Many authors in recent years have suggested that increases in water productivity are needed to ensure food security in 2050 and beyond. Some authors call for increasing the ‘crop per drop’ or ‘value per drop,’ generated with water in agriculture in rainfed and irrigated settings.5,6 Those phrases are essentially analogous to the notion of increasing water productivity, which often is defined as the ratio of some measure of output (either crop mass or value) to some measure of water input (either water applied or transpired).
At first glance, the call to increase water productivity seems appropriate and compelling, given the obvious need to increase crop production to meet increasing food demands. Yet, water productivity, as defined in the literature, is simply a measure of total output or total value, divided by the amount of a single input used in production. The resulting ratio describes the average amount of output or value associated with the water applied or consumed. The ratio does not describe the incremental productivity of water, and it does not account for the contributions of other inputs in crop production. For these reasons, water productivity cannot serve as an indicator of economic efficiency, which requires consideration of incremental gains and costs, including opportunity costs.
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