Advances in water science, management, and conservation

Abstract

Water will become increasingly limited as the world’s population increases to a projected 8 billion by 2025. Agriculture is by far the biggest user of water, but as population increases and industries grow, the competition among users will intensify. Although about 70% of the world’s food is produced from rainfall, irrigation expansion during the past few decades is the primary reason that food production has increased at a faster rate than population. Worldwide, about 17% of the cropland is irrigated and accounts for 40% of the food and fiber production. Irrigation, however, uses much of the water abstracted from aquifers and surface water supplies, such as lakes and rivers. Irrigation accounts for nearly 70% of world water abstraction, and it is more than 90% in some agricultural economies in the arid and semiarid tropics. Agriculture must become more efficient in the use of both rainfall and irrigation water. There are essentially five ways that improvements can progress. The first, but perhaps the most challenging, is to increase photosynthetic efficiency. The second is to increase the amount of evapotranspiration, which is the sum of the water transpired through plant leaves and the water evaporated from the soil surface while the crop is growing. The third is to reduce evaporation of water during the period when a crop is not growing so that more water will be retained in the soil for use by a subsequent crop. Fourth, the harvestable portion of the crop can be increased. The fifth is to increase the portion of the field water supply that is used for evapotranspiration by reducing percolation, runoff, and evaporation.