 marks]

• Water is borrowed and returned, so condition and rate of return decide availability
• Timing, storage and geology matter more than annual rainfall totals
• Water stress and water scarcity are figures rather than adjectives
• Impacts: health, food, industry, politics
• Explain circumstances rather than blaming anybody