The recently completed Comprehensive Assessment of Water Management in Agriculture concluded that globally there are sufficient land and water resources to produce food for a growing population over the next 50 years. But it is probable that today’s trends, if continued, will lead to water crises in many parts of the world. Yearly some 7 100 billion cubic meters (m3) of water are evaporated by crops to meet global food demand, equivalent to more than 3 000 litres per person per day. With a growing population, rising incomes and changes in diets, food demand will increase rapidly. Demand for biomass for biofuels will further drive the demand for agricultural products and hence agricultural water. Some forecasts foresee a doubling of agricultural water demand in the coming 50 years. This is reason for concern as already 1.2 billion people live in areas where water is insufficient to meet all demands. Fortunately, there seems much scope to improve productive use of water and get more out of a unit of water. This paper explores forecasts of global agricultural water demand and scenarios to meet this. It concludes with challenges in future water supply.

This paper describes the challenges facing irrigated agriculture today and in the future, with a focus on recent challenges, including rapid increases in non-irrigation water demands, growing water pollution, competition from biofuels, and growing impact from climate variability and change. Increased agricultural productivity is suggested as a key investment to counteract growing water shortages for food production and food security.

Rapid increase in world population during the 20th century, along with the conversion of land to non-agricultural uses, have drastically decreased the availability of finite soil resources for agricultural use. Per capita soil area for agricultural use is also decreasing because of soil degradation. Four related but different terms, often used interchangeably with erroneous and confusing interpretations, are soil degradation, land degradation, desertification and vulnerability to desertification. Global area subject to different degradation processes is estimated at 1 965 Mha by soil degradation, 3 506 Mha by land degradation, 3 592 Mha by land desertification of which 1 137 Mha is soil desertification, and 4 324 Mha by vulnerability to land desertification. Urbanisation and conversion to industrial land uses and development of infrastructure are also competing land uses. In 2005, 3.16 billion people lived in urban centres over a globally urbanised land area of 351 Mha. In the United States, 79% of the total population of about 300 million lives in urban centres over a land area of 18.6 Mha, or 2% of the total US land area. In rapidly urbanising China, India and other Asian countries, brick making uses topsoil to 1-m depth equivalent to 0.5%-0.7% of cropland area per year in some regions. Policy interventions are needed to limit conversion of prime farmland to nonagricultural uses.

Soil resources are being degraded primarily by nutrient mining in poor countries and by nutrient loading and other excesses in rich counties. Both are reversible, by applying science-based policies to counteract them. Food production can drastically increase in Africa with the proper use of donor funding, limiting food aid to starvation situations, and enabling chronically hungry small farm households in Africa to have access to improved hybrid seeds and appropriate mineral fertilisers. The fertiliser and improved seed required to produce an additional tonne of maize grain by Millennium Village farmers cost six times less than the same tonne of US food aid.