Hydrological, engineering, agronomical, breeding and physiological pathways for the effective and efficient use of water in agriculture

In water-scarce environments, water conservation is a means to increase agricultural production and preserve the environment. Water conservation can be approached at different scales with different objectives. Water productivity (the ratio between crop production and water use or water consumption) is increased by reducing non-beneficial use and by regulating water applications as to reduce transpiration in smaller proportion than yield. Water productivity can also be modified through other agronomical practices and breeding. The consumptive use coefficient (consumptive use to supply ratio) can be enhanced through various engineering solutions that reduce the use of irrigation water and energy although they do not necessarily reduce consumptive use. Therefore, engineering solutions for water conservation at farm level do not imply basin-scale water conservation. However, agronomical solutions such as regulated deficit irrigation are directly linked to basin water conservation because they goal is to reduce transpiration with little or no yield penalty. Optimising water productivity is a plausible goal in some situations; however, in others it clashes with profit maximization, food production and food security. Concerning breeding, past evidence shows that except for very harsh environments, selection for greater transpiration efficiency, evaluated physiologically as the ratio of photosynthetic assimilation versus transpiration or indirectly through the stable carbon isotope signature of plant dry matter, may have a negative effect by limiting yield potential as well as productivity under a wide range of water stress conditions.