Evaluation of Ability to Determine Transpiration Fraction from Stable Water Isotopes by Synthesis of Models and Observations in an Alpine Forest

The partitioning of latent heat flux into contributions from ground evaporation and plant transpiration in land surface models is a key feature of any hydrological scheme, but notoriously difficult to resolve. Global scale models show significant differences in the terrestrial energy balance, which can be traced to poor constraints on the pathways of water that control evapotranspiration (ET). In addition, the depth of water uptake has been shown to be correlated with ecosystem ET partitioning, but vegetation rooting profiles are difficult to observe and therefore present a significant source of uncertainty. Stable isotope ratios in water from soil, leaves and water vapor have been used to help constrain the ET partitioning and to track water movement in ecosystems, but many previous studies have been limited in two regards: 1) lack of sufficient data to provide true closure, and 2) lack of an adequate modeling framework to incorporate many of the processes which control ET. We present results for the partitioning of ET from an isotopically-enabled land surface model (ISOLSM) that is driven by meteorological, hydrological and isotopic data collected at the Manitou Experimental Forest during the summer and autumn of 2011. An ensemble of 394 realizations has been produced, constrained by observations of latent heat, sensible heat and CO2 fluxes. Even when all three flux constraints are met, the transpiration fraction is not well-constrained. These experiments show that previous work using isotopic observations likely has significant uncertainty in their ET partitioning estimates. However, root-weighted soil isotope values have proven to sufficiently provide a final constraint on transpiration fraction, and an investigation of the time scales associated with this constraint is conducted. A sensitivity analysis of rooting profile and ET partitioning reveals the soil isotope ratios, and therefore the estimate of transpiration fraction, depend strongly on the rooting profile.