Temps de lecture :5 minutes
Irrigation consists of artificially bringing water to cultivated crops to increase their yield. Irrigation can compensate for a lack of rainwater, but necessitates the removal of groundwater, and water from rivers and lakes. Not all arable land is irrigated: only 17% was in 2003, but this land could provide up to 40% of the world’s harvest. (1)
Water as a Resource
Water is a renewable resource: the amount present on Earth is virtually constant. At any one time, water is flowing in the ocean, evaporating, and falling again over the earth. However, certain water stores, in particular aquifers (beds of water-bearing rock), take somewhere on a timescale of thousands of years to replenish. On a human timescale, they could therefore be depleted before they can regenerate, and come to be missed.
Furthermore, only 2.5% of total water reserves are fresh water. Fresh water from watercourses and subterranean groundwater forms 1% of the total volume of fresh water, and 0.01% of the total water on Earth. There is therefore a limited supply of water that can be directly used for irrigation. (2)
According to the World Meteorological Organization, “there is a risk of a serious water shortage within 50 years”. In some regions, water is being pumped away faster than it can be replaced. This is the case in Tamil Nadu in India, where excessive pumping has caused a decade long fall of 25-30 metres in levels of groundwater supplying wells.(3)
The volume of water extracted in the world has risen to around 3 600 km³ each year. 70% of this is used for agriculture. (4). The amount of water used for irrigation varies dramatically according to country, climate, availability of water, and agricultural techniques used (rice needs more water to grow than wheat, for example). Europe dedicates 33% of its water to agriculture, while sub-Saharan Africa devotes 88%. (5)
The Water Strategy Issue
Water is shared in a very unevenly across the planet. Numerous regions have been exploited to their maximum capacity. Although agriculture remains the most water intensive industry, industrial and domestic uses are equally important. This resource consumption can create conflicts of interest between users, and also between countries (California gained 30% of the water from urban areas in Colorado, while Mexico got only 9%). [See Conflicts page]
There is a risk that these tensions will rise, as according to the FAO (Food and Agricultural Organization of the United Nations), the food needs of the world’s population will be 50% higher than they are today by 2030. [Debate]
Producing More with Less Water
The predicted growth in demand requires more to be produced using less water, but irrigated agriculture with a low level of inputs (fertilizer, pesticides etc.) remains more productive than naturally irrigated (rain) agriculture with high levels of inputs. (6)
The poor state of pumping and irrigation equipment leads to serious losses: leaks in irrigation canals, discharge, infiltration and evaporation. In developing countries, the average quantity of water used on plants represents only 38% of the water drawn. If a part of this water returns to water courses and aquifers it could be reused. Losses from evaporation due to irrigation techniques used could represent 65% of water withdrawn. (7)
To reduce water losses during irrigation, different techniques have been developed. Alongside water provision techniques used by plants, the drop-by-drop technique allows a reduction in water loss caused by evapotranspiration, while allocating each plant the water it needs. It is also possible optimise irrigation efficiency by reacting to soil levels, watering only when appropriate, or by choosing plant varieties better adapted to a dry environment. (8)
In dry areas, the collection of rain water reduces the risks stemming from irregular precipitation and increases yield two to three times more than for those crops without it. It leads to a better feeding of groundwater supplies and regulates the provision of water over a longer period than a rain shower. Using another example, Israel collects 75% of its drain water, purifies it in processing plants, and then uses it for crop irrigation. (9).
Another solution is to grow less water intensive species or varieties of food producing crops, using cultivation techniques that preserve soils (covering vegetables to reduce water evaporation, for example). (10). The World Water Council proposes a target of a 40% growth in production, but with only a 9% increase in water consumption. (11) This implies a type of cultivation that is very productive and highly technological, utilisation of plants chosen for their high yield, and the use of fertilizer and plant protection products.
Impacts on the Environment
At a local level, excessive irrigation can cause a considerable reduction in the volume of water available, and lead to desertification in some regions. Today, Lake Chad is 20 times smaller than it was 30 years ago, due to a decline in precipitation and an increase in pressure on water resources in the region. Similarly, water taken from the two rivers that feed the Aral Sea have caused sea levels to fall by 15 metres, and reduced its surface area by 40%. [See text box]
Some large rivers cannot join the sea for considerable periods of time, such as the HuangHe (China), the Colorado (United States) and the Shebelle (Somalia). The Yellow River in China (HuangHe) could not reach the sea for seven months in 1997.
Poor irrigation management can have equally dramatic consequences on soils: if the water is not drained, it stagnates in the fields, then evaporates leaving mineral and salt deposits that were dissolved within it. These sterilise the earth. Poorly controlled irrigation can lead to soil saturation that leaves the soil compacted, starving plant roots of oxygen.
Finally, uncontrolled agricultural irrigation tends to worsen soil erosion, causing more sediment to build up in rivers, floods in middle course areas, greatly reduced flow rates downstream, and the silting up of reservoirs. The FAO estimates that 1% of large reservoirs are lost like this each year. [Debate]
Overexploitation of freshwater resources compels fresh consideration of alternative sources of water supply. Desalinisation is one proposal. In Kuwait, 43% of water used for crop cultivation comes from desalinised water. This expensive technique remains reserved for richer countries for the moment. Further, desalinisation is very energy intensive and so not very environmentally friendly.
Before 1960, 50 000 million cubic metres of water flowed into the Aral Sea every year. Water diverted for cotton irrigation and the construction of reservoirs to store floodwater led to a fall in the annual flow rate entering the Sea, which dropped to 1 300 million cubic metres between 1981 and 1990.
Of the 24 species that populated the sea, 20 disappeared, and the stocks of fish that had reached 44 000 tonnes per year in the fifties and guaranteed 60 000 jobs, were reduced to nothing. The toxic blend of dust and salt emanating from the dried marine sub soil was dumped on nearby agricultural land. The weakly flowing rivers contain concentrated salts and toxic chemical products that render the water dangerous to use, and are partly responsible for the raised levels of various illnesses (renal problems, deformities etc) in the region. Recently, the construction of a dam has allowed water levels to rise and the situation has improved slightly.
(4) FAO Water
(5) L’eau et ses enjeux, de Françoise Anctil, Presses internationales Polytechnique
(11) World Water Council
Voir aussi :
– Atlas mondial de l’eau, 2007, Salif Diop, Philippe Rakacewicz, Editions Autrement