Over the past 50 years, the yields of cereal crops in the developing world have tripled, with only a 30 % increase in the total land area cultivated. This intensification of agriculture was achieved by improved crop varieties (hybridisation), fertilisation and irrigation, three of the main pillars of the Green Revolution. In fact, during the past 40-50 years, there has been a ~70 % increase in irrigated cropland area and, consequently, global water consumption via irrigation has more than doubled from ~650 to ~1 400 km3 yr. Irrigation enables farmers to increase crop production by reducing their dependence on natural rainfall. Worldwide, it is estimated that only 18 % of all cultivated land is irrigated, yet these lands produce 40 % of all food. For example, without irrigation it is estimated that the global production of rice, cotton, citrus and sugar cane would decrease by 31 % to 39 % and cereal production would decrease by 47 %, representing a 20 % loss of total cereal production worldwide.
An expansion of irrigation, along with improved management of water supplies, is considered a vital part of ensuring food security in the future. Irrigation is practiced in almost all countries of the world but to widely divergent extents. India and China have the highest percentage of irrigation land and are also countries with high population density and low per capita availability of cropland. Water resources or adequate water management (harvesting, collection and reservoirs) have kept pace so far. However, pumping water in excess of recharge is increasingly a serious problem in parts of China, India (see case studies) and Bangladesh.
Irrigation is widely practiced in arid and semi-arid regions, where the response of crop yields to irrigation is highest, such as the Arabian Peninsula, Iran, central Asia, northern Africa, Australia and western North America. In these regions, water is scarce, competition for its use is large and maintaining per capita food production from irrigated land might fail in the near future. Even under temperate climate conditions (e.g. eastern United States, Canada, western and central Europe) additional irrigation is used on high value crops, such as maize, to compensate for rainfall variability during the growing season that causes unpredictable annual yields. Although irrigation has led to dramatic increases in food production, it has also caused extensive environmental damage and undermines human resilience to water scarcity. Irrigation is responsible for 70 % of all freshwater withdrawals in the globe. In many parts of the world, irrigation poses a major threat to water resources. This is due to excessive groundwater extraction, inefficient irrigation methods, the salinisation of aquifers and water pollution resulting from an overuse of fertilisers15. Salinisation causes the worldwide loss of ~1.5 million hectares of arable land per year, affecting about 16 % of all agriculture lands16. Soil salinisation, which is a complex phenomenon involving the movement of salts and water in soils, interacting with groundwater, may result from the use of brackish water, poor drainage and/or leaching and poor land management. Human induced salinisation is a widespread problem as around 30 % of irrigated land are affected and becoming commercially unproductive.
According to definitions used by the FAO, agricultural land is equipped for irrigation only if there are permanent structures for off-farm water management (green colours in background map). This means that areas with water harvesting, deep water rice cultivation, cropping in natural wetlands and even watering of crops with temporary water management infrastructure are considered as areas that are under agricultural water management but are not considered as being equipped for irrigation. The statistics provided in the next section follow the FAO classification. Differences between these figures and irrigated land reported by other institutions (e.g. national statistical offices) are often the result of definitions and classification schemes that do not align with the FAO classification system.
Trends in irrigation expansion
It has been argued that some form of irrigation has been used as long as crops have been cultivated. However, it is evident that the development of irrigation infrastructure has grown considerably during the past three centuries. According to the best estimates, the area equipped for irrigation increased from about 5 Mha in year 1700 to 11 Mha in year 1800, 63 Mha in year 1900, 112 Mha in year 1950 and 230 Mha in year 1980.
Predicting the future extent of irrigated land is challenging. One scenario suggested an increase of area equipped for irrigation by 120 Mha for the period 2000–2080. The area of land that might be irrigated is about twice the current extent of irrigated land (graph above). However, estimates are incomplete and methodologies to estimate irrigation potential differ between countries. More importantly, the threat of water scarcity and the inevitable competition among other water-use sectors are growing in arid and semi-arid regions around the globe. Increases in irrigation extent in these regions will depend, to a large degree, on increases in water-use efficiency.