The Water Resource for Agriculture [and Discussion]

The relation between the yield of a crop and its water loss to the atmosphere is examined and the dependence of this relation on the climatically determined potential rate of water loss is stressed. Current methods for assessing the response of crop production to relative water loss are described, with their limitations. The high overall ratio of water loss to dry matter yield harvested in crop production - estimated to average 5.6 kg g$^{-1}$ globally - is contrasted with the much lower values calculated theoretically and measured in good field practice, demonstrating the opportunities that exist to improve the efficiency with which water is currently used in agriculture. The technological methods available to improve this efficiency before the end of this century are reviewed together with the opportunities of extending the size of the agricultural water resource. Special attention is paid to the large potential for doing so in irrigated agriculture and the costs involved. The influence of climatic change at the level anticipated in the next few decades, on water supply, requirements and efficiency of use in irrigated and dryland food production is examined and the uncertainties involved are emphasized.