Providing the water needed to produce food for more than 9 billion people by 2050 seems simple: agriculture must produce more food with less water. However, three complex issues are involved: First, water, food production and rural development do not have a simple correlation. Second, there are interactions between processes at local, basin and global scales. Third, change involves people in complex networks of institutions. The Challenge Program on Water and Food brings together agriculturalists, hydrologists and development specialists in a global-to-local programme that focuses on change through institutions. We believe that this scale, complexity and involvement are necessary to deliver plausible change.

Water resources are already very scarce. With further demand from population growth, dietary changes, biofuel production, urbanisation and climate change, it will be extremely difficult to find enough supply to enable an increase in global food production by 70 per cent. There are, however, potential solutions that involve increasing water productivity, improved water storage, more irrigation and re-using waste water. But current investment levels in overseas development aid and spending at country level are unlikely to be sufficient to ensure food security in the relatively short- term, let alone by 2050, when the global population is forecast to be nine billion. This article describes some of the issues that have to be faced to deliver food security and overcome water scarcity, and how these improvements can be achieved through a combination of science, policy and investment.

Agricultural water use includes a continuum from purely rainfed to fully irrigated systems. Growing pressures on limited water supplies from domestic, industrial, and environmental uses will likely lead to a decline in water availability for food production. Similarly, income growth and urbanization lead to dietary shifts that require more water resources per calorie consumed, putting further pressures on water supplies. As a result, semiarid and arid countries continue to increase net imports of food. Crop water use for sugarcane (Saccharum officinarum L.), maize (Zea mays L.), soybean [Glycine max (L.) Merr.], and fruits are expected to grow over time, whereas water use for wheat (Triticum aestivum L.) and rice (Oryza sativa L.) are expected to decline after 2030. These projections include substantial improvements in water use efficiency at the field, farm, and river basin scale over the coming decades in response to growing water scarcity. If these efficiency improvements are not achieved, future crop water demands would be even larger. Although water resources are a key limiting factor for future food security, policy and investment options to reduce agricultural water use exist on both the water supply and demand side; but political will and ingenuity are needed for their implementation.