This article looks at the many ways in which water is important for the future sustainability of the community of life on our planet.
Where did it come from?
Water is abundant in space: it is made up of hydrogen created in the Big Bang and oxygen released from dying stars.
Earth has vast oceans today, but our planet was a dry rock when it first formed . . . or so we assume. The planets of our solar system were created around 4.6 billion years ago from clumps of rocks spinning around the Sun.
Earth, it was long believed, was moulded from rocks that came from the inner solar system where the fierce heat of the Sun would have boiled away any water. We do not know for sure. Perhaps the Earth was actually born ‘wet’, made of meteorites that contain the building blocks of water?
The global water cycle
Apart from being a component of living organisms, water has a major influence on climate, the global carbon cycle and other biogeochemical cycles.
The hydrosphere is the Earth’s total supply ofwater and the cycling of water between theatmosphere, oceans, waterbodies and biosphere is called the global water cycle or hydrological cycle.
Salty ocean water makes up 97.5% of all theEarth’s water. Of the remaining 2.5%freshwater component, the majority, about 40–80%, is locked up in polar ice (most in Antarctica) and underground water, mostly aquifers.
The volume of water in aquifers isunknown but estimated at about half that in the polar caps. Of the water in soil, about halfis estimated to be in the root zone. Only about0.27% of all fresh water is found in lakes,rivers and wetlands and this is the main sourceof water used by humans. The annual volumes of the water flows between the various watersinks is summarised in Figure 5.2 whichincludes estimates of the residence time forwater in each sink. Greatest rainfall occurs inthe tropics but high temperatures here meanthat evapotranspiration is also high.
Global water use
From the 1950s, global water withdrawal for human consumption has escalated as a
result of scientific and technological developments, notably the increase in irrigated land, growth in industrial and powersectors, and intensive dam construction.
Irrigation using dams and extraction from river systems leads to fragmentation of the
catchment areas and their associated ecosystems. This has altered the water cycle of
rivers and lakes, affected their water quality and potential as a human resource, and
altered the global water budget. Wetlands, inland water bodies and fossil water aquifers
are drying up and renewable aquifers are being drawn down faster than they replenish.
Chemical contamination of groundwater, lakes, rivers and the oceans is threatening the
quality of the water supply in many parts of the world. Six major threats to the biology of global river systems can be listed: dams and infrastructure,excessive water extraction, climate change, invasive species, overfishing and pollution. Freshwater ecosystems are among the most
species rich on Earth so species extinctionssoon follow water depletion.
About 70% of global water use is divertedfrom the environment to Agriculture where,
in much of the world, only about 10% reaches the crops. More efficient use of
water includes: reducing evaporation; more efficient watering systems; developing
agriculture away from sites with little water; and reviving old reservoirs and local water collecting are as rather than depending on large centralised water supplies.
Water shortages, especially those related to food security, can lead to economic and
political tensions. In many regions there is competition between urban and rural water
use and potential for conflict when systems are shared by two or more nations especially
where ‘downstream’ countries must use the water left by ‘upstream’ countries. For
example, the Nile is shared by Ethiopia, Egyptand Sudan; the Ganges by Pakistan, India and Bangladesh; Tigris-Euphrates by Turkey, Syria, and Iraq; Mekong by China, Vietnam, Laos and Cambodia. indicates those are as with the greatest unsustainable water
consumption. The map shows freshwater withdrawals as a percentage of a country’s
annual renewable water resources in 2001. If withdrawals exceed a threshold (generally
reckoned at about 20–40%), ecosystems will be put under stress. Many countries already
exceed this threshold with some withdrawing more than 100% of their annual renewable
resources. This is only possible by withdrawingfossil water from underground aquifers, a
resource that can only be used once. Largerivers such as the Nile, Huang He (Yellow),
Colorado, Amu Darya and Syr Darya are oftenso depleted by withdrawals for irrigation that in dry periods, they fail to reach the sea.The effect of human activity on the water
cycle can be explained in terms of the‘colour’ of the water. Humans have altered
the global balance between harvestablesurface and groundwater (blue water), nonharvestable
soil moisture (green water) andwater produced by evapotranspiration (white
water). Most of the world’s crops are grownusing natural rainfall. Only 15% are
produced using blue water for irrigation.Global water conservation is about harvesting
more blue water from rainfall, recycling asmuch of this as possible (lilac water), and
using green water more efficiently. Thisprinciple is equally applicable to urban
landscapes. Until recently water managementand monitoring has concentrated on blue
irrigation water. But effective management ofgreen water has major implications for global
food production. Green water productivitycan be improved by increasing rain
infiltration and decreasing run-off. We shallconsider four issues relating to global water
sustainability: deforestation, urbanisation,climate change and world trade.
Changes in one part of the global water cyclecan affect situations large distances away.
Deforestation especially has been implicated in quite large-scale climate modification.
Forests are climate regulators. Dense tropical rainforest in particular absorbs incoming
radiation, keeping the air cool. Trees can absorb and store large volumes of moisture,
up to two-thirds of the available water in some regions. This stored water is more than
just recycled rainwater; it is the way ecosystems’ dissipate the large quantities of
heat collected and stored each day. Through the moisture released by evapotranspiration,
tropical rainforests redistribute absorbed heat. Most of the world’s heat redistribution
occurs through ocean currents but theatmosphere also spreads heat around the
globe and tropical rainfall drives thisprocess. The intense heat energy needed to
convert water to vapour by evaporation isreleased back into the atmosphere when the
water vapour condenses into clouds and rain.About 75% of the energy that drives
atmospheric circulation comes from the heatreleased during tropical rainfall.
A large forest such as that in the AmazonRiver Basin has global importance so it is
hardly surprising that the Amazon’s cloud and rainfall characteristics have led to the
nickname ‘the green ocean.’ During the past15 years more than 494 000 km2 of forest have
been cleared from the Amazon Basin with thecurrent rate being about 20–200 km2 each
year. Deforestation alters the exchanges of notonly water but also carbon, and energy with the atmosphere, cycles that, even now, we onlypartially understand. Indonesia and Brazil together contribute about 10% of globalgreenhouse gases because of their clearance of tropical forest.
More people concentrated in urban environments increases the demand for
domestic and industrial water. As surface water is depleted the search for new supplies
moves to underground aquifers and the transfer of surface water large distances away
from its source. Following the model of large, centralised water supplies, vast multibillion
dollar water re-location engineering projects are underway or planned to divert water from
regions of high rainfall to those of low rainfall: for China (Yangtze diversion of water
from wet south to dry north), India (dams and canals to link 14 rivers that drain the
Himalaya, transporting water to the dry south), North America (north-western
Canadian Rockies to Denver and Mexico). As environmental flows decline, apart from
displaced populations there is: transport of pollution, land degradation in the areas
where water has been sourced, increase in soil salinity, decline in freshwater fisheries, and
the destabilisation of ecosystems. A quarter of the world now obtains its water
from deep aquifers. ‘Closed’ aquifers contain ‘fossil water’ that may have been stored there
for thousands or millions of years, and this water can only be used once – it is not being
replaced. India’s aquifers are being used faster than they can be replenished. The
overuse of underground aquifers (e.g. Ogalalla Aquifer under six states in the USA)
has lead to subsidence, sink holes and water intrusion. From China to Iran, and Indonesia
to Pakistan, rivers are running dry through increasing extraction and these impacts are sometimes being enhanced by the effects of climate change.
Climatologists are currently researching how climate change will alter weather patterns, rainfall and water regimes – and much remains to be done. However, a number of general trends have been established.
In Australia, as a result of climate change, rainfall is expected to increase in the northwest and decrease in the south-east. This will affect both ground and surface water supplies and therefore how much water can be harvested. Factors linked to rainfall include surface wetness, reflectivity, also vegetation quantity and species composition, which in turn affect evapotranspiration and cloud formation, and therefore rainfall .
Water and trade
Globally, agriculture accounts for about two thirds of all water use but the proportions of water allocated to agriculture and industry vary considerably from country to country.
In Australia the proportion of total agricultural water use for food eaten within the country is about 21%. When the water embodied in exported agricultural products is included the total use rises to 67% (i.e. 46% of Australia’s agricultural water use is embodied in exported food).
Global trade tends to distribute resources from areas of abundance to areas of shortage, so it makes sense for ‘dry’ countries to import water-intense products. Globally, about 16% of the world’s blue and greenwater is embodied in exports. However, 50–70% of the world’s blue (harvested) wateris used to produce food for export. Most significantly for the environment, by importing food, a country is exporting the environmental problems associated with its production. Green water represents the largest share of ‘virtual’ (embodied) water in the international trade of agricultural commodities (mostly embodied in maize, soybean and wheat exports from the USA, Canada, Australia and Argentina) with exports going from highly productive rainfed rich countries towards generally bluewater based ones. Green water flows have rarely been estimated even though they can
A redistribution of rainfall and water patterns:
• More intense rainfall with more cyclones andfloods increasing run-off while at the same
time reducing infiltration.• When the climate is dry, small changes in temperature and rainfall could cause relativelylarge changes in run-off.
• Arid and semi-arid regions will therefore beparticularly sensitive to reduced rainfall and to increased evaporation and plant transpiration. Changes at the surface due to changes in the amount and timing of rainfall include:
• Recharging of groundwater supplies and aquifers.
• Changes in water quality.
• Changes in run-off, groundwater flows and evaporation, and therefore changes to dependent natural ecosystems.
• Altered flood regimes and water levels in lakes and streams will affect nutrients and dissolved organic oxygen, and therefore the quality and clarity of the water. Changes in water temperatures and in the thermal structure of fresh waters could affect:
• The survival and growth of certain organisms.
• Species diversity and productivity. Rising seas due to melting ice caps would result in:
• Salt water invading coastal freshwater supplies.
• Coastal aquifers may be damaged by saline intrusion as salty groundwater rises.
• The movement of the salt-front up estuaries would affect freshwater pumping plants upriver.
• Reduced water supplies would place additional stress on people, agriculture and the environment.
• Regional water supplies, particularly in developing countries, will come under many stresses in the 21st century. play a major role in ensuring global water security. However, the potential to save water by managing green water trade is limited by many factors.
Australian water use
The water footprint of a country can bedefined as the volume of water needed for the
production of the goods and services consumed by the inhabitants of that country.
It is often pointed out that Australia is the driest continent with frequent droughts. It hasthe lowest percentage of rainfall as run-off of any continent, and the lowest volume of waterin its rivers (on average 12%). Australia is the driest continent on average only. Tully, a town in tropical Queensland, has an average annual rainfall of a staggering 4.5 m, the highest in Australia, although over
the year rainfall ranges from over 700 mm in February and March to about 100 mm in September and October. Some areas have rainfall that, over part of the year, exceeds evapotranspiration, and that includes the urbanised areas of the south-west and east coast where, as we are often told, we are not short of water, we simply need to be smarter in the way we harvest and use it.
Rainfall distribution and availability
Australia’s water resources are divided into 12
drainage regions, 246 river basins and 325
Table 5.1 Summary of annual Australian water use for the
Population 19 071 705
Wa ter use Water volume
National water footprint 26.56
National water use
Internal water use
Urban (domestic) water withdrawal 6.51
Crop water use for Australian
Crop water consumption for export 68.67
Industrial water use for Australian
Industrial water use for export 0.12
Imported water use
Agricultural goods for national
Industrial goods for national
For re-export of imported products 4.21
Per capita water use Water volume
Domestic water use 341
Internal water for agricultural goods 736
Imported water in agricultural goods 41
Internal water for industrial goods 64
Imported water in industrial goods 211
Total per capita water footprint 1393 kL/yr (=
Source: Hoekstra & Chapagain (2007).6
management areas for surface water (i.e.streams, rivers, lakes and wetlands). Regions
north of the Tropic of Capricorn, togetherwith Tasmania, receive over 50% of Australia’s
divertible water but contain a smallproportion of the population, whereas about
65% of the population lives in coastalVictoria, New South Wales and Queensland in
an area that receives about 23% of thedivertible water (Table 5.2).
About 33% of usable water is diverted tohuman use although this proportion varies
across the continent, being low in the northand high in the Murray–Darling Basin
(Figure 5.3).In many regions, the sustainable extractionlevel is being approached or exceeded, this
being indicated by reduced water quality,algal blooms, increasing salinity and
threatened biodiversity. Between 1983 and1996, the area of irrigated land increased by
26% and use of groundwater increased by88%. In 2000, 84 (26%) of 325 surface water
management areas were either overused orclose to it, and 168 (30%) of 538 groundwater
management units were totally allocated orover-allocated.
Eastern states in general and the Murray–Darling Basin in particular are showing signs
of stress and this is expected to worsen asclimate change produces a reduction in
rainfall over south-eastern Australia. TheWWF lists the Murray–Darling in the world’s
top 10 most threatened river systems. Thesein order are: the Salween, La Plata, Danube,
Rio Grande, Ganges, Murray–Darling, Indus,Nile, Yangtze and Mekong.
Overall, water use for the period 1985–1996/7increased 65% and over the same period
water for irrigation increased 70%.Total surface water run-off is calculated at
about 440 000 GL per annum, of which about25 000 GL is captured for human use, about
70% used for rural irrigation, about 20% forurban and industrial use, and about 10% for
other rural uses.
Table 5.3 Australian water consumption (GL), 2000–01 and 2004–05 (all states)
2000–1 2004–5 NSW Vic Qld SA WA Tas NT ACT
Agriculture 14 989 12 191 4133 3281 2916 1020 535 258 47 1
Forestry and fishing 44 51 11 8 3 1 25 4 1 –
Mining 321 413 63 32 83 19 183 16 17 –
Manufacturing 549 589 126 114 158 55 81 49 6 1
Electricity/gas 255 271 75 99 81 3 13 – 1 –
Water supply 2165 2083 631 793 426 71 128 20 8 5
Other industries 1102 1059 310 262 201 52 168 18 30 17
Household 2278 2108 572 405 493 144 362 69 31 31
Total 21 703 18 767 5922 4993 4361 1365 1495 434 141 56
Source: ABS (2006).7
• Climate, forests and global biogeochemical cycles are all linked to the global water cycle.
• Current levels of human water use and diversion threaten food security and
• Water and environmental impacts are all embodied in commercial products and world trade.
• We must plan for increased urbanisation and climate change.
• Water efficiencies come from managing lilac, blue and green water.
In urban space this means water-sensitive design that includes:
• Improving rainwater harvesting and storage linked into buildings.
• More recycling of greywater and stormwater.
• Improved management of overall water flows including water storage in the soil.