The role of aerodynamic resistance in thermal remote sensing-based evapotranspiration models

&amp;lt;p&amp;gt;&amp;amp;#8216;Aerodynamic resistance&amp;amp;#8217; (hereafter r&amp;lt;sub&amp;gt;a&amp;lt;/sub&amp;gt;) is a preeminent variable in the modelling of evapotranspiration (ET), and its accurate quantification plays a critical role in determining the performance and consistency of thermal remote sensing-based surface energy balance (SEB) models for estimating ET at local to regional scales. Atmospheric stability links r&amp;lt;sub&amp;gt;a&amp;lt;/sub&amp;gt; with land surface temperature (LST) and the representation of their interactions in the SEB models determines the accuracy of ET estimates.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;The present study investigates the influence of r&amp;lt;sub&amp;gt;a&amp;lt;/sub&amp;gt; and its relation to LST uncertainties on the performance of three structurally different SEB models by combining nine OzFlux eddy covariance datasets from 2011 to 2019 from sites of different aridity in Australia with MODIS Terra and Aqua LST and leaf area index (LAI) products. Simulations of the latent heat flux (LE, energy equivalent of ET in W/m&amp;lt;sup&amp;gt;2&amp;lt;/sup&amp;gt;) from the SPARSE (Soil Plant Atmosphere and Remote Sensing Evapotranspiration), SEBS (Surface Energy Balance System) and STIC (Surface Temperature Initiated Closure) models forced with MODIS LST, LAI, and in-situ meteorological datasets were evaluated using observed flux data across water-limited (semi-arid and arid) and radiation-limited (mesic) ecosystems.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Our results revealed that the three models tend to overestimate instantaneous LE in the water-limited shrubland, woodland and grassland ecosystems by up to 60% on average, which was caused by an underestimation of the sensible heat flux (H). LE overestimation was associated with discrepancies in r&amp;lt;sub&amp;gt;a&amp;lt;/sub&amp;gt; retrievals under conditions of high atmospheric instability, during which errors in LST (expressed as the difference between MODIS LST and in-situ LST) apparently played a minor role. On the other hand, a positive bias in LST coincides with low r&amp;lt;sub&amp;gt;a&amp;lt;/sub&amp;gt; and causes slight underestimation of LE at the water-limited sites. The impact of r&amp;lt;sub&amp;gt;a&amp;lt;/sub&amp;gt; on the LE residual error was found to be of the same magnitude as the influence of errors in LST in the semi-arid ecosystems as indicated by variable importance in projection (VIP) coefficients from partial least squares regression above unity. In contrast, our results for mesic forest ecosystems indicated minor dependency on r&amp;lt;sub&amp;gt;a&amp;lt;/sub&amp;gt; for modelling LE (VIP&amp;lt;0.4), which was due to a higher roughness length and lower LST resulting in dominance of mechanically generated turbulence, thereby diminishing the importance of atmospheric stability in the determination of r&amp;lt;sub&amp;gt;a&amp;lt;/sub&amp;gt;.&amp;lt;/p&amp;gt;</jats:p

An introduction to the Australian and New Zealand flux tower network - OzFlux

Published: 31 October 2016OzFlux is the regional Australian and New Zealand flux tower network that aims to provide a continental-scale national research facility to monitor and assess trends, and improve predictions, of Australia's terrestrial biosphere and climate. This paper describes the evolution, design, and current status of OzFlux as well as provides an overview of data processing. We analyse measurements from all sites within the Australian portion of the OzFlux network and two sites from New Zealand. The response of the Australian biomes to climate was largely consistent with global studies except that Australian systems had a lower ecosystem water-use efficiency. Australian semi-arid/arid ecosystems are important because of their huge extent (70 %) and they have evolved with common moisture limitations. We also found that Australian ecosystems had a similar radiation-use efficiency per unit leaf area compared to global values that indicates a convergence toward a similar biochemical efficiency. The two New Zealand sites represented extremes in productivity for a moist temperate climate zone, with the grazed dairy farm site having the highest GPP of any OzFlux site (2620 gC m⁻² yr⁻¹) and the natural raised peat bog site having a very low GPP (820 gC m⁻² yr⁻¹). The paper discusses the utility of the flux data and the synergies between flux, remote sensing, and modelling. Lastly, the paper looks ahead at the future direction of the network and concludes that there has been a substantial contribution by OzFlux, and considerable opportunities remain to further advance our understanding of ecosystem response to disturbances, including drought, fire, land-use and land-cover change, land management, and climate change, which are relevant both nationally and internationally. It is suggested that a synergistic approach is required to address all of the spatial, ecological, human, and cultural challenges of managing the delicately balanced ecosystems in Australasia.Jason Beringer ... Wayne Meyer ... et al

An introduction to the Australian and New Zealand flux tower network - OzFlux

© Author(s) 2016. OzFlux is the regional Australian and New Zealand flux tower network that aims to provide a continental-scale national research facility to monitor and assess trends, and improve predictions, of Australia's terrestrial biosphere and climate. This paper describes the evolution, design, and current status of OzFlux as well as provides an overview of data processing. We analyse measurements from all sites within the Australian portion of the OzFlux network and two sites from New Zealand. The response of the Australian biomes to climate was largely consistent with global studies except that Australian systems had a lower ecosystem water-use efficiency. Australian semi-arid/arid ecosystems are important because of their huge extent (70 %) and they have evolved with common moisture limitations. We also found that Australian ecosystems had a similar radiation-use efficiency per unit leaf area compared to global values that indicates a convergence toward a similar biochemical efficiency. The two New Zealand sites represented extremes in productivity for a moist temperate climate zone, with the grazed dairy farm site having the highest GPP of any OzFlux site (2620 gC m-2 yr-1) and the natural raised peat bog site having a very low GPP (820 gC m-2 yr-1). The paper discusses the utility of the flux data and the synergies between flux, remote sensing, and modelling. Lastly, the paper looks ahead at the future direction of the network and concludes that there has been a substantial contribution by OzFlux, and considerable opportunities remain to further advance our understanding of ecosystem response to disturbances, including drought, fire, land-use and land-cover change, land management, and climate change, which are relevant both nationally and internationally. It is suggested that a synergistic approach is required to address all of the spatial, ecological, human, and cultural challenges of managing the delicately balanced ecosystems in Australasia

Author Correction: The FLUXNET2015 dataset and the ONEFlux processing pipeline for eddy covariance data

The following authors were omitted from the original version of this Data Descriptor: Markus Reichstein and Nicolas Vuichard. Both contributed to the code development and N. Vuichard contributed to the processing of the ERA-Interim data downscaling. Furthermore, the contribution of the co-author Frank Tiedemann was re-evaluated relative to the colleague Corinna Rebmann, both working at the same sites, and based on this re-evaluation a substitution in the co-author list is implemented (with Rebmann replacing Tiedemann). Finally, two affiliations were listed incorrectly and are corrected here (entries 190 and 193). The author list and affiliations have been amended to address these omissions in both the HTML and PDF versions

The FLUXNET2015 dataset and the ONEFlux processing pipeline for eddy covariance data.

The FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible

Reduced throughfall decreases autotrophic respiration, but not heterotrophic respiration in a dry temperate broadleaved evergreen forest

Climate change may have major implications on soil respiration dynamics and the carbon sink strength of forest soils. To assess the effect of climate change on soil respiration (RS), it is crucial to understand individual responses of autotrophic (RA) and heterotrophic (RH) components. We investigated the effect of continuously (20 months) reduced throughfall (TFR, −40%) and the influence of seasonal changes in soil temperature and moisture on RS, RA and RH, partitioned by root exclusion, in a dry temperate broadleaved evergreen eucalypt forest in south-eastern Australia. TFR decreased mean RS from 4.7±0.1 (Control) to 3.8±0.1 (TFR) μmolCO2m−2s−1 (−19%). TFR indicated a strong decrease in RA from 2.5±0.1 (Control) to 1.5±0.1 (TFR) μmolCO2m−2s−1 (−40%), but had no effect on RH. The mean relative contribution of RH to RS was 47% in the Control and increased to 61% under TFR. RS was the result of distinct seasonal patterns and dependencies of RH and RA on environmental variables. Soil temperature was a good predictor of RH (Control: R2=0.72, TFR: R2=0.75), but not of RA. In contrast, RH was not limited by soil moisture, while RA was partly influenced by soil moisture (Control: R2=0.29, TFR: R2=0.56). The lack of response of RH to changes in soil moisture (seasonal and under TFR) was likely influenced by the high rainfall conditions such that soil moisture did not decrease to a point where it limited soil microbial decomposition processes. Our results show that TFR implied the strongest effect on RA and that changes in soil temperature and moisture alone do not sufficiently explain seasonal changes in RA and RS. This indicates that biotic factors, such as plant internal carbon allocation, may exert a stronger influence on RA and hence, RS. In short-term a reduction in rainfall will lead to a decrease of soil respiration in dry temperate broadleaved evergreen eucalypt forests. The magnitude of this decrease and its persistence under extended drought will be greatly influenced by seasonal and inter-annual climate variability and potential changes in plant carbon allocation

Soil Methane Uptake Increases under Continuous Throughfall Reduction in a Temperate Evergreen, Broadleaved Eucalypt Forest

Soils in temperate forests ecosystems are the greatest terrestrial CHâ sink globally. Global and regional circulation models predict decreased average rainfall, increased extreme rainfall events and increased temperatures for many temperate ecosystems. However, most studies of soil CHâ uptake have only considered extended periods of drought rather than an overall decrease in rainfall amount. We measured soil CHâ uptake from March 2010 to March 2012 after installing passive rainfall reduction systems to intercept approximately 40% of throughfall in a temperate broadleaf evergreen eucalypt forest in south-eastern Australia. Throughfall reduction caused an average reduction of 15.1 ± 6.4% (SE) in soil volumetric water content, a reduction of 19.8 ± 6.9% in soil water-filled pore space (%WFPS) and a 20.1 ± 6.8% increase in soil air-filled porosity. In response to these changes, soil CHâ uptake increased by 54.7 ± 19.3%. The increase in soil CHâ uptake could be explained by increased diffusivity in drier soils, whilst the activity of methanotrophs remained relatively unchanged. It is likely that soil CHâ uptake will increase if rainfall reduces in temperate broadleaf evergreen forests of Australia as a consequence of climate change

Bridge to the future: Important lessons from 20 years of ecosystem observations made by the OzFlux network.

In 2020, the Australian and New Zealand flux research and monitoring network, OzFlux, celebrated its 20th anniversary by reflecting on the lessons learned through two decades of ecosystem studies on global change biology. OzFlux is a network not only for ecosystem researchers, but also for those 'next users' of the knowledge, information and data that such networks provide. Here, we focus on eight lessons across topics of climate change and variability, disturbance and resilience, drought and heat stress and synergies with remote sensing and modelling. In distilling the key lessons learned, we also identify where further research is needed to fill knowledge gaps and improve the utility and relevance of the outputs from OzFlux. Extreme climate variability across Australia and New Zealand (droughts and flooding rains) provides a natural laboratory for a global understanding of ecosystems in this time of accelerating climate change. As evidence of worsening global fire risk emerges, the natural ability of these ecosystems to recover from disturbances, such as fire and cyclones, provides lessons on adaptation and resilience to disturbance. Drought and heatwaves are common occurrences across large parts of the region and can tip an ecosystem's carbon budget from a net CO2 sink to a net CO2 source. Despite such responses to stress, ecosystems at OzFlux sites show their resilience to climate variability by rapidly pivoting back to a strong carbon sink upon the return of favourable conditions. Located in under-represented areas, OzFlux data have the potential for reducing uncertainties in global remote sensing products, and these data provide several opportunities to develop new theories and improve our ecosystem models. The accumulated impacts of these lessons over the last 20 years highlights the value of long-term flux observations for natural and managed systems. A future vision for OzFlux includes ongoing and newly developed synergies with ecophysiologists, ecologists, geologists, remote sensors and modellers