Simulations of global evapotranspiration using semiempirical and mechanistic schemes of plant hydrology

Some of the plant hydrology schemes implemented in global land surface models (LSMs) are relatively simple. This is despite evidence that simulated carbon, water, and energy fluxes are sensitive to both the availability of soil moisture and the formulation of plant hydrology. The current study introduces a mechanistic scheme of plant hydrology (soil-plant-atmosphere (SPA)) into a global LSM (Joint U. K. Land Environmental Simulator (JULES)) in order to compare with a traditional, semiempirical plant hydrology scheme (SiB) based on the Ball-Berry stomatal relation. The SPA scheme simulates explicitly the physical processes which change leaf water potential and account for the flow of water through the soil-plant-atmosphere media. Using both plant hydrology schemes, the annually averaged global evaporation-to-precipitation ratio is 0.58+/-0.9. The annually averaged global transpiration-to-precipitation ratio is 0.22+/-0.09 and 0.28+/-0.08 using the SPA and SiB plant hydrology schemes, respectively. The output from the two plant hydrology schemes typically differs less than the systematic errors associated with the different observational data sets (eddy covariance fluxes, continental runoff, etc.) employed to calibrate and validate the LSM. However, SPA is more conservative with respect to plant water use efficiency compared to SiB. This is partly due to the exhaustion of stored leaf water (not accounted for with the SiB scheme) which acts to limit afternoon transpiration when diurnal vapor pressure deficit is greatest. The trend in global runoff simulated with both plant hydrology schemes for the latter half of the 20th century (-1% per 50 years) agrees well with recent observational estimates that sample 80% of the global runoff network.