Welcome to the blog of the course and textbook Facing up to global warming: What is going on and what you can do about it. This course is run by Professor Nick Gray of the Trinity Centre for the Environment at Trinity College Dublin.

Tuesday, June 2, 2015

Peak Water...Is Water a Renewable Resource?

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. 

1 comment:

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