The Water Cycle
Published on SSWM (http://www.sswm.info/)

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The Water Cycle

Compiled by:
Katharina Conradin (seecon international gmbh)

The amount of water on earth has been constant over centuries. It is in permanent circulation and regeneration and is therefore a renewable resource. However, most of the water on earth is salty (97%) and thus not suitable for the majority of uses. Only roughly 2.5% is freshwater in rivers, lakes, groundwater, fixed in soil or frozen in icecaps and a mere 0.5% of the total is easily accessible for human use (INFORESOURCES FOCUS 2006). This liquid water travels the earth constantly in the water cycle. Humans largely influence the water cycle today, be it quantitatively, by using large parts of the water available, or be it qualitatively, by changing the quality of water (e.g. pollution). On the earth, water never occurs in pure form – it always contains dissolved substances – be it minerals, nutrients, and also pollutants. The water cycle is thus inherently linked to the nutrient cycle.

Water – Essential for Life!

Every schoolchild knows that water is essential for life. Yet, many are only just now beginning to grasp how essential it is to everything in life: Growing our foods, cooking and personal hygiene are just the most imminent uses of water. We also use it to produce energy, to transport goods around the globe, as a cultural and social asset – it is necessary for the development of any forms of human life (WBCSD 2010).

The Global Water Cycle

The amount of water on earth has been constant for a very long time. Water travels the earth in a constant cycle through atmosphere, land mass and oceans. Precipitation is the main source of water for all human uses and for ecosystems; its main driver is the sun. Precipitation is taken up by plants and soils, evaporates into the atmosphere via evapotranspiration, and runs off to the sea via rivers, and to lakes and wetlands. This water supports forests, rain-fed cultivated and grazing lands, and ecosystems. Some of the precipitation can be stored for centuries in snow and ice, and some of it can also be stored temporarily in aquifers before it again comes out (springs, pumps etc.). However, most water falls back into the oceans or lands as rain, where it flows over the ground as surface runoff and again flows into rivers and eventually the sea (WWAP 2003).

Freshwater Resources

The Water Cycle. Water travels the earth in a constant cycle, which is ultimatively driven by the sun. Source: OWENS 2006

The Water Cycle. Water travels the earth in a constant cycle, which is ultimatively driven by the sun. Source: OWENS (2006)

The earth is called the blue planet – more than 70% of its surface is covered by water (PIDWIRNY 2006). Although water is the most widely occurring substance on earth, only ca. 2.5% is freshwater while the remainder is salt water. Some two thirds of this freshwater is locked up in glaciers and permanent snow cover (see graph below). In addition to the accessible freshwater in lakes, rivers and aquifers, man-made storage in reservoirs adds a further 8,000 cubic kilometres (km3). Water resources are renewable (except some ancient aquifers), with huge differences in availability in different parts of the world and wide variations in seasonal and annual precipitation in many places (WWAP 2003).

Freshwater Distribution


The world’s freshwater resources. Source: WBCSD 2009

The world’s freshwater resources. Source: WBCSD (2009)

Freshwater is distributed very unevenly over the world. Fewer than 10 countries possess 60% of the world’s available freshwater supply: Brazil, Russia, China, Canada, Indonesia, U.S., India, Columbia and the Democratic Republic of Congo. And even in those countries, the availability varies greatly (WBCSD 2009). Only 80% of the worlds population are served by renewable and accessible water; a fifth of the world population relies on ancient aquifers (aquifers that are not renewed any more presently), interbasin transfers or on desalinised seawater (WWAP 2009).

Water Scarcity Index: The above maps show the exploitation of water resources on the world. The darker the colour, the more water is exploited and the higher is the water stress in that area. Source: REKACEWICZ 2009

Water Scarcity Index: The above maps show the exploitation of water resources on the world. The darker the colour, the more water is exploited and the higher is the water stress in that area. Source: REKACEWICZ (2009)

Today, a quarter of the world’s population is affected by physical water scarcity – mainly in northern Africa, the Middle East, and in parts of India, Pakistan and China. This means that there is not enough water for all uses, whether agricultural, industrial or domestic. Defining thresholds for stress in terms of available water per capita is more complex, however, entailing assumptions about water use and its efficiency. Nevertheless, it has been proposed that when annual per capita renewable freshwater availability is less than 1,700 cubic meters, countries begin to experience periodic or regular water stress. Below 1,000 cubic meters, water scarcity begins to hamper economic development and human health and well-being (WWAP 2009).

Human Influences on the Water Cycle

Humans significantly influence the global water cycle, both quantitatively and qualitatively. Concerning quantity, humans withdraw 8% of the total annual renewable freshwater, and appropriate 26% of annual evapotranspiration and 54% of accessible runoff. Humankind’s control of runoff is now global and we are significant players in the hydrological cycle. Per capita use is increasing (with better lifestyles) and population is growing. Thus the percentage of appropriated water is increasing. Together with spatial and temporal variations in available water, the consequence is that water for all our uses is becoming scarce and leading to a water crisis (WWAP 2003).

Local water cycles are also influenced by the way we plan our cities: When land is paved, water cannot soak into the soil. It runs off these hard surfaces very rapidly, so pipes are needed to rapidly carry the resulting large volumes of stormwater to the nearest stream or beach. The result is that streams carry less water or dry up when it is not raining and flood when it does. Pollutants on roads and yards are also swept into waterways (see also sustainable sanitation).

Influences on the water cycle in cities through sealed surfaces. Source: AUCKLAND CITY COUNCIL 2010

Influences on the water cycle in cities through sealed surfaces. Source: AUCKLAND CITY COUNCIL (2010)

The more humans use water, the more they also influence water quality: Nutrients from fertiliser and from sanitation systems are carried directly into aquatic ecosystems (see nutrient cycle), and also pesticides, industrial pollutants, or medical substances eventually end up in water. As all these substances can be soluble, they also become part of the water cycle (WWAP 2003). Hence, freshwater resources are further reduced by pollution. Some 2 million tons of waste per day are disposed of within receiving waters, including industrial wastes and chemicals, human waste and agricultural wastes (fertilisers, pesticides and pesticide residues) (UNEP & UN HABITAT 2010) (see also soil degradation).

Water Uses

The main uses of water are for agriculture, industry and household use. Industrial use of water increases with country income, going from 10% for low- and middle-income countries to 59% for high-income countries. Source: WBCSD (2009)

Agriculture: In many developing nations, irrigation accounts for over 90% of water withdrawn from available sources for use. In England where rain is abundant year round, water used for agriculture accounts for less than 1% of human usage. Yet even on the same continent, water used for irrigation in Spain, Portugal and Greece exceeds 70% of total usage.

 

Industrial use: After agriculture, industry is the second largest user of water – be it for the production of electricity, as process water, for mining, as a means to dispose or discharge waste etc. The amount varies heavily depending on the type of industries, and so does the pollution resulting from its use.

 

Domestic use: By now, the water we use directly in our homes accounts for the smallest part of water use – yet it is essential for our survival: for drinking, preparing foods, and for washing ourselves. The consumption varies greatly – for example, from as little as 10 L per person an day in low-income areas in Ulan Bator (Mongolia) (World Bank 2010) to almost 600 L per person and day in the United States (WBCSD 2009). A minimum of 50 L per day as a basic water need is recommended (FALKENMARK 2005; WATER ENCYCLOPAEDIA 2010).
Other uses: Furthermore, also other stakeholders such as fisheries, tourism, or transport use water resources.

See also approaches to optimise water use.

Consequences 

Map showing freshwater withdrawal by the different sectors. Source: WORLD RESOURCES INSTITUTE 2000

Map showing freshwater withdrawal by the different sectors. Source: WORLD RESOURCES INSTITUTE (2000)

Today, a fifth of the world’s people, more than 1.2 billion, live in areas of physical water scarcity, where there is simply not enough water to meet all demands, including environmental flows (WBCSD 2009). “Population growth, changes in lifestyles and living standards, uncontrolled pollution and the effect of climate change result in growing water scarcity in various regions of the world. Competition between humans and between sectors for this essential resource is increasing steadily. It is estimated that by 2030, 60% more food will have to be produced which has important implications on water use and availability. This demand for food, and thus water, is not negotiable” (INFORESOURCES FOCUS 2006).


Outlook


Water-Stress This map shows populations living in water-stressed (red) and relatively unstressed (blue) conditions highlighting substantial within-country differences that national estimates often obscure. higher stress occurs when water availability is less than water demand; lower stress occurs when water availability is higher than demand. Source: WWAP (2006)

Water scarcity is a complex problem - it is one of unsustainable use, sectoral thinking, mismanagement and a lacking and holistic water governance. As so often, it is the life of the poor who are affected most: by water related disease, by degraded and dangerous environments, by a lack of food and water for hygiene. Almost one billion people do not have access to improved water sources, and 2.6 billion do not use improved sanitation options.

 

Single sector approaches such as wastewater treatment or water management as such are limited in their actions. To save and recycle water, regain resources, to protect ecosystems and to provide mankind with a prosperous and healthy environment, the whole water cycle needs to be taken into account in an integrated, holistic way – linking up sustainable water management sanitation and agriculture.

References

AUCKLAND CITY COUNCIL (Editor) (2010): Stormwater. Auckland (New Zealand): Auckland City Council. URL [Accessed: 25.01.2012].

CORCORAN, E. (Editor); NELLEMANN, C. (Editor); BAKER, E. (Editor); BOS, R. (Editor); OSBORN, D. (Editor); SAVELLI, H. (Editor) (2010): Sick Water? The central role of wastewater management in sustainable development. A Rapid Response Assessment. United Nations Environment Programme (UNEP), UN-HABITAT, GRID-Arendal. URL [Accessed: 05.05.2010].

INFORESOURCES FOCUS (Editor) (2006): Water for Food – a Matter of Survival. Inforesources Focus. URL [Accessed: 24.01.2011].

OWENS, M. (2006): The Global Water Cycle has Intensified. Reston: U.S. Geological Survey (USGS). URL [Accessed: 25.01.2012].

PIDWIRNY, M. (2006): Fundamentals of Physical Geography (2nd Edition). Okanagan: University of British Columbia. URL [Accessed: 20.04.2010].

REKACEWICZ, P. (2009): Water Scarcity Index. In: UNEP/GRID-Arendal Maps and Graphics Library. URL [Accessed: 20.04.2010].

WBCSD (Editor) (2009): Facts and Trends. Water (Version 2). Geneva: World Business Council for Sustainable Development (WBDSC). URL [Accessed: 20.04.2010].

KAMATA, T.; REICHERT, J.A.; TSEVEGMID, T.; KIM, Y.; SEDGEWICK, B.; WORLD BANK (Editor) (2010): Mongolia. Enhancing Policies and Practices for Ger Area Development in Ulaanbaatar. Washington: World Bank. URL [Accessed: 17.04.2012].

WORLD RESOURCES INSTITUTE (Editor) (2000): World Resources 2000-2001: People and ecosystems: The Fraying Web of Life. Washington DC: World Resources Institute.

WORLD WATER ASSESSMENT PROGRAMME WWAP (Editor) (2003): Water for people – water for life. Executive Summary of the United Nations World Water Development Report 1: Water for people – water for life. Paris and Oxford: UNESCO and Berghahn Books. URL [Accessed: 17.04.2012].

WWAP (Editor) (2006): Water – the challenges. Educational Poster Prepared for the United Nations World Water Development Report 1. Paris and London: UNESCO and Earthscan. URL [Accessed: 27.04.2010].

VELASQUEZ ORTA, S. ; FURLONG, C. (2009): Messages for Youth. The United Nations World Water Development Report 3: Water in a Changing World. Perugia: United Nations World Water Assessment Programme (WWAP). URL [Accessed: 17.04.2012].

WATER ENCYCLOPAEDIA (Editor) (2010): Survival Needs. Water Encylopaedia. URL [Accessed: 20.04.2010].

For further readings, case studies, awareness raising material, training material, important weblinks or the related powerpoint presentation, see www.sswm.info/category/concept/water-cycle