Bridging Thermal Infrared Sensing and Physically-Based Evapotranspiration Modeling : From Theoretical Implementation to Validation Across an Aridity Gradient in Australian Ecosystems

Thermal infrared sensing of evapotranspiration (E) through surface energy balance (SEB) models is challenging due to uncertainties in determining the aerodynamic conductance (g(A)) and due to inequalities between radiometric (T-R) and aerodynamic temperatures (T-0). We evaluated a novel analytical model, the Surface Temperature Initiated Closure (STIC1.2), that physically integrates T-R observations into a combined Penman-Monteith Shuttleworth-Wallace (PM-SW) framework for directly estimating E, and overcoming the uncertainties associated with T0 and gA determination. An evaluation of STIC1.2 against high temporal frequency SEB flux measurements across an aridity gradient in Australia revealed a systematic error of 10-52% in E from mesic to arid ecosystem, and low systematic error in sensible heat fluxes (H) (12-25%) in all ecosystems. Uncertainty in TR versus moisture availability relationship, stationarity assumption in surface emissivity, and SEB closure corrections in E were predominantly responsible for systematic E errors in arid and semi-arid ecosystems. A discrete correlation (r) of the model errors with observed soil moisture variance (r = 0.33-0.43), evaporative index (r = 0.77-0.90), and climatological dryness (r = 0.60-0.77) explained a strong association between ecohydrological extremes and T-R in determining the error structure of STIC1.2 predicted fluxes. Being independent of any leaf-scale biophysical parameterization, the model might be an important value addition in working group (WG2) of the Australian Energy and Water Exchange (OzEWEX) research initiative which focuses on observations to evaluate and compare biophysical models of energy and water cycle components. Plain Language Summary Evapotranspiration modeling and mapping in arid and semi-arid ecosystems are uncertain due to empirical approximation of surface and atmospheric conductances. Here we demonstrate the performance of a fully analytical model which is independent of any leaf-scale empirical parameterization of the conductances and can be potentially used for continental scale mapping of ecosystem water use as well as water stress using thermal remote sensing satellite data.dPeer reviewe

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.</p

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\u27s 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

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.Peer reviewe

Evergreen and ever growing : stem and canopy growth dynamics of a temperate eucalypt forest

Irregular growth rings and long-lived leaves present challenges to quantify stem growth and canopy dynamics of evergreen eucalypt trees, which has impeded understanding of seasonal and inter-year biomass allocation in temperate eucalypt forests. To address this knowledge gap, we examined the subannual dynamics of stem and canopy growth by tree canopy class of three co-occurring species over 35 months in a temperate eucalypt forest of southeastern Australia. We used dendrometers to monitor basal area increments and daily terrestrial lidar scans to monitor integrated and height-specific canopy dynamics. Associations with concomitant weather data and stand-level carbon fluxes were used to enhance understanding of biomass allocation patterns. Comparisons of growth patterns indicated seasonal asynchronicity of stem and crown growth: the canopy expanded mainly in summer and early autumn, and the stems grew mainly in spring and autumn, but also to a lesser degree in winter. Eucalypts in the dominant crown classes grew all year, with growth allocated to crown expansion and thickening in the summer months, or to stem growth in all other months. Canopy volume was stable during the first part of the study period and subsequently increased by 20%. However, stratum-specific dynamics indicated a distinct seasonality of canopy turnover, characterised by volume gains at the top of the canopy and concurrent volume losses in the middle stratum during summer. Growth patterns, as either canopy expansion or stem increment, were not clearly associated with ecosystem-scale carbon dynamics. In addition, relationships of growth with climatic variables appeared to be associative rather than causative, indicating that allocation dynamics to root growth and nonstructural carbohydrate pools will be essential in explaining ecosystem carbon dynamics in temperate eucalypt forests

Field-Programmable Smart-Pixel Arrays: Design, VLSI Implementation, and Applications

A smart-pixel array is a two-dimensional array of optoelectronic devices that combine optical inputs and outputs with electronic processing circuitry. A field-programmable smart-pixel array (FP-SPA) is a smart-pixel array capable of having its electronic functionality dynamically programmed in the field. Such devices could be used in a diverse range of applications, including optical switching, optical digital signal processing, and optical image processing. We describe the design, VLSI implementation, and applications of a first-generation FP-SPA implemented with the 0.8-μm complementary metal-oxide semiconductor–self-electro-optic effect device technology made available through the Lucent Technologies–Advanced Research Projects Agency Cooperative (Lucent/ARPA/COOP) program. We report spice simulations and experimental results of two sample applications: In the first application, we configure this FP-SPA as an array of free-space optical binary switches that can be used in optical multistage networks. In the second, we configure the device as an optoelectronic transceiver for a dynamically reconfigurable free-space intelligent optical backplane called the hyperplane. We also describe the testing setup and the electrical and the optical tests that demonstrate the correct functionality of the fabricated device. Such devices have the potential to reduce significantly the need for custom design and fabrication of application-specific optoelectronic devices in the same manner that field-programmable gate arrays have largely eliminated the need for custom design and fabrication of application-specific gate arrays, except in the most demanding applications

Effects of inhomogeneities within the flux footprint on the interpretation of seasonal, annual, and interannual ecosystem carbon exchange

Carbon flux measurements using the eddy covariance method rely on several assumptions, including reasonably flat terrain and homogeneous vegetation cover. An increasing number of flux sites are located over partially or completely inhomogeneous areas, but the implication of such inhomogeneities on carbon budgets, and particularly the influence of year-to-year variations in wind patterns on annual budgets, remains unclear. Moreover, directional homogeneity of climatic drivers of carbon fluxes is often assumed, although climatic variables vary with wind direction at many locations. In this study, we examined the directional flux characteristics, incorporating the combined effects of variable surface characteristics and climatic drivers on the annual carbon budgets of an evergreen forest. Our study area was characterized by moderate variation in surface characteristics (leaf area index: 1.5-2; topographic wetness index: 4-16), and significant variation in the key drivers of carbon fluxes with wind direction (such as temperature, VPD and turbulence). Interactions among surface characteristics and climatic variables resulted in carbon uptake 'hotspots'. These localized hotspots influenced mean CO2 fluxes from several wind directions, and were most distinctive during the summer months. Hotspot contributions to yearly budgets varied from year to year, depending on prevailing weather conditions. Consequently, directional variations in flux characteristics affected quarterly estimates of carbon budgets by up to 22%, and annual budgets by up to 25%. We present a procedure to quantify and adjust for the effects of year-to-year variations in directional flux characteristics on interannual comparisons of carbon budgets. Any remaining differences in budgets (after the adjustment) can then be linked more accurately to variations in ecophysiological drivers. Our study clearly highlights that directional variations in flux characteristics can have a significant influence on annual carbon budgets, and that these should be accounted for in interannual comparisons

Trading water for carbon : maintaining photosynthesis at the cost of increased water loss during high temperatures in a temperate forest

Carbon and water fluxes are often assumed to be coupled as a result of stomatal regulation during dry conditions. However, recent observations evidenced increased transpiration rates during isolated heatwaves across a range of eucalypt species under experimental and natural conditions, with inconsistent effects on photosynthesis (ranging from increases to stark declines). To improve the empirical basis for understanding carbon and water fluxes in forests under hotter and drier climates, we measured the water use of dominant trees and ecosystem‐scale carbon and water exchange in a temperate eucalypt forest over three summer seasons. The forest maintained photosynthesis within 16% of baseline rates during hot and dry conditions, despite ~70% reductions in canopy conductance during a 5‐day heatwave. While carbon and water fluxes both decreased by 16% on exceptionally dry days, gross primary productivity only decreased by 5% during the hottest days and increased by 2% during the heatwave. However, evapotranspiration increased by 43% (hottest days) and 74% (heatwave), leading to ~40% variation in traditional water use efficiency (water use efficiency = gross primary productivity/evapotranspiration) across conditions and approximately two‐fold differences between traditional and underlying or intrinsic water use efficiency on the same days. Furthermore, the forest became a net source of carbon following a 137% increase in ecosystem respiration during the heatwave, highlighting that the potential for temperate eucalypt forests to act as net carbon sinks under hotter and drier climates will depend not only on the responses of photosynthesis to higher temperatures and changes in water availability, but also on the concomitant responses of ecosystem respiration

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

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