Impact of Water Use Efficiency Parameterization on Partitioning Evapotranspiration with the Eddy Covariance Flux Variance Method.

Partitioned observations of evapotranspiration (ET) into its constituent components of soil and canopy evaporation (E) and plant transpiration (T) are needed to validate many agricultural water use models. E and T observations are also useful for assessing management practices to reduce crop water use by suppressing soil E or reducing T by plant stress. However, the most commonly used approaches to measure crop water use, such as weighing lysimeters, soil water balance, or eddy covariance (EC), measure ET and cannot directly partition ET into E and T. Recent work with EC data has led to a new method to partition ET based on the variance of high frequency CO₂ and H₂O data and leaf level water use efficiency (WUE). This method, hereafter called the flux variance partitioning (FVP) approach, is advantageous because it can use standard EC observations without additional constraints such as isotopic data, sap flow, or soil microlysimeters or Bowen Ratio systems to partition ET. However, FVP relies on parameterized leaf temperature and intercellular CO₂ data to correctly estimate WUE and thus E and T. In this presentation we evaluate several differing approaches to estimate leaf level WUE for partitioning ET with FVP. We test these approaches against a two source energy balance (TSEB) approach in a peach orchard in California, USA. Initial results show that a WUE parameterization using an intercellular CO₂ that depends on vapor pressure deficit has a transpiration flux that is 15 W m^-2 (0.53 mm day^-1) lower than a WUE parameterization with a constant intercellular to atmospheric CO₂ ratio. The frequency of model convergence with FVP is also slightly higher with the vapor pressure deficit parameterization. The results indicate that FVP is a useful tool to partition ET, but that the robustness of the partitioning depends on constraining leaf-level WUE.