Partitioning eddy covariance-measured evapotranspiration into transpiration and evaporation: the opportunities outweigh the challenges
Paul Stoy
University of Wisconsin-Madison
Evaporation and transpiration are two of the most uncertain terms in the global water balance. They are relatively difficult to measure, respond differently to ongoing climate changes, and transpiration in particular is closely coupled to vegetation processes like photosynthesis. At the same time, ecosystem evapotranspiration is relatively easy to measure using eddy covariance, and multiple methods have been proposed to partition transpiration and evaporation using flux data. Here, I briefly describe the theoretical bases for partitioning evaporation and transpiration using eddy covariance-measured evapotranspiration and discuss how each is promising for understanding global changes to water cycling. I then discuss the opportunities created by flux variance partitioning – which uses turbulence theory to directly partition transpiration and evaporation from evapotranspiration measurements – and present initial transpiration and evaporation estimates from 17 towers from the CHEESEHEAD19 campaign (PI: A. Desai) in northern Wisconsin, USA. Results point to key simplifications for understanding evaporation across seasonal transitions across the study domain that may help understand water resources in temperate forest ecosystems.
How well can we quantify evapotranspiration?
Matthias Mauder
Technische Universität Dresden
The eddy-covariance method provides the most direct estimates for fluxes between ecosystems and the atmosphere, and hence also evapotranspiration. However, dispersive fluxes can occur in the presence of secondary circulations, which can inherently not be captured by such single-tower measurements. In this study, we present options to correct local flux measurements for such large-scale transport based on a non-local parametric model that has been developed from a set of idealized large-eddy simulations. This method is test for the sites DK-Sor, DE-Fen, and DE-Gwg, representing typical conditions in the mid-latitudes with different measurement height, different terrain complexity and different landscape-scale heterogeneity. Two options to determine the boundary-layer height, which is a necessary input variable for modelling the dispersive fluxes, are applied, either based on operational radio-soundings and local in-situ measurements for the flat site or from backscatter-intensity profile obtained from collocated ceilometers for the two sites in complex terrain. The adjusted total fluxes are evaluated by assessing the improvement in energy balance closure and by comparing the resulting latent heat fluxes with evapotranspiration rates from nearby lysimeters.
Evapotranspiration-vegetation biomass relationship in a South American salt marsh, and its importance for livestock management
Natalia Tonti
Universidad de Buenos Aires
Salt marshes have gained interest because of their importance for water and carbon cycles. Among the ecosystem services they provide is their use as pastures for livestock production. As a result, the prediction of biomass productivity can be of great interest for the sustainable management of these environments. Evapotranspiration is one of the variables most used to estimate the yield of green biomass in pastures and crops, which to date has not been examined in detail for natural environments. One reason is that species diversity makes the estimation of evapotranspiration with Penman-Monteith equation troublesome. In this lecture I am going to show some results from studies carried out at Mar Chiquita salt marsh (Buenos Aires, Argentina) dominated by Spartina densiflora. We studied the aboveground biomass and species cover variability for two categories (erect and sward plants) in three plots affected by low, moderate, and high cattle grazing during a 2-year period. Evapotranspiration was estimated with a coupled surface resistance Penman-Monteith model. Plant categories show different growth response according to livestock impact. S. densiflora has a slow-growing response after cattle consumption, even with high evapotranspiration. On the other hand, sward plants respond with biomass overproduction to livestock consumption, and a significantly positive relationship to evapotranspiration rate.
Evaluating Terrestrial Biosphere Model ET and ET partitioning in SW US Semiarid Ecosystems and Interactions with Vegetation and Carbon Cycling
Natasha MacBean, Kashif Mahmud, Russ Scott
Indiana University Bloomington
Semiarid regions are hotspots of land-atmosphere coupling and play an important role in global carbon cycle interannual variability. Accurate modeling of semiarid ecosystem processes is therefore key for both reliable short-term weather forecasts as well as longer-term projections of carbon-climate feedbacks. As water availability is a dominant control on semiarid ecosystem processes, it is crucial that the global terrestrial biosphere models (TBMs) that form the land component of numerical weather prediction and earth system models can accurately capture spatio-temporal dynamics of evapotranspiration (ET) and partitioning into its component bare soil evaporation (E) and plant transpiration (T). Here, we evaluated time series of soil moisture, ET and T/ET simulated by the ORCHIDEE TBM against observations from 6 Ameriflux sites across semi-arid grass, shrub and forest sites in the southwestern US. We show that models that include a mechanistic representation of vertical soil moisture diffusion capture well diurnal to seasonal temporal dynamics. However, modeled T/ET ratios were generally lower than estimated values across all sites, particularly during the monsoon season. Furthermore, discrepancies remain in the timing of the transition from minimum T/ET during the hot, dry May–June period to increasing values during the start of the monsoon in July–August. Evaluating the simulated ecosystem water use efficiency points towards a too weak response of modeled carbon uptake to increasing water availability, which we hypothesize could be due to poor model representation of the variety of phenological and physiological semiarid plant strategies for dealing with high fluctuations in changing water availability. We present a set of ongoing and planned model optimization and model development studies that seek to explore this hypothesis, with the ultimate goal of improving TBM predictions of semiarid ecosystem coupled vegetation-carbon-water interactions.
Physical and biophysical constraints to changes in evapotranspiration
Simone Fatichi
National University of Singapore
Distributed and accurate observations of precipitation are becoming common, however estimates of evapotranspiration (ET) from the forest stand to larger scales are particularly challenging and uncertain, despite increasing availability of flux-tower observations, remote sensing products, and process-based models. Accurate quantification of ET and its spatial-temporal variability is however increasingly demanded by the community to both understand fundamental processes related to the hydrological budget as well as to answer practical management questions.
In this presentation, I review how the dominant controls affecting ET, namely, air temperature, atmospheric CO2, solar radiation, wind speed, vapor pressure deficit, water availability and vegetation cover, have been changing in last few decades and how this might have affected the trends in ET and their uncertainty reported in published regional and global scales studies. A particular emphasis is given to the potential pitfalls of using simple methods to compute ET and on the importance to properly consider water and energy constraints at the appropriate scales, as these can dominate the evapotranspiration response also in presence of considerable land cover changes. Finally, I discuss by means of modeling studies, the compensating responses of vegetation to increasing CO2, i.e., increased leaf area and reduced stomatal conductance, which have likely constrained ET changes in recent decades despite considerable increases in Gross Primary Production and Water Use Efficiency.