Water resources for human requirements are the sources of fresh water that can be used for agriculture, industry, households, recreational use and ecosystems. 97 per cent of global water supply is fresh, sweet water. 3 per cent fresh water that can be used for human or environmental consumption. Out of these 3 per cent, two thirds are frozen located in glaciers and ice caps. As a result, only 1 per cent of the world’s water resources are usable for human beings. However, fresh water is a renewable resource due to the hydrological cycle and therefore water is a “space-temporal” resource (USGS 2012). In order to illustrate the different sources of water (soil moisture, surface and groundwater) for agricultural production, water sources are being distinguished in different colours according to their use.
Water resources on the planet are constantly renewed thanks to the water cycle. Water from oceans, plants, wetlands and surface waters evaporates into the atmosphere where it condensates and finally precipitates back onto the earth’s surface. Precipitation or rainfall then is either used by plants for consumptive use or the runoff flows back into the sea or surface waters. Furthermore, water infiltrates into the soil to recharge groundwater aquifers (USGS 2012). 520,000 km3 (cubic kilometres) a year, of which 412,000 km3 returns to the oceans as direct precipitation and 108,000 km3 falls on land. What happens to the precipitation depends on many factors such as rate of rainfall, soil types, topography of land, density of vegetation cover and levels of urbanization that all influence the destination of the runoff (Malesu et al 2007).
Colours of water
A number of scientists around the Swedish global authority on water management, Malin Falkenmark, have conceived a chromatic framework to illustrate the different sources of water in the hydrological cycle for human and ecosystems uses to bridge the gap between hydrology and policy-makers in communicating the different sources of water.
Blue water is the water from surface and groundwater. In agriculture, blue water plays a paramount role due to its use in irrigation agriculture. Approximately 40% of global agriculture is irrigated with blue water. It is also the water that is transportable and thus used for industrial, recreational and household consumption. Although blue water only provides 30% of global water supply, this source of water is often perceived as the main source of water. Blue water has the highest water productivity as it can be used in modern irrigation systems. It is also the water that is “produced” by engineers through dams and pumps (Falkenmark and Rockström 2006).
Green water is the water infiltrated into the soil from precipitation. It provides a large natural storage of water, similar to groundwater but accessible to natural and agricultural vegetation (Hoff 2006). The runoff of precipitation recharges aquifers and lakes in the water cycle. The term green water relates to the chlorophyll of plant growth. However, green water may be the water mostly affected by climate change due to changing precipitation patterns. Nevertheless, a more efficient use of green water is viewed as a cornerstone in agricultural water management to attain food security in particular in drylandareas (Wani et al 2009). Green water is ignored by engineers because they can't pipe or pump it, by economists because they can't price it, and by governments because they can't tax it (Zaks and Monfreda 2006).
Grey water is the recycled blue water that was previously used in households or industries. It is either recycled for further use oder applied for irrigation. The term stems from the cloudy appearance and from its position within potable water and sewage water that are also referred to as white water and black water. Grey water reuse is an ecologically futile option to recycle blue water and to decrease pressures on blue water resources. For example, an arid country like Egypt that almost entirely relies on blue water (the river Nile) for its water consumption reuses 7 billion cubic metres of water per year , which accounts for approximately 13 per cent of its available water resources (MWRI 2010) Grey water can be either purified through mechanical or biological systems or discharged on agricultural land or in rivers or streams without purification (Malesu 2007). However, 90 per cent of grey water remains untreated globally, hence it is viewed as a major cornerstone in development assistance to increase purified grey water reuse in developing countries (Corcoran et al 2010). Bilateral development assistance, for example, focuses in its development assistance to developing countries on financing wastewater reuse plants and capacity development strategies (see links).
Corcoran, E., C. Nellemann, E. Baker, R. Bos, D. Osborn, H. Savelli (eds). 2010. Sick Water? The central role of waste- water management in sustainable development. A Rapid Re- sponse Assessment. United Nations Environment Pro- gramme, UN-HABITAT, GRID-Arendal.
Hoff, H. (2006). Challenges in upland watershed manage- ment: for what? for whom? Proc 5th Rosenberg Int Forum on Water Policy, Banff, Alberta, Canada. September.
Falkenmark, M. and Rockström, J. (2006). The New Blue and Green Water Paradigm: Breaking New Ground for Water Resources Planning and Management. Journal of Water Resources Planning and Management May/June 2006.
Maimbo M. Malesu, Alex R. Oduor and Orodi J. Odhiambo (Eds). (2007). Green water management handbook: Rainwater harvesting for agricultural production and ecological sustainability. Technical Manual No. 8 Nairobi, Kenya: World Agroforestry Centre (ICRAF), Netherlands Ministry of Foreign Affairs.
United States Geological Survey (2012). The Water Cycle. Available online: http://ga.water.usgs.gov/edu/watercycle.html (accessed 16 September 2013).
Wani, S., Rockström, J. and Oweis, T. (2009). Rainfed agriculture: unlocking the potential. Comprehensive assessment of water management in agriculture series 7. CABI, ICRISAT and IWMI.
Zaks, D. and Monfreda, C. (2006). Green Water and Sustainable Agriculture. Available online: http://www.worldchanging.com/archives/004494.html (16 September 2013).