Showing papers by "Mathew Williams published in 2019"
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University of Sheffield1, Paul Sabatier University2, University of Toulouse3, Technical University of Denmark4, University of Edinburgh5, University of Montpellier6, Polytechnic University of Milan7, University of Bordeaux8, German Aerospace Center9, Jet Propulsion Laboratory10, European Space Research and Technology Centre11, University of Virginia12, University of Tasmania13, Hobart Corporation14, Chalmers University of Technology15
TL;DR: The European Space Agency's 7th Earth Explorer mission, BIOMASS, is to determine the worldwide distribution of forest above-ground biomass (AGB) in order to reduce the major uncertainties in calculations of carbon stocks and fluxes associated with the terrestrial biosphere, including carbon fluxe associated with Land Use Change, forest degradation and forest regrowth as mentioned in this paper.
160 citations
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University of Maryland, College Park1, Ghent University2, Wageningen University and Research Centre3, ETH Zurich4, Colorado State University5, Brown University6, Centre national de la recherche scientifique7, Aberystwyth University8, United States Forest Service9, California Institute of Technology10, Norwegian University of Life Sciences11, Goddard Space Flight Center12, Commonwealth Scientific and Industrial Research Organisation13, University of Leeds14, Universities Space Research Association15, International Institute for Applied Systems Analysis16, European Space Research and Technology Centre17, University of Massachusetts Amherst18, University of Edinburgh19
TL;DR: A wide range of anticipated user requirements for product accuracy assessment are outlined and recommendations for the validation of biomass products are provided, including the collection of new, high-quality in situ data and the use of airborne lidar biomass maps as tools toward transparent multi-resolution validation.
Abstract: Several upcoming satellite missions have core science requirements to produce data for accurate forest aboveground biomass mapping. Largely because of these mission datasets, the number of available biomass products is expected to greatly increase over the coming decade. Despite the recognized importance of biomass mapping for a wide range of science, policy and management applications, there remains no community accepted standard for satellite-based biomass map validation. The Committee on Earth Observing Satellites (CEOS) is developing a protocol to fill this need in advance of the next generation of biomass-relevant satellites, and this paper presents a review of biomass validation practices from a CEOS perspective. We outline the wide range of anticipated user requirements for product accuracy assessment and provide recommendations for the validation of biomass products. These recommendations include the collection of new, high-quality in situ data and the use of airborne lidar biomass maps as tools toward transparent multi-resolution validation. Adoption of community-vetted validation standards and practices will facilitate the uptake of the next generation of biomass products.
93 citations
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TL;DR: In this paper, the authors review how satellite observations of the biosphere have helped improve our understanding of the terrestrial carbon cycle and present examples of studies which have used satellite products to evaluate and improve simulations from global vegetation models, focusing on model data integration approaches ranging from bottomup extrapolation of single variables to carbon cycle data assimilation system able to ingest multiple types of observations.
Abstract: Our understanding of the terrestrial carbon cycle has been greatly enhanced since satellite observations of the land surface started. The advantage of remote sensing is that it provides wall-to-wall observations including in regions where in situ monitoring is challenging. This paper reviews how satellite observations of the biosphere have helped improve our understanding of the terrestrial carbon cycle. First, it details how remotely sensed information of the land surface has provided new means to monitor vegetation dynamics and estimate carbon fluxes and stocks. Second, we present examples of studies which have used satellite products to evaluate and improve simulations from global vegetation models. Third, we focus on model data integration approaches ranging from bottom-up extrapolation of single variables to carbon cycle data assimilation system able to ingest multiple types of observations. Finally, we present an overview of upcoming satellite missions which are likely to further improve our understanding of the terrestrial carbon cycle and its response to climate change and extremes.
30 citations
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TL;DR: The value of the Sentinel-2 spectral characteristics for retrieving critical variables that can support more sustainable crop management practices are demonstrated.
Abstract: Leaf Area Index (LAI) and chlorophyll content are strongly related to plant development and productivity. Spatial and temporal estimates of these variables are essential for efficient and precise crop management. The availability of open-access data from the European Space Agency’s (ESA) Sentinel-2 satellite—delivering global coverage with an average 5-day revisit frequency at a spatial resolution of up to 10 metres—could provide estimates of these variables at unprecedented (i.e., sub-field) resolution. Using synthetic data, past research has demonstrated the potential of Sentinel-2 for estimating crop variables. Nonetheless, research involving a robust analysis of the Sentinel-2 bands for supporting agricultural applications is limited. We evaluated the potential of Sentinel-2 data for retrieving winter wheat LAI, leaf chlorophyll content (LCC) and canopy chlorophyll content (CCC). In coordination with destructive and non-destructive ground measurements, we acquired multispectral data from an Unmanned Aerial Vehicle (UAV)-mounted sensor measuring key Sentinel-2 spectral bands (443 to 865 nm). We applied Gaussian processes regression (GPR) machine learning to determine the most informative Sentinel-2 bands for retrieving each of the variables. We further evaluated the GPR model performance when propagating observation uncertainty. When applying the best-performing GPR models without propagating uncertainty, the retrievals had a high agreement with ground measurements—the mean R2 and normalised root-mean-square error (NRMSE) were 0.89 and 8.8%, respectively. When propagating uncertainty, the mean R2 and NRMSE were 0.82 and 11.9%, respectively. When accounting for measurement uncertainty in the estimation of LAI and CCC, the number of most informative Sentinel-2 bands was reduced from four to only two—the red-edge (705 nm) and near-infrared (865 nm) bands. This research demonstrates the value of the Sentinel-2 spectral characteristics for retrieving critical variables that can support more sustainable crop management practices.
26 citations
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TL;DR: Soil carbon has been measured for over a century in applications ranging from understanding biogeochemical processes in natural ecosystems to quantifying the productivity and health of managed syst....
Abstract: Soil carbon has been measured for over a century in applications ranging from understanding biogeochemical processes in natural ecosystems to quantifying the productivity and health of managed syst...
26 citations
01 Apr 2019
TL;DR: In this paper, the authors use the CARbon DAta MOdel (CARDAMOM) data-assimilation system to produce pan-Arctic terrestrial C-cycle analyses for the period 2000-2015.
Abstract: . There is a significant knowledge gap in the current state
of the terrestrial carbon (C) budget. Recent studies have highlighted a poor
understanding particularly of C pool transit times and of whether productivity
or biomass dominate these biases. The Arctic, accounting for approximately
50 % of the global soil organic C stocks, has an important role in the
global C cycle. Here, we use the CARbon DAta MOdel (CARDAMOM) data-assimilation system to
produce pan-Arctic terrestrial C cycle analyses for 2000–2015. This approach
avoids using traditional plant functional type or steady-state assumptions.
We integrate a range of data (soil organic C, leaf area index, biomass, and
climate) to determine the most likely state of the high-latitude C cycle at
a 1 ∘ × 1 ∘ resolution and also to provide general
guidance about the controlling biases in transit times. On average, CARDAMOM
estimates regional mean rates of photosynthesis of 565 g C m −2 yr −1
(90 % confidence interval between the 5th and 95th percentiles: 428, 741),
autotrophic respiration of 270 g C m −2 yr −1 (182, 397) and
heterotrophic respiration of 219 g C m −2 yr −1 (31, 1458),
suggesting a pan-Arctic sink of −67 ( −287 , 1160) g Cm −2 yr −1 , weaker in tundra and stronger in taiga. However, our
confidence intervals remain large (and so the region could be a source of C),
reflecting uncertainty assigned to the regional data products. We show a
clear spatial and temporal agreement between CARDAMOM analyses and different
sources of assimilated and independent data at both pan-Arctic and local
scales but also identify consistent biases between CARDAMOM and validation
data. The assimilation process requires clearer error quantification for leaf area index (LAI) and biomass products to resolve these biases. Mapping of vegetation C stocks
and change over time and soil C ages linked to soil C stocks is required
for better analytical constraint. Comparing CARDAMOM analyses to global
vegetation models (GVMs) for the same period, we conclude that transit times
of vegetation C are inconsistently simulated in GVMs due to a combination of
uncertainties from productivity and biomass calculations. Our findings
highlight that GVMs need to focus on constraining both current vegetation C
stocks and net primary production to improve a process-based understanding of
C cycle dynamics in the Arctic.
24 citations
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TL;DR: In this article, a coupled photosynthesis evapotranspiration model of intermediate complexity is proposed to reduce computational load and parameter numbers by operating at canopy scale and daily time step, which can be used for terrestrial ecosystem model-based data fusion.
Abstract: . Photosynthesis (gross primary production, GPP) and evapotranspiration (ET) are ecosystem processes with global significance for climate, the global carbon and hydrological cycles and a range of ecosystem services.
The mechanisms governing these processes are complex but well understood. There is strong coupling between these processes, mediated directly by stomatal conductance and indirectly by root zone soil moisture content and its accessibility.
This coupling must be effectively modelled for robust predictions of earth system responses to global change. Yet, it is highly demanding to model leaf and cellular processes, like stomatal conductance or electron transport, with response times of minutes, over decadal and global domains.
Computational demand means models resolving this level of complexity cannot be easily evaluated for their parameter sensitivity nor calibrated using earth observation information through data assimilation approaches requiring large ensembles.
To overcome these challenges, here we describe a coupled photosynthesis evapotranspiration model of intermediate complexity.
The model reduces computational load and parameter numbers by operating at canopy scale and daily time step. Through the inclusion of simplified representation of key process interactions, it retains sensitivity to variation in climate, leaf traits, soil states and atmospheric CO2 .
The new model is calibrated to match the biophysical responses of a complex terrestrial ecosystem model (TEM) of GPP and ET through a Bayesian model–data fusion framework.
The calibrated ACM-GPP-ET generates unbiased estimates of TEM GPP and ET and captures 80 %–95 % of the sensitivity of carbon and water fluxes by the complex TEM. The ACM-GPP-ET model operates 3 orders faster than the complex TEM. Independent evaluation of ACM-GPP-ET at FLUXNET sites, using a single global parameterisation, shows good agreement, with typical R2∼0.60 for both GPP and ET.
This intermediate complexity modelling approach allows full Monte Carlo-based quantification of model parameter and structural uncertainties and global-scale sensitivity analyses for these processes and is fast enough for use within terrestrial ecosystem model–data fusion frameworks requiring large ensembles.
21 citations
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TL;DR: In this article, the authors use the CARbon DAta MOdel (CARDAMOM) data-assimilation system to produce pan-Arctic terrestrial C-cycle analyses for the period 2000-2015.
Abstract: . There is a significant knowledge gap in the current state
of the terrestrial carbon (C) budget. Recent studies have highlighted a poor
understanding particularly of C pool transit times and of whether productivity
or biomass dominate these biases. The Arctic, accounting for approximately
50 % of the global soil organic C stocks, has an important role in the
global C cycle. Here, we use the CARbon DAta MOdel (CARDAMOM) data-assimilation system to
produce pan-Arctic terrestrial C cycle analyses for 2000–2015. This approach
avoids using traditional plant functional type or steady-state assumptions.
We integrate a range of data (soil organic C, leaf area index, biomass, and
climate) to determine the most likely state of the high-latitude C cycle at
a 1 ∘ × 1 ∘ resolution and also to provide general
guidance about the controlling biases in transit times. On average, CARDAMOM
estimates regional mean rates of photosynthesis of 565 g C m −2 yr −1
(90 % confidence interval between the 5th and 95th percentiles: 428, 741),
autotrophic respiration of 270 g C m −2 yr −1 (182, 397) and
heterotrophic respiration of 219 g C m −2 yr −1 (31, 1458),
suggesting a pan-Arctic sink of −67 ( −287 , 1160) g Cm −2 yr −1 , weaker in tundra and stronger in taiga. However, our
confidence intervals remain large (and so the region could be a source of C),
reflecting uncertainty assigned to the regional data products. We show a
clear spatial and temporal agreement between CARDAMOM analyses and different
sources of assimilated and independent data at both pan-Arctic and local
scales but also identify consistent biases between CARDAMOM and validation
data. The assimilation process requires clearer error quantification for leaf area index (LAI) and biomass products to resolve these biases. Mapping of vegetation C stocks
and change over time and soil C ages linked to soil C stocks is required
for better analytical constraint. Comparing CARDAMOM analyses to global
vegetation models (GVMs) for the same period, we conclude that transit times
of vegetation C are inconsistently simulated in GVMs due to a combination of
uncertainties from productivity and biomass calculations. Our findings
highlight that GVMs need to focus on constraining both current vegetation C
stocks and net primary production to improve a process-based understanding of
C cycle dynamics in the Arctic.
18 citations
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TL;DR: In this article, the authors evaluated the relative importance of key climate constraints to GPP, comparing direct plant physiological responses to water availability and indirect structural and trait responses via changes to leaf area index (LAI), roots and photosynthetic capacity.
Abstract: . The capacity of Amazon forests to sequester carbon is threatened by climate-change-induced shifts in precipitation patterns. However, the relative
importance of plant physiology, ecosystem structure and trait composition
responses in determining variation in gross primary productivity (GPP)
remain largely unquantified and vary among models. We evaluate the relative
importance of key climate constraints to GPP, comparing direct plant
physiological responses to water availability and indirect structural and
trait responses (via changes to leaf area index (LAI), roots and
photosynthetic capacity). To separate these factors we combined the
soil–plant–atmosphere model with forcing and observational data from seven
intensively studied forest plots along an Amazon drought stress gradient. We
also used machine learning to evaluate the relative importance of individual
climate factors across sites. Our model experiments showed that variation in
LAI was the principal driver of differences in GPP across the gradient,
accounting for 33 % of observed variation. Differences in photosynthetic
capacity ( Vcmax and Jmax ) accounted for 21 % of variance, and
climate (which included physiological responses) accounted for 16 %.
Sensitivity to differences in climate was highest where a shallow rooting
depth was coupled with a high LAI. On sub-annual timescales, the relative
importance of LAI in driving GPP increased with drought stress
( R2=0.72 ), coincident with the decreased importance of solar radiation
( R2=0.90 ). Given the role of LAI in driving GPP across Amazon
forests, improved mapping of canopy dynamics is critical, opportunities for
which are offered by new satellite-based remote sensing missions such as
GEDI, Sentinel and FLEX.
14 citations
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TL;DR: In this article, the authors present a method to derive atmospheric-observation-based estimates of carbon dioxide (CO2 ) fluxes at the national scale, demonstrated using data from a network of surface tall-tower sites across the UK and Ireland over the period of 2013-2014.
Abstract: . We present a method to derive atmospheric-observation-based estimates of carbon dioxide ( CO2 ) fluxes
at the national scale, demonstrated using data from a network of surface
tall-tower sites across the UK and Ireland over the period 2013–2014. The
inversion is carried out using simulations from a Lagrangian chemical
transport model and an innovative hierarchical Bayesian Markov chain Monte
Carlo (MCMC) framework, which addresses some of the traditional problems
faced by inverse modelling studies, such as subjectivity in the
specification of model and prior uncertainties. Biospheric fluxes related to
gross primary productivity and terrestrial ecosystem respiration are solved
separately in the inversion and then combined a posteriori to determine net
ecosystem exchange of CO2 . Two different models, Data
Assimilation Linked Ecosystem Carbon (DALEC) and Joint UK Land Environment Simulator (JULES),
provide prior estimates for these fluxes. We carry out separate inversions
to assess the impact of these different priors on the posterior flux
estimates and evaluate the differences between the prior and posterior
estimates in terms of missing model components. The Numerical Atmospheric
dispersion Modelling Environment (NAME) is used to relate fluxes to the
measurements taken across the regional network. Posterior CO2 estimates
from the two inversions agree within estimated uncertainties, despite large
differences in the prior fluxes from the different models. With our method,
averaging results from 2013 and 2014, we find a total annual net biospheric
flux for the UK of 8±79 Tg CO2 yr −1 (DALEC prior) and
64±85 Tg CO2 yr −1 (JULES prior), where negative values represent an
uptake of CO2 . These biospheric CO2 estimates show that annual UK
biospheric sources and sinks are roughly in balance. These annual mean
estimates consistently indicate a greater net release of CO2 than the
prior estimates, which show much more pronounced uptake in summer months.
14 citations
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TL;DR: In this article, the authors used a calibrated and locally validated BGC model to simulate direct soil N2O emissions along with NO3 leaching and crop N uptake in fields of barley, wheat and oilseed rape.
Abstract: . The application of nitrogenous fertilisers to agricultural soils is a major
source of anthropogenic N2O emissions. Reducing the nitrogen (N)
footprint of agriculture is a global challenge that depends, among other
things, on our ability to quantify the N2O emission intensity of the
world's most widespread and productive agricultural systems. In this context,
biogeochemistry (BGC) models are widely used to estimate soil N2O
emissions in agroecosystems. The choice of spatial scale is crucial because
larger-scale studies are limited by low input data precision, while
smaller-scale studies lack wider relevance. The robustness of large-scale model
predictions depends on preliminary and data-demanding model
calibration/validation, while relevant studies often omit the performance of
output uncertainty analysis and underreport model outputs that would allow a
critical assessment of results. This study takes a novel approach to these
aspects. The study focuses on arable eastern Scotland – a data-rich region
typical of northwest Europe in terms of edaphoclimatic conditions, cropping
patterns and productivity levels. We used a calibrated and locally validated
BGC model to simulate direct soil N2O emissions along with
NO3 leaching and crop N uptake in fields of barley, wheat and oilseed
rape. We found that 0.59 % ( ±0.36 ) of the applied N is emitted as
N2O while 37 % ( ±6 ) is taken up by crops and 14 %
( ±7 ) is leached as NO3 . We show that crop type is a key
determinant of N2O emission factors (EFs) with cereals having a low
(mean EF %), and oilseed rape a high (mean
EF=2.48 %), N2O emission intensity. Fertiliser
addition was the most important N2O emissions driver suggesting that
appropriate actions can reduce crop N2O intensity. Finally, we
estimated a 74 % relative uncertainty around N2O predictions
attributable to soil data variability. However, we argue that
higher-resolution soil data alone might not suffice to reduce this uncertainty.
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TL;DR: In this paper, the authors used a calibrated and locally-validated BGC model to simulate direct soil N2O emissions along with NO3 leaching and crop N uptake in fields of barley, wheat and oilseed rape.
Abstract: Abstract. The application of nitrogenous fertilisers to agricultural soils is a major source of anthropogenic N2O emissions. Reducing the nitrogen (N) footprint of agriculture is a global challenge that depends on our ability to quantify the N2O emission intensity of the world's most widespread and productive agricultural systems. In this context, biogeochemistry (BGC) models are widely used to estimate soil N2O emissions in agroecosystems. The choice of spatial scale is crucial because larger scale studies are limited by low input data precision while smaller scale studies lack wider relevance. The robustness of large-scale model predictions depends on preliminary and data-demanding model calibration/validation while relevant studies often omit the performance of output uncertainty analysis and underreport model outputs that would allow a critical assessment of results. This study takes a novel approach on these aspects. The study focuses on arable Eastern Scotland; a data-rich region typical of Northwest Europe in terms of edaphoclimatic conditions, cropping patterns and productivity levels. We used a calibrated and locally-validated BGC model to simulate direct soil N2O emissions along with NO3 leaching and crop N uptake in fields of barley, wheat and oilseed rape. We found that 0.59 % (±0.36) of the applied N is emitted as N2O while 37 % (±6) is taken up by crops and 14 % (±7) is leached as NO3. We show that crop type is a key determinant of N2O emission factors (EF) with cereals having a low (mean EF
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TL;DR: In this article, the surface fluxes of CO2 from both marine areas and terrestrial surfaces were investigated together with their impact in atmospheric CO2 concentrations by the usage of a high-resolution modelling framework.
Abstract: . Although coastal regions only amount to 7 % of the global oceans, their
contribution to the global oceanic air–sea CO2 exchange is
proportionally larger, with fluxes in some estuaries being similar in
magnitude to terrestrial surface fluxes of CO2 . Across a heterogeneous surface consisting of a coastal marginal sea with
estuarine properties and varied land mosaics, the surface fluxes of
CO2 from both marine areas and terrestrial surfaces were investigated
in this study together with their impact in atmospheric CO2
concentrations by the usage of a high-resolution modelling framework. The
simulated terrestrial fluxes across the study region of Denmark experienced
an east–west gradient corresponding to the distribution of the land cover
classification, their biological activity and the urbanised areas. Annually,
the Danish terrestrial surface had an uptake of approximately
−7000 GgC yr −1 . While the marine fluxes from the North Sea and the
Danish inner waters were smaller annually, with about −1800 and
1300 GgC yr −1 , their sizes are comparable to annual terrestrial fluxes
from individual land cover classifications in the study region and hence are not
negligible. The contribution of terrestrial surfaces fluxes was easily
detectable in both simulated and measured concentrations of atmospheric
CO2 at the only tall tower site in the study region. Although, the
tower is positioned next to Roskilde Fjord, the local marine impact was not
distinguishable in the simulated concentrations. But the regional impact from
the Danish inner waters and the Baltic Sea increased the atmospheric
concentration by up to 0.5 ppm during the winter months.
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TL;DR: The authors thank Dr. Chen for his correspondence and will address the queries raised in sequence and hope they provide further clarity with respect to the findings described in the paper.
Abstract: We thank Dr. Chen for his correspondence. We will address the queries raised in sequence and hope that they provide further clarity with respect to the findings described in our paper ([1][1]).
Regarding pre-release angiographic depth and post-release migration, indeed, the pre-release depth of