Peak Water...Is water a renewable Resource?
Peak
water is reached when the rate of water demand exceeds the rate at which
water resources used for supply can be replenished. Therefore, all water
supplies can be considered finite as they can all be depleted by over
exploitation. So while the total
volume of water in the hydrological cycle remains the same, the availability of
water does alter. This is particularly
true of aquifers (groundwater) and static water bodies such as lakes and reservoirs
where the water may take a long time to replenish. So water availability is
strongly linked to rainfall and the ability to retain this water within
resources which is difficult as increasing intensity of water reduces
infiltration.
Due
to increasing demand from population growth, migration to urban centres and for
agriculture, it is possible that a state of peak water could be reached in many
areas if present trends continue. By 2025
it is estimated that 1.8 billion people will be living with absolute water
scarcity and in excess of 4 billion of the world’s population may be
subject to water stress.
A
question I am often asked is how does a renewable resource become finite? The answer is not as straight forward as
first appears. Water availability is
governed by a number of possible factors: Over-abstraction (i.e. using it before it can be replenish
thereby exhausting the supply and causing significant and often permanent
ecological damage), not returning water to hydrological resources, saltwater
intrusion often caused by over-abstraction, pollution of resources and
finally climate change effects (glacier loss, reduced stream flow, evaporation
of lakes). Comparatively only a very
small amount of water is regularly renewed by rain and snowfall, resulting in
only a small volume of water available on a sustainable basis. So all water supplies have an optimal
abstraction rate to ensure they are sustainable, but once exceeded then
supplies are doomed to failure. As we
saw in Section 3.2, a modified Hubbert curve applies to any resource that can
be harvested faster than it can be replaced. This applies to all water
resources but especially to groundwaters.
Peak water is defined in three different ways
according to the impact on the resource as: peak renewable, peak non-renewable or peak
ecological water.
Peak Renewable Water comes from resources that
are quickly replenished such as rivers and streams, shallow aquifers that
recharge relatively quickly and rainwater systems. These resources are constantly renewed by
rainfall or snow melt; however this does not mean these resources can provide unlimited
supplies of water. If demand exceeds
100% of the renewable supply then the “peak renewable” limit is reached. For many major river catchments globally, the
peak renewable water limit has been reached.
For example, in excess of 100% of the average flow of the Colorado River
is already allocated through legal agreements with the seven US States and
Mexico. So in a typical year the river flow can now fall to zero before it reaches
the sea. Similarly the River Thames can during
periods of low flow fall below the volume of water abstracted. The river is prevented from drying up due to
over-abstraction by returning wastewater after treatment to the river which is
then reused numerous times as it approaches London. Due to the high population within the catchment, the
Environment Agency has classified the area as seriously water stressed with
towns and cities along the length of the Thames such as Swindon, Oxford and
London itself, at risk of water shortages and restrictions during periods of
dry weather.
Peak Non-renewable Water comes from resources that are effectively
non-renewable aquifers that have very
slow recharge rates , or contain ancient water that was captured and
stored hundreds or thousands of years
ago and is no longer being recharged ( a
problem that will be exacerbated by climate change), or groundwater systems
that have been damaged by compaction or other physical changes.
Abstraction
in excess of natural recharge rates becomes increasingly difficult and
expensive as the water table drops which results in a peak of production,
followed by diminishing abstraction rates and accompanied by a rapid decline in
quality as deeper more mineralized waters (i.e. increasingly salty to the
taste) are accessed. Worldwide, a significant fraction of current agricultural
production depends on non-renewable groundwater (e.g. North China plains,
India, Ogallala Aquifer in the Great Plains of the United States) and the loss
of these through over-exploitation threatens the reliability of long-term food
supplies in these regions.
When
the use of water from a groundwater aquifer far exceeds natural recharge rates,
this stock of groundwater will be depleted or fall to a level where the cost of
extraction exceeds the value of the water when used, very much like oil fields.
The problem is that climate change often results in less rainfall creating a
greater dependence on aquifers for supply.
Peak Ecological Water is water abstracted for
human use which leads to ecological damage greater than the value of the water
to humans. The human population already uses almost 50% of all renewable and
accessible freshwater leading to serious ecological effects to both freshwater
resources and transitional habitats such as wetlands. Since 1900, half of the world’s wetlands have
disappeared while approximately 50% of freshwater species have become extinct
since 1970, faster than the decline of species either on land or in the sea. Water
supports both man’s need and that of its natural flora and fauna. These fragile environments need to be
preserved for overall planet health. The simple fact that water supply quality
is often a close relationship with the ecosystem, with most water bodies able
to self purify its water constantly removing pollutants and improving quality
overall. However, the problem has been in putting an economic value on
ecological systems (sometimes referred to as ecological services) and nature as
a whole; whereas water used by humans can be easily quantified economically. In the mistaken assumption that such values
are zero has led to them being highly discounted, underappreciated, or ignored
in water policy decisions in many areas. Over-abstraction is a major problem in many
rivers in southern England that are fed from the aquifer below. As more groundwater is abstracted then the
water table falls as does the water level in the river.
It is not only rivers that are drying up due
to over abstraction and global warming but some of the largest freshwater lakes
in the world such as the Aryl Sea and Lakes Chad and Victoria in Africa. Link In
the USA, water abstraction and water use peaked during 1975 to 1980 but has
stabilized since. This should have
affected economic growth but has been able to continue to grow by implementing better water
management strategies to satisfy the new needs of industry. This has been achieved through water
conservation, stricter regulations, water efficient and improved technology,
education, water pricing etc. So US
citizens are now using less water per capita than ever before. However,
many regions of the U.S. face water scarcity (e.g. the arid west) and new areas
of water scarcity continue to develop due to climate change (e.g. southeast and
Great Lakes region) which all indicate that peak water has been reached . The
key question is how long can economic growth be sustained without water
becoming a limiting factor? More information.
Will
water shortages affect us in Ireland and the UK? The straight answer is yes, and to some
extent already is. No one is exempt from
the peak water crisis. Due to global warming
most arid regions will probably run out of water in less than two decades. In wetter areas, peak water has been reached
due to: heavy use of water; pollution of resources (often associated with
urbanization); infrastructure not being completed to keep up with demand
(China, India) and finally inadequate infrastructure (London, Dublin).
Agriculture
represents at least 70% of freshwater use worldwide and with the demand for
food soaring, especially as a result of climate change and increasing crop failure
(e.g. China rice failure in 2011) then demand for irrigation and livestock
watering will continue to be a major drain on supplies
Extract from: Gray, N.F. (2015) Facing up to
Global Warming: What is Going on and How You Can Make a Difference, Springer,
New York.
@nickgraytcd
