Virtual water content of temperate cereals and maize: Present and potential future patterns

Abstract

Knowledge of the virtual water content (VWC) of crops and especially its possible future developments is helpful for improvements in water productivity and water management, which are necessary at global scale due to rising demand for food, the necessity to ease present and future water scarcity, and the reduction of poverty. Using a dynamic global vegetation and water balance model (LPJmL), this study quantifies the VWC of two of the most important crop types worldwide, temperate cereals and maize, at high spatial resolution (0.5°). We analyzed present conditions (1999–2003) and also for the first time also for scenarios of future climate and increasing atmospheric CO2 concentrations (2041–2070; HadCM3, ECHAM5 and CCSM3 climate models, A2 emissions scenario). VWC presently differs significantly among regions: highest values are common in large parts of Africa (>2 m3 kg−1), and lowest values were found e.g. for Central Europe (<0.5 m3 kg−1), indicating that water-use efficiency of crops is much higher in the latter region. The regional patterns of VWC result from complex and interactive processes; the dominant factor is the crop yield level (high VWC values occur most frequently in regions with low yields). Climate change and rising atmospheric CO2 concentration will have non-uniform effects on crop yields and evapotranspiration. Worldwide VWC patterns will change significantly, with a pronounced regional pattern that reflects primarily the changes in yields as driven mainly by regionally decreasing precipitation, increasing temperature and increasing atmospheric CO2 concentration. Although globally the water-use efficiency is projected to increase, many regions—including parts of the US, East and Mediterranean Europe, South Africa, Argentina, Australia and South East Asia—are projected to become less water efficient (higher VWC) for at least one of the crop types. CO2 fertilisation was simulated to generally reduce VWC, though realisation of this effect in the field will depend, for example, on the intensity of nutrient management in the future. The potentially adverse future changes in VWC found here pose a challenge to water management efforts and eventually global trade policies.