Critical water requirements for food, methodology and policy consequences for food security

Food security and increasing water scarcity have a dominant place on the food policy agenda. Food security requires sufficient water of adequate quality because water is a prerequisite for plant growth. Nowadays, agriculture accounts for 70% of the worldwide human fresh water use. The expected increase of global food demand requires a great deal of effort to supply sufficient fresh water. If a doubling of agricultural production goes along with a doubling of the use of water, current fresh water resources are probably not sufficient in the long run. The objective of this study is to develop a generally applicable method for the assessment of crop growth-related water flows or ‘‘transpirational water’’ requirements of agricultural crops. Traditionally, agricultural studies have made assessments of water requirements for specific situations to provide a yield. This study uses the agricultural information the other way around. Water had to be present for a growth to occur. Based on the strong linearity of processes taking place in all green plants, the study develops a method to calculate the growth-related factor of crop water requirements, assesses the impact of crop characteristics on water requirements, and evaluates options to reduce the use of water by changing food consumption patterns. The study calculates ‘‘transpirational water’’ requirements for a representative group of crops with different functions for human nutrition, such as staple crops, vegetables, and livestock fodder. For C3 crops in a temperate climate, 0.11 l are needed to produce 1 g of glucose; for C4 crops in a tropical climate, 0.09 l. Water requirements per unit of dry mass differ by a factor of two. These differences arise from differences among harvest indices of crops and their chemical composition. Differences in requirements per unit of nutritional energy, however, are low. Therefore, options to reduce the use of water by qualitative changes of food consumption patterns are few. The study distinguished between site-specific and crop growth specific water flows. In this way, it quantified a central flow of the hydrological system, the water flow that actually passes a crop and is directly related to the photosynthesis process. If yields increase, this water demand increases with the same factor. The results show critical water requirements in agriculture. However, these results are intended to improve the insight into hydrological systems and must always be used in combination with locally, variable water needs. The results have two important consequences for food policy issues. Firstly, the study shows only small differences in water requirements among crops. Secondly, results indicate that under rainfed conditions, a doubling of food production on existing land areas does not imply a doubling of water use but only of ‘‘transpirational water’’ use. This flow forms a minor part of total water requirements in agriculture