Demand for both food and water are projected to increase substantially in the next four decades. Water scarcity is also projected to increase in scale and complexity. Climate change is projected to increase temperatures, spatio-temporal variability in rainfall, frequency and severity of droughts and soil water stresses to crops. Due to the crucial role of water in crop growth and yield formation, prolonged or severe soil water deficits in crop producing areas can result in substantial yield penalties. The potential of food trade to help address food insecurity as a result of insufficient water availability for crop production has been rationalized in the virtual water concept. The aim of this thesis was to improve the evidence base for understanding and evaluating the relationships between future water availability for crop production and food trade (or virtual water flows), and the utility of the virtual water concept to inform policy and management decisions on water-food security.
The UK and barley were used as a model country and crop, respectively. Three crop growth simulation models (AquaCrop, CropWat and WaSim) were evaluated for their abilities to estimate the water use of 10 barley genotypes. Subsequently, the effect of projected climate change on UK barley yields in the 2030s, 2040s and 2050s was simulated using the high, medium and low emission scenarios data from the UK Climate Projections 2009 (UKCP09). Projections of total UK feed barley supply and demand were performed to quantify potential virtual water flows and to analyse the implications for food security and policy.
The results show that the predicted water use of barley differed between the models but not among the genotypes. Predicted seasonal water use of the barley genotypes ranged from 241.4 to 319.2 mm. Based on the root mean square error (RMSE) and the index of agreement (D-Stat) values, CropWat performed poorly while AquaCrop and WaSim performed excellently. Barley yields under projected climate change increased substantially over baseline yields in all UK regions. Projected mean barley yields for the UK ranged from 6.04 tons ha-1 (2030s) to 7.77 tons ha-1 (2050s). In spite of the projected increase in yields, the UK faces the risk of large deficits in feed barley and meat supply from the 2030s to the 2050s due to a combination of population growth, increased per capita meat demand and reductions in land area allocated to barley production. Finally, current water scarcity concepts were found to be incompatible with water availability and consumption in crop producing areas, a situation that diminishes the usefulness of the virtual water concept for policy. To address this deficiency, a framework for making water scarcity compatible with crop production was proposed.
In conclusion, the poor performance of CropWat has implications for its wider use in quantifying global virtual water flows associated with crop trade. Even though UK barley yields are projected to increase under projected climate change, the projected deficits in feed barley and meat supply threatens to destabilize future UK food security. The UK can rely on import to offset the large deficits in feed barley and meat supply but can use the proposed framework to reduce the effect of its imports on water scarcity in the exporting countries. The proposed framework improves understanding and evaluation of the role and usefulness of the virtual water concept in water-food security policy and management decisions.