Measurement and Partitioning of Evapotranspiration for Application to Vadose Zone Studies

Abstract

Partitioning evapotranspiration (ET) into its constituent components, evaporation (E) and transpiration (T), is important for numerous hydrological purposes including assessing impacts of management practices on water use efficiency and improved validation of vadose zone models that parameterize E and T separately. However, most long-established observational techniques have short observational timescales and spatial footprints, raising questions about the representativeness of these measurements. In the past 15 yr, new approaches have allowed ET partitioning at spatial scales ranging from the pedon to the globe and at long timescales. In this update, we review some recent methodological developments for partitioning ET. These include micrometeorological approaches involving the flux variance partitioning of high-frequency eddy covariance observations and proxies for photosynthesis and transpiration such as measurements of isotopic fractionation and carbonyl sulfide uptake. We discuss advances in partitioning the energy balance between canopy and soil using remote sensing. We conclude that the flux variance partitioning with raw eddy covariance data and the two-source energy balance approaches with remote sensing platforms may have the greatest potential for partitioning ET, in part because large public repositories of eddy covariance and satellite data could be readily reprocessed to partition ET.Abbreviations: COS, carbonyl sulfide; CRDS, cavity ring-down spectroscopy; ECV, eddy covariance; ET, evapotranspiration; FVP, flux variance partitioning; ICOS, integrated cavity output spectroscopy; TSEB, two-source energy balance; UAV, unmanned aerial vehicle; WUE, water use efficiency.Evapotranspiration (ET) is one of the largest hydrologic fluxes on Earth, accounting for approximately 60% of terrestrial precipitation globally (Haddeland et al., 2011) or about 6.5 to 6.8 ´ 10 4 km 3 water yr −1 (Oki and Kanae, 2006;Jung et al., 2010;Miralles et al., 2011). Evapotranspiration is clearly one of the most important boundary conditions for vadose zone studies, and it is well recognized that there are numerous controls on the amount of ET, including meteorological and vegetation conditions (Williams et al., 2012;Puma et al., 2013) and soil moisture Jung et al., 2010). Depending on the temporal and spatial scale, ET can be observed with a wide variety of techniques including lysimetry (Gee and Hillel, 1988;Poss et al., 2004;Johnson et al., 2005), micrometeorology (including eddy covariance) (Snyder et al., 1996;Todd et al., 2000;Hemakumara et al., 2003;Williams et al., 2004), satellite remote sensing (Bastiaanssen et al., 1998;Allen et al., 2007;Senay et al., 2013), water balance (Wilson et al., 2001;Zeng et al., 2014), or a combination approach (Nagler et al., 2005;Anderson and Goulden, 2009;Goulden et al., 2012;Zhang et al., 2015). Evapotranspiration can be modeled with a variety of models, ranging from complex, mechanistic equations (Monteith, 1965) to simple, empirical formulae (Makkink, 1957;Hargreaves and Samani, 1985...

Partitioning Eddy Covariance Water Flux Components Using Physiological and Micrometeorological Approaches

“…Alternatively, the flux variance similarity (FVS) method employs a combination of turbulence scaling arguments and prior information of WUE, typically from leaf observations, to numerically solve water fluxes from high‐frequency EC data (Scanlon & Kustas, ; Scanlon & Sahu, ). Although the method has been successfully applied to different ecosystem types (Scanlon & Kustas, ; Sulman et al, ; Wang et al, ), the accuracy of the method remains dependent on prior knowledge of plant WUE (Anderson et al, ). Also, the method assumes that the turbulent Schmidt numbers for CO 2 and water vapor are identical (Reynolds analogy), which may be questionable in nonideal meteorological conditions due to variability in sources and sinks near the ground and dissimilarity in entrainment fluxes.…”

Reviews and syntheses: Turning the challenges of partitioning ecosystem evaporation and transpiration into opportunities

“…Ongoing efforts to synthesize measurements of ecosystem water cycle components -for example, SAPFLUXNET (Poyatos et al, 2016) -are a promising approach to build an understanding of different terms of the ecosystem water balance across global ecosystems. Multiple reviews and syntheses of E and T measurements have been written (e.g., Abtew and Assefa, 2012;Anderson et al, 2017b;Blyth and Harding, 2011;Kool et al, 2014;Shuttleworth, 2007;Wang and Dickinson, 2012) and have provided the key insights that ecosystem models use to simulate ecosystem-atmosphere water flux (De Kauwe et al, 2013). Rather than reiterate the findings of these studies, we focus on existing and emerging approaches to partition E and T at the ecosystem scale on the order of tens of meters to kilometers at temporal resolutions on the order of minutes to hours, with a particular emphasis on new observational and methodological techniques.…”

“…For full water balance accounting, observations of drainage from the rooting zone using drainage lysimeters, soil moisture at multiple soil levels spanning the root zone, the flow of water down plant stems (stemflow), leaf wetness sensors, measurements of the amount of water held in plants themselves, and of course multiple precipitation gauges are required. Such a multi-measurement approach would also create an opportunity to compare the performance of emerging technologies like distributed temperature sensing from fiber-optic cables (Schilperoort et al, 2018), modeling cosmic ray neutron fields for soil water source estimation (Andreasen et al, 2016), and global navigation satellite system reflectometry (GNSS-R) for soil moisture estimation (Zribi et al, 2018). It remains difficult to assimilate E and T measurements into models using conventional data assimilation techniques because observations may contain substantial bias error yet still provide valuable information (Williams et al, 2009).…”

“…Recent statistical ET partitioning approaches (Rigden et al, 2018) are similarly limited by the lack of direct E and T observations for evaluation. Interest in partitioning E and T from ecosystem ET measurements has grown in recent years (Anderson et al, 2017b), and many new measurements and modeling approaches seek to do so but often rely on assumptions that need further testing. We begin with a brief research review that notes recent updates to our theoretical understanding of ET and outlines the challenges in measuring E and T at the ecosystem scale.…”

Deep soil water <sup>18</sup>O and <sup>2</sup>H measurements preserve long term evaporation rates on China's Loess Plateau

“…Water loss from soil by evaporation is an essential component of the terrestrial water cycle (Or and Lehmann, 2019). Estimation of soil evaporation, not only the amount but also the ratios of it to precipitation (f) or evapotranspiration, within the 25 different landscapes helps understanding ecohydrological processes (Sprenger et al, 2017b;Sprenger et al, 2017a), quantifying the water balance (Skrzypek et al, 2015), partitioning evapotranspiration (Anderson et al, 2017;Kool et al, 2014), and calibrating rainfall-runoff models (Birkel et al, 2014). Because of the dynamic nature due to variation of climate, surface soil water capacity, and vegetation conditions (Or and Lehmann, 2019), a more reliable assessment of soil evaporation or f needs to determine the long-term average values (Stoy et al, 2019), which remains a serious technical constraint (Kool 30 et al, 2014).…”