Water productivity and water footprints are not helpful in determining optimal water allocations or efficient management strategies

In a recent paper in Water International, Amarasinghe and Smakhtin suggest that crop–water production functions provide conceptual support for using water productivity and water footprints to guide water allocation and to determine optimal management strategies. Their analysis is thoughtful and interesting. Yet, there is no conceptual foundation supporting the use of water productivity or water footprints in an optimizing context. Production functions do not provide conceptual support. Policy makers wishing to achieve the optimal allocation of water must consider incremental gains and costs, including opportunity costs and externalities. Ratios depicting average output or average value do not contain sufficient information for determining optimal water allocations or efficient management strategies.