Agriculture is the World's largest water consuming sector, which combined with conventional water resources assessments has led to serious concern whether there actually is enough water to support rapidly growing populations with food, especially in tropical developing countries.

This paper challenges to conventional assessments on water resource availability, which tend to be strongly biased towards water resources in perennial rivers, accessible groundwater and lakes, so-called blue water.

It is argued that the by far largest contributing water flow to global livelihood security is direct return flow of vapour, so-called green water, sustaining growth of economic and natural biomass.

A shift in hydrological thinking is outlined that opens the window for new opportunities and challenges in the field of water for food security. System options for improved crop yields among smallholder farmers in water scarce tropical regions are presented. System trade-off's between natural ecosystems and farming systems are analysed.

It is hypothesised that a major future challenge in the field of water resources management may be how to balance vapour flow availability between different biomes.

Johan Rockstrom's research is focused on integrated soil and water management in tropical agro-ecosystems, with special attention to linking land management with ecohydrology at a watershed scale. He is involved in several research projects on water management systems for improved food security among small-holder farmers in semi-arid savannah environments in sub-Saharan Africa.