Out of the total precipitation over the continents, only one third becomes runoff in rivers and recharges aquifers, so-called blue water (see Box 1), which takes the liquid route to the sea. Two thirds infiltrate into the soil, forming the so-called green water (see Box 1) that supplies the plant cover, and returns to the atmosphere as vapour flow. In spite of the dominance of green water in plant production, it is still common to seek solutions to water deficits in crop production mainly by increasing irrigation, i.e. adding blue water. At the same time, soil moisture has generally been interpreted merely as a component of the soil (Falkenmark and Lundqvist, 1996). Hydrologists’ interest in green water as a resource originally grew out of studies by Soviet hydrologists in the early days of the International Water Decade (1965–1974). L’vovich (1974), for instance, quantified the amount of water involved in terrestrial biomass production and developed from that comprehensive water balances for all continents and major ecological regions.
Today, the new focus on providing more water for food production for a growing population results in warnings – in particular from the ecological community – that the limits of irrigation expansion have been reached in many regions. As a result of increasing water withdrawal, primarily for agriculture, a growing number of river basins are “closing” with no uncommitted river flow left (Falkenmark and Molden, 2008). Integrated Water Resources Management (IWRM) with a focus on blue water only, can no longer provide sustainable solutions. This has generated interest in the potential of the invisible green water resource for additional crop production, and in shifting more of the green water flow from unproductive evaporation to productive transpiration. The new paradigm of managing precipitation as the key resource, including both green and blue water, provides an additional degree of freedom to help close the water gap (Falkenmark and Rockström, 2004). The integrated green–blue water approach opens up new avenues for research as well as for sustainable development and poverty alleviation.
Recent model developments enable a global, spatially explicit, consistent and process-based assessment of green and blue water availability, flow paths, and productivity, particularly in agriculture (Alcamo et al., 2007a, Liu et al., 2007, Rost et al., 2008, Vörösmarty et al., 2005). With the advanced models at hand, the full water resource, i.e. blue and green water, can be addressed, together with a wide range of possible interventions from soil and water conservation all the way to large-scale water infrastructure, and associated potentials for increasing food security and environmental sustainability (Rost et al., submitted for publication).Definition of green and blue water.
Following the definition of Rockström et al. (2009), green water is the soil water held in the unsaturated zone, formed by precipitation and available to plants, while blue water refers to liquid water in rivers, lakes, wetlands and aquifers, which can be withdrawn for irrigation and other human uses. Consistent with this definition, irrigated agriculture receives blue water (from irrigation) as well as green water (from precipitation), while rainfed agriculture only receives green water.
Rainwater harvesting, as addressed by Wisser et al. (this issue), is at the interface of blue and green water. Catching runoff and storing it in small reservoirs (or possibly underground) is interpreted as blue water management, enhancement of soil infiltration as green water management.
The papers in this special issue consistently use this resource definition, and separately calculate green and blue consumptive crop water use and green and blue virtual water content in irrigated and rainfed agriculture.
This special issue synthesises green and blue water simulations from a wide range of global models with different origins, ranging from hydrological, vegetation and crop models, to partial and general equilibrium economic models. Accordingly, the focus of the different authors varies.
Menzel and Matovelle (this issue) simulated future global and regional blue water scarcity for a range of different climate and socio-economic scenarios and the relative importance of changes in water availability versus changes in demand.
Fader et al. studied consumptive crop water use and resulting virtual water content of crops in rainfed and irrigated systems, revealing significant differences between regions and also for future climate and CO2 scenarios.
Siebert & Döll studied crop water productivity and virtual water content of various crops, showing a large dominance of green water in average virtual water content (1100 m3/ton) compared to blue water (291 m3/ton). They also calculated the hypothetical loss in total cereal production if there was no irrigation (−20%).
Hanaski et al. analysed the global virtual water trade, i.e. the amount of real water demand substituted by virtual water imports, the water footprint left in the exporting countries and the global water savings from trade. They also quantified green and blue water contributions to virtual water trade as well as contributions from non-renewable sources and from medium-sized reservoirs.
Calzadilla et al. focus on the role of green and blue water in agriculture and international trade. They compared a water crisis scenario with a sustainable water use scenario, the latter eliminating groundwater overdraft and increasing allocations for the environment. They quantified the contribution of irrigation to short-term economic welfare, and the difficult tradeoffs with long-term sustainability goals in countries with groundwater overdraft, as well as knock-on effects in other countries.
The study by Sulser et al. combines blue and green water management strategies with other complementary agricultural investments. They show for different scenarios how a combined approach has the potential to positively impact the lives of many more poor people around the world.
Wisser et al. quantified the potential of different intensities of small-scale rainwater harvesting, water storage and supplemental irrigation, for increasing global cereal production, with the largest potential increases being found in Asia and Africa. They also show the potential negative impacts on downstream river flow.
Finally, Liu & Yang simulated the consumptive water use in croplands, and compared the blue water fraction with national and sub-national statistics. They show that during the growing period, the croplands globally consumed a total of 5940 km3 year−1 (84% of which was green water), and over the entire year 7323 km3 year−1 (87% green). They demonstrate the potential for better management of this resource, in particular in combination with nutrient management.