4. How can human actions seriously affect water resources?
- 4.1 How are aquatic ecosystems threatened by sediment in water?
- 4.2 How can different kinds of pollution affect water resources?
- 4.3 What are the consequences of excessive water withdrawal?
- 4.4 How is climate change affecting water resources?
Our water resources face a host of serious threats, all of
which are caused primarily by human activity. They include
landscape changes, and
One of the most serious threats to water resources is the
degradation of ecosystems,
which often takes place through changes to landscapes such as
the clearance of forests, the conversion of natural landscapes
to farmland, the growth of cities, the building of roads, and
surface mining. Each type of change to a landscape will have its
own specific impact, usually directly on natural ecosystems and
directly or indirectly on water resources.
Although it is difficult to integrate the intricacies of
ecosystems into traditional
assessment and management processes a holistic ecosystem
approach to water management is strongly recommended.
4.1 How are aquatic ecosystems threatened by sediment in water?
Sediments can occur in water bodies naturally, but they are
also produced in large amounts as a result of land-use change
Activities such as farming, clearing forests, building roads,
and mining can put too much soil and particulate matter in
rivers. This sediment can harm plants and animals by carrying
toxic chemicals into the water, smothering fish eggs and small
organisms used by fish as food, raising water temperature, and
reducing the amount of sunlight penetrating the water.
Sediment can also reduce the capacity of reservoirs and make
it difficult for ships to navigate in waterways. It can also
damage equipment used in water supply installations and
hydroelectric plants, thus increasing their maintenance costs.
Table 4.4 Principal sources and impacts of
Sedimentation should be factored into strategies to protect
4.2 How can different kinds of pollution affect water resources?
Wastes that people dispose of can pollute the air, the land,
and water resources. They affect the quality of rainwater and of
water resources both above and below ground, and damage natural
The causes of freshwater
pollution are varied and include industrial wastes, sewage,
runoff from farmland, cities, and factory effluents, and the
build-up of sediment.
Emissions from factories and vehicles are released into the
air. They can travel long distances before falling to the
ground, for instance in the form of
acid rain. The emissions
create acidic conditions that damage
forests and lakes. The pollution that passes directly into water
from factories and cities can be reduced through treatment at
source before it is discharged. It is harder to reduce the
varied forms of pollution that are carried indirectly, by
runoff, from a number of widely spread non-point sources, into
freshwater and the
Only a small percentage of chemicals are regulated, and
concern is growing about contamination by unregulated chemicals.
A variety of pharmaceutical products, such as painkillers and
antibiotics, are having an
impact on water resources above and below ground. Conventional
water treatment does not work for many of them.
In general, it takes much longer to clean up polluted water
bodies than for pollution to occur in the first place, and there
is thus a need to focus on protecting water resources. In many
cases, clean-up takes more than 10 years. Although
underground water is less
easily polluted than water above ground, cleaning it once it is
polluted takes longer and is more difficult and expensive. Ways
are being found to assess where and how underground water is
most vulnerable to pollution. The findings are important in
cases where aquifers supply
drinking water, and where natural
ecosystems depend on
Sewage and runoff from farms, farmlands and gardens can
contain nutrients, such as nitrogen and phosphorus, that cause
excessive aquatic plant growth, and this in turn has a range of
damaging ecological effects.
Complicating the problem of water pollution is the overall
lack of adequate information about the quality of water around
the world. Many countries do not collect enough data, and most
of them are not prepared to sharing it. But this is changing
because of growing awareness of the need for such information,
and due to the availability of an international database,
GEMSTAT, that went online in March 2005.
4.3 What are the consequences of excessive water withdrawal?
Around the world certain lakes, rivers and inland seas are in
the process of drying up because too much water is being drawn
from them or from their tributaries.
Groundwater, too, is used
faster than it is replenished, as is clear from a growing number
of reports documenting sharp drops in
aquifer levels. In many
cases, drought periods have compounded this well-documented
The Niger, the Nile, the Ganges, the Tigris, the Euphrates,
the Yangtze, the Colorado, and the Rio Grande are just some of
the major rivers suffering substantial reductions in flow.
Numerous lakes and inlands seas are shrinking dramatically in
many geographic regions. The Aral Sea and Lake Chad have
decreased dramatically in size over the last few decades.
These problems persist though their causes have been evident
for quite some time. Foremost are the very inefficient ways in
which water is supplied to farms and cities,
deforestation, and the
failure to properly manage and control the withdrawal of water,
and to think of more economic ways to use water.
Not enough consideration has been given to minimizing use and
conserving water resources. Instead the supply has been further
strained by the construction of new reservoirs and inappropriate
diversions. While some towns and cities are taking action, only
broad-based and fundamental change in national and regional
practices can reverse the impact.
The threat to groundwater
is not as obvious as that to lakes and rivers. There is less
visual evidence and the effects of withdrawing too much
groundwater take longer to recognise. In the last half-century,
pumping from aquifers
increased globally. But often the benefits—bigger harvests for
example—were short-lived, ultimately resulting in lower water
tables, drilling of deeper wells, and, sometimes, even the
depletion of the groundwater source.
Cases from all climatic regions illustrate that excessive use
of groundwater is
relatively common practice. The consequences can be seen in
reduced spring yields, diminished river flow, poorer water
quality, damage to natural habitats such as wetlands, and the
gradual sinking of land, known as subsidence.
4.4 How is climate change affecting water resources?
Exactly how global warming is affecting water resources is not
altogether clear. New research suggests that
climate change is
increasing existing stress, for example by reducing runoff in
areas already suffering from water shortages. Scientists agree
that extreme weather events stemming from global warming, such
as storms and floods, are likely to be more frequent in the
future. However, based on current knowledge, scientists can only
make general predictions about the impact of climate change on
water resources. More information is needed, for instance, about
impacts on water resources in specific regions and under
different policy scenarios.
One type of water resource that has been clearly affected by
climate change is
glaciers. Scientists have
long observed that land and mountain glaciers are shrinking, and
this trend has accelerated considerably in recent years. For
example, it has been predicted that most glaciers in Tibet could
melt by 2100. And while it was initially thought that the water
released could benefit China’s arid north and west, it now
appears that the additional runoff evaporates long before it
reaches drought-stricken farmers downstream.