Disentangling ecosystem transpiration from evapotranspiration observations employing simplified vegetation-substrate energy balance model
Authors: Mallick Kaniska , Baldocchi Dennis , Jarvis Andrew , Trebs Ivonne , Sulis Mauro , Berry Joseph 
Published: March 2020

&lt;p&gt;Evapotranspiration (E&lt;sub&gt;ET&lt;/sub&gt;) observed by eddy covariance (EC) towers is composed of physical evaporation (E&lt;sub&gt;E&lt;/sub&gt;) from wet surfaces and biological transpiration (E&lt;sub&gt;T&lt;/sub&gt;), that involves soil moisture uptake by roots and water vapor transfer regulated through the canopy-stomatal conductance (g&lt;sub&gt;C&lt;/sub&gt;) during photosynthesis. E&lt;sub&gt;T&lt;/sub&gt; plays a dominant role in the global water cycle and represents 80% of the total terrestrial E&lt;sub&gt;ET&lt;/sub&gt;. Understanding the magnitude and variability of E&lt;sub&gt;T&lt;/sub&gt; are critical to assess the ecophysiological responses of vegetation to drought. While separating E&lt;sub&gt;T&lt;/sub&gt; signals from lumped E&lt;sub&gt;ET&lt;/sub&gt; observations and/or simulating E&lt;sub&gt;T&lt;/sub&gt; by terrestrial systems models is insufficiently constrained owing to the large uncertainties in disentangling g&lt;sub&gt;C&lt;/sub&gt; from the aggregated canopy-substrate conductance (g&lt;sub&gt;cS&lt;/sub&gt;), evaluating ecosystem E&lt;sub&gt;T&lt;/sub&gt; derived through partitioning E&lt;sub&gt;ET&lt;/sub&gt; observations (or model simulation) is also challenging due to the absence of any ecosystem-scale measurements of this biotic flux and g&lt;sub&gt;C&lt;/sub&gt;. To date, the main methods for partitioning EC-E&lt;sub&gt;ET&lt;/sub&gt; observations are largely based on regressing E&lt;sub&gt;ET&lt;/sub&gt; with gross photosynthesis (P&lt;sub&gt;g&lt;/sub&gt;) and atmospheric vapor pressure deficit (D&lt;sub&gt;A&lt;/sub&gt;) observations. However, such methods ignore the essential feedback of the surface energy balance (SEB) and canopy temperature (T&lt;sub&gt;C&lt;/sub&gt;) on g&lt;sub&gt;C&lt;/sub&gt; and E&lt;sub&gt;T&lt;/sub&gt;.&lt;/p&gt;&lt;p&gt;This study demonstrates partitioning E&lt;sub&gt;ET&lt;/sub&gt; observations into E&lt;sub&gt;T&lt;/sub&gt; and E&lt;sub&gt;E&lt;/sub&gt; [soil evaporation (E&lt;sub&gt;Es&lt;/sub&gt;) and interception evaporation (E&lt;sub&gt;Ei&lt;/sub&gt;)] through an &amp;#8216;analytical solution&amp;#8217; of g&lt;sub&gt;C&lt;/sub&gt;, T&lt;sub&gt;C&lt;/sub&gt; and canopy vapor pressures by employing a Shuttleworth-Gurney vegetation-substrate energy balance model with minimal complexity. The model is called TRANSPIRE (Top-down partitioning evapotRANSPIRation modEl), which ingests remote sensing land surface temperature (LST) and leaf area index (L&lt;sub&gt;ai&lt;/sub&gt;) information in conjunction with meteorological, sensible heat flux (H) and E&lt;sub&gt;ET&lt;/sub&gt; observations from EC tower into the SEB equations for retrieving canopy and soil temperatures (T&lt;sub&gt;S&lt;/sub&gt;, T&lt;sub&gt;C&lt;/sub&gt;), g&lt;sub&gt;C&lt;/sub&gt;, and E&lt;sub&gt;T&lt;/sub&gt;.&lt;/p&gt;&lt;p&gt;E&lt;sub&gt;T&lt;/sub&gt; estimates from TRANSPIRE were tested and evaluated with a remote sensing based E&lt;sub&gt;T&lt;/sub&gt; estimate from an analytical model (STIC1.2), where lumped E&lt;sub&gt;ET&lt;/sub&gt; was partitioned by employing a moisture availability constraints across an aridity gradient in the North Australian Tropical Transect (NATT) by using time-series of 8-day MODIS Terra LST and LAI products in conjunction with EC measurements from 2011 to 2018. Both methods captured the seasonal pattern of E&lt;sub&gt;T&lt;/sub&gt;/E&lt;sub&gt;ET&lt;/sub&gt; ratio in a very similar way. While E&lt;sub&gt;T&lt;/sub&gt; accounted for 60&amp;#177;10% of the annual E&lt;sub&gt;ET&lt;/sub&gt; in the tropical savanna, E&lt;sub&gt;T&lt;/sub&gt; in the arid mulga contributed 75&amp;#177;12% of the annual E&lt;sub&gt;ET&lt;/sub&gt;. Seasonal variation of E&lt;sub&gt;T&lt;/sub&gt; was higher in the arid, semi-arid ecosystems (50 - 90%), as compared to the humid tropical ecosystem (10 - 50%). The TRANSPIRE model reasonably captured E&lt;sub&gt;T&lt;/sub&gt; variations along with soil moisture and precipitation dynamics in both sparse and homogeneous vegetation and showed the potential of partitioning E&lt;sub&gt;ET&lt;/sub&gt; observations for cross-site comparison with a variety of models.&lt;/p&gt;