No abstract

Abstract:Water footprint (WF) accounting as proposed by the Water Footprint Network (WFN) can potentially provide important information for water resource management, especially in water scarce countries relying on irrigation to help meet their food requirements. However, calculating accurate WFs of short-season vegetable crops such as carrots, cabbage, beetroot, broccoli and lettuce presented some challenges. Planting dates and inter-annual weather conditions impact WF results. Joining weather datasets of just rainfall, minimum and maximum temperature with ones that include solar radiation and wind-speed affected crop model estimates and WF results. The functional unit selected can also have a major impact on results. For example, WFs according to the WFN approach do not account for crop residues used for other purposes, like composting and animal feed. Using yields in dry matter rather than fresh mass also impacts WF metrics, making comparisons difficult. To overcome this, using the nutritional value of crops as a functional unit can connect water use more directly to potential benefits derived from different crops and allow more straightforward comparisons. Grey WFs based on nitrogen only disregards water pollution caused by phosphates, pesticides and salinization. Poor understanding of the fate of nitrogen complicates estimation of nitrogen loads into the aquifer.

Food production in water-scarce countries like South Africa will become more challenging in the future because of the growing population and intensifying water shortages. Reducing food wastage is one way of addressing this challenge. The wastage of carrots, cabbage, beetroot, broccoli and lettuce, produced on the Steenkoppies Aquifer in Gauteng, South Africa, was estimated for each step along the supply chain from the farm to the consumer. Water footprints for these vegetables were used to determine the volume of water lost indirectly as a result of this wastage. Highest percentage wastage occurs at the packhouse level, which is consistent with published literature. Some crops like lettuce have higher average wastage percentages (38%) compared to other crops like broccoli (13%) and cabbage (14%), and wastage varied between seasons. Care should therefore be taken when applying general wastage values reported for vegetables. The classification of "waste" presented a challenge, because "wasted" vegetables are often used for other beneficial purposes, including livestock feed and composting. It was estimated that blue water lost on the Steenkoppies Aquifer due to vegetable crop wastage (4 Mm 3 year −1) represented 25% of the estimated blue water volume that exceeded sustainable limits (17 Mm 3 year −1).

The lack of sustainability of our water resources threatens food security in many places worldwide. Different water footprint (WF) methodologies were investigated for their ability to improve water management at various scales. Methodologies according to the Water Footprint Network (WFN), life cycle assessment (LCA) and hydrological‐based approaches were assessed, and a working example is given for apples produced in South Africa. A fundamental viewpoint was defined and the knowledge hierarchy applied to investigate the approaches and information generated. WFs reported simply as a volume of water used per mass of crop produced cannot indicate the sustainability of the water use unless interpreted within the local hydrological and environmental context. The WFN methodology appears most useful to resource managers due to its quantitative nature and ability to compare blue and green water consumption versus water availability. The LCA approach may be best for comparisons of the impact of different products. None of the methodologies provides a single metric that can be used to inform wise consumer choices as it is not possible to incorporate all the complexities associated with water use into a single number that can be used to inform sustainable water use. © 2018 John Wiley & Sons, Ltd.

No abstract