Water for food and nature in drought-prone tropics:: vapour shift in rain-fed agriculture

This paper quantifies the eco-hydrological challenge up until 2050 of producing food in balance with goods and services generated by water-dependent ecosystems in nature. Particular focus is given to the savannah zone, covering 40% of the land area in the world, where water scarcity constitutes a serious constraint to sustainable development. The analysis indicates an urgent need for a new green revolution, which focuses on upgrading rain-fed agriculture. Water requirements to produce adequate diets for humans are shown to be relatively generic irrespective of hydro-climate, amounting to a global average of 1300 m(3) cap(-1) yr(-1). Present food production requires an estimated 6800 km(3) yr(-1) of consumptive green water (5000 km(3) yr(-1) in rain-fed agriculture and 1800 km(3) yr(-1) from irrigated crops). Without considering water productivity gains, an additional 5800 km(3) yr(-1) of water is needed to feed a growing population in 2050 and eradicate malnutrition. It is shown that the bulk of this water will be used in rain-fed agriculture. A dynamic analysis of water productivity and management options indicates that large 'crop per drop' improvements can be achieved at the farm level. Vapour shift in favour of productive green water flow as crop transpiration could result in relative water savings of 500 km(3) yr(-1) in semi-arid rain-fed agriculture.