Principles of Transpiration and Crop Water Production Functions.

Increasing crop water productivity (mass of marketable yield per unit water use) can improve land productivity and support food security, given increased global demands on freshwater sources. Foundational studies of 'crop water requirement' demonstrated influences of crop physiological type and atmospheric demand on the transpiration ratio. Analysis of the soil-plant-atmosphere energy balance established potential evaporation as a thermodynamic benchmark for crop water use. Irrigation management applications use crop coefficients and potential evapotranspiration to forecast irrigation requirements. In semiarid regions, regional advection can further increase the evaporative demand. Corresponding production functions for crop water productivity involve evaporation and transpiration; shoot biomass primary productivity; grain fraction of shoot biomass; carbohydrate equivalents for oil, protein, and starch components of grain; C3 and C4 carbon assimilation pathways; and vapor pressure deficits. Opportunities to improve crop water productivity include crop management which matches light utilization to available water supply, reduces the likelihood of water deficits for critical yield-formation processes, and utilizes crop residues to create favorable microclimate effects. Adaptive crop traits include increased transpiration efficiency and harvest index, and root geometries which favor timely soil water acquisition and exploit root-shoot signaling.