Plant‐based predictions of canopy transpiration instead of meteorological approximations

Estimation of plant water use is critical, both now and under future climate‐changed environments, to understanding water limitation on plant production and hydrologic cycling. Currently, such predictions are based on meteorological approaches that are inherently empirical and should not be extrapolated beyond the empirical database. An alternative approach to calculating transpiration that relies on fundamental physiological and physical descriptors of canopy water use efficiency is reviewed. Re‐arrangement of the water use efficiency expression results in a definition of transpiration based on explicitly defined and readily observed, if necessary, parameters. The key plant parameters are photosynthate conversion to plant mass, photosynthesis pathway (C4 vs. C3), and canopy radiation use efficiency. The parameters tend to be stable within a species under non‐stressed conditions, although fully defined differences exist among species. Also, the consequences of stresses such as temperature, water deficit, and nitrogen deficit can be readily accounted for by their effect on canopy carbon accumulation. The capability of this approach in predicting canopy transpiration is illustrated in a comparison between calculated and measured transpiration by turf grasses. Water use predictions by natural and managed plant canopies need not to be handicapped by empirical meteorological approaches, but rather explicit parameters associated with various species traits can be used to define transpiration rate.