Comments on 'Potential evaporation at eddy-covariance sites across the globe'

Abstract. Potential evaporation (Ep) is a crucial variable for hydrological forecasting and drought monitoring. However, multiple interpretations of Ep exist, and these reflect a diverse range of methods to calculate it. As such, a comparison of the performance of these methods against field observations in different global ecosystems is urgently needed. In this study, potential evaporation was defined as the rate of evaporation (or evapotranspiration – sum of transpiration and soil evaporation) that the actual ecosystem would attain if it evaporates at maximal rate. We use eddy-covariance measurements from the FLUXNET2015 database, covering eleven different biomes, to parameterize and inter-compare the most widely used Ep methods and to uncover their relative performance. For each site, we isolate the days for which ecosystems can be considered as unstressed based on both an energy balance approach and a soil water content approach. Evaporation measurements during these days are used as reference to calibrate and validate the different methods to estimate Ep. Our results indicate that a simple radiation-driven method calibrated per biome consistently performs best, with a mean correlation of 0.93, unbiased RMSE of 0.56 mm day−1, and bias of −0.02 mm day−1 against in situ measurements of unstressed evaporation. A Priestley and Taylor method, calibrated per biome, performed just slightly worse, yet substantially and consistently better than more complex Penman, Penman–Monteith-based or temperature-driven approaches. We show that the poor performance of Penman–Monteith-based approaches relates largely to the fact that the unstressed stomatal conductance cannot be assumed to be constant in time at the ecosystem scale. Contrastingly, the biome-specific parameters required for the simple radiation-driven methods are relatively constant in time and per biome type. This makes these methods a robust way to estimate Ep and a suitable tool to investigate the impact of water use and demand, drought severity and biome productivity.

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Potential evaporation at eddy-covariance sites across the globe

Hydrology and Earth System Sciences ◽

10.5194/hess-23-925-2019 ◽

2019 ◽

Vol 23 (2) ◽

pp. 925-948 ◽

Cited By ~ 15

Author(s):

Wouter H. Maes ◽

Pierre Gentine ◽

Niko E. C. Verhoest ◽

Diego G. Miralles

Keyword(s):

Eddy Covariance ◽

Poor Performance ◽

Drought Severity ◽

Atmospheric Conditions ◽

Potential Evaporation ◽

Diverse Range ◽

Eddy Covariance Measurements ◽

Priestley And Taylor Method ◽

The Impact ◽

The Given

Abstract. Potential evaporation (Ep) is a crucial variable for hydrological forecasting and drought monitoring. However, multiple interpretations of Ep exist, which reflect a diverse range of methods to calculate it. A comparison of the performance of these methods against field observations in different global ecosystems is urgently needed. In this study, potential evaporation was defined as the rate of terrestrial evaporation (or evapotranspiration) that the actual ecosystem would attain if it were to evaporate at maximal rate for the given atmospheric conditions. We use eddy-covariance measurements from the FLUXNET2015 database, covering 11 different biomes, to parameterise and inter-compare the most widely used Ep methods and to uncover their relative performance. For each of the 107 sites, we isolate days for which ecosystems can be considered unstressed, based on both an energy balance and a soil water content approach. Evaporation measurements during these days are used as reference to calibrate and validate the different methods to estimate Ep. Our results indicate that a simple radiation-driven method, calibrated per biome, consistently performs best against in situ measurements (mean correlation of 0.93; unbiased RMSE of 0.56 mm day−1; and bias of −0.02 mm day−1). A Priestley and Taylor method, calibrated per biome, performed just slightly worse, yet substantially and consistently better than more complex Penman-based, Penman–Monteith-based or temperature-driven approaches. We show that the poor performance of Penman–Monteith-based approaches largely relates to the fact that the unstressed stomatal conductance cannot be assumed to be constant in time at the ecosystem scale. On the contrary, the biome-specific parameters required by simpler radiation-driven methods are relatively constant in time and per biome type. This makes these methods a robust way to estimate Ep and a suitable tool to investigate the impact of water use and demand, drought severity and biome productivity.

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Potential evaporation at eddy-covariance sites across the globe

10.5194/hess-2017-682 ◽

2018 ◽

Cited By ~ 4

Author(s):

Wouter H. Maes ◽

Pierre Gentine ◽

Niko E. C. Verhoest ◽

Diego G. Miralles

Keyword(s):

Eddy Covariance ◽

Poor Performance ◽

Drought Severity ◽

Potential Evaporation ◽

Diverse Range ◽

Eddy Covariance Measurements ◽

Priestley And Taylor Method ◽

The Impact ◽

Energy Balance Approach

Abstract. Potential evaporation (Ep) is a crucial variable for hydrological forecast and in drought monitoring systems. However, multiple interpretations of Ep exist, and these reflect a diverse range of methods to calculate Ep. As such, a comparison of the performance of these methods against field observations in different global ecosystems is badly needed. In this study, we used eddy-covariance measurements from 107 sites of the FLUXNET2015 database, covering 11 different biomes, to parameterize and compare the main Ep methods and uncover their relative performance. For each site, we extracted the days for which ecosystems are unstressed based on both an energy balance approach and on a soil water content approach. The evaporation measurements during these days were used as reference to validate the different methods to estimate Ep. Our results indicate that a simple radiation-driven method calibrated per biome consistently performed best, with a mean correlation of 0.93, an unbiased RMSE of 0.56 mm day−1, and a bias of −0.02 mm day−1 against in situ measurements of unstressed evaporation. A Priestley and Taylor method, calibrated per biome, performed just slightly worse, yet substantially and consistently better than more complex Penman, Penman-Monteith-based or temperature-based approaches. We show that the poor performance of Penman-Monteith based approaches relates largely to the fact that the unstressed stomatal conductance was assumed constant. Further analysis showed that the biome-specific parameters required for the simple radiation-driven methods are relatively constant per biome. This makes this simple radiation-driven method calibrated per biome a robust method that can be incorporated into models for improving our understanding of the impact of global warming on future global water use and demand, drought severity and ecosystem productivity.

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Assessments of gross primary productivity estimations with satellite data-driven models using eddy covariance observation sites over the northern hemisphere

Agricultural and Forest Meteorology ◽

10.1016/j.agrformet.2019.107771 ◽

2020 ◽

Vol 280 ◽

pp. 107771 ◽

Cited By ~ 2

Author(s):

Xinyao Xie ◽

Ainong Li ◽

Jianbo Tan ◽

Huaan Jin ◽

Xi Nan ◽

...

Keyword(s):

Eddy Covariance ◽

Northern Hemisphere ◽

Satellite Data ◽

Primary Productivity ◽

Gross Primary Productivity ◽

Data Driven

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Correction of Eddy Covariance Based Crop ET Considering the Heat Flux Source Area

Atmosphere ◽

10.3390/atmos12020281 ◽

2021 ◽

Vol 12 (2) ◽

pp. 281

Author(s):

Stuart L. Joy ◽

José L. Chávez

Keyword(s):

Heat Flux ◽

Eddy Covariance ◽

Agricultural Land ◽

Flux Measurement ◽

Source Area ◽

Mean Bias Error ◽

Bias Error ◽

Crop Water ◽

The Past ◽

Measurement Results

Eddy covariance (EC) systems are being used to measure sensible heat (H) and latent heat (LE) fluxes in order to determine crop water use or evapotranspiration (ET). The reliability of EC measurements depends on meeting certain meteorological assumptions; the most important of such are horizontal homogeneity, stationarity, and non-advective conditions. Over heterogeneous surfaces, the spatial context of the measurement must be known in order to properly interpret the magnitude of the heat flux measurement results. Over the past decades, there has been a proliferation of ‘heat flux source area’ (i.e., footprint) modeling studies, but only a few have explored the accuracy of the models over heterogeneous agricultural land. A composite ET estimate was created by using the estimated footprint weights for an EC system in the upwind corner of four fields and separate ET estimates from each of these fields. Three analytical footprint models were evaluated by comparing the composite ET to the measured ET. All three models performed consistently well, with an average mean bias error (MBE) of about −0.03 mm h−1 (−4.4%) and root mean square error (RMSE) of 0.09 mm h−1 (10.9%). The same three footprint models were then used to adjust the EC-measured ET to account for the fraction of the footprint that extended beyond the field of interest. The effectiveness of the footprint adjustment was determined by comparing the adjusted ET estimates with the lysimetric ET measurements from within the same field. This correction decreased the absolute hourly ET MBE by 8%, and the RMSE by 1%.

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Assessing evapotranspiration and crop coefficients of potato in a semi-arid climate using Eddy Covariance techniques

Agricultural Water Management ◽

10.1016/j.agwat.2021.107029 ◽

2021 ◽

Vol 255 ◽

pp. 107029

Author(s):

A.T.B. Machakaire ◽

J.M. Steyn ◽

A.C. Franke

Keyword(s):

Eddy Covariance ◽

Arid Climate ◽

Crop Coefficients ◽

Semi Arid

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Howland Forest, ME, USA: Multi-Gas Flux (CO2, CH4, N2O) Social Cost Product Underscores Limited Carbon Proxies

Land ◽

10.3390/land10040436 ◽

2021 ◽

Vol 10 (4) ◽

pp. 436

Author(s):

Bruno D. V. Marino ◽

Nahuel Bautista ◽

Brandt Rousseaux

Keyword(s):

Global Warming ◽

Eddy Covariance ◽

Direct Measurement ◽

Social Cost ◽

Forest Products ◽

Forest Carbon ◽

Carbon Offsets ◽

Carbon Accounting ◽

Climate Action ◽

Forest Carbon Sequestration

Forest carbon sequestration is a widely accepted natural climate solution. However, methods to determine net carbon offsets are based on commercial carbon proxies or CO2 eddy covariance research with limited methodological comparisons. Non-CO2 greenhouse gases (GHG) (e.g., CH4, N2O) receive less attention in the context of forests, in part, due to carbon denominated proxies and to the cost for three-gas eddy covariance platforms. Here we describe and analyze results for direct measurement of CO2, CH4, and N2O by eddy covariance and forest carbon estimation protocols at the Howland Forest, ME, the only site where these methods overlap. Limitations of proxy-based protocols, including the exclusion of sink terms for non-CO2 GHGs, applied to the Howland project preclude multi-gas forest products. In contrast, commercial products based on direct measurement are established by applying molecule-specific social cost factors to emission reductions creating a new forest offset (GHG-SCF), integrating multiple gases into a single value of merit for forest management of global warming. Estimated annual revenue for GHG-SCF products, applicable to the realization of a Green New Deal, range from ~$120,000 USD covering the site area of ~557 acres in 2021 to ~$12,000,000 USD for extrapolation to 40,000 acres in 2040, assuming a 3% discount rate. In contrast, California Air Resources Board compliance carbon offsets determined by the Climate Action Reserve protocol show annual errors of up to 2256% relative to eddy covariance data from two adjacent towers across the project area. Incomplete carbon accounting, offset over-crediting and inadequate independent offset verification are consistent with error results. The GHG-SCF product contributes innovative science-to-commerce applications incentivizing restoration and conservation of forests worldwide to assist in the management of global warming.

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Is Agriculture Always a GHG Emitter? A Combination of Eddy Covariance and Life Cycle Assessment Approaches to Calculate C Intake and Uptake in a Kiwifruit Orchard

Sustainability ◽

10.3390/su13126906 ◽

2021 ◽

Vol 13 (12) ◽

pp. 6906

Author(s):

Federica Rossi ◽

Camilla Chieco ◽

Nicola Di Virgilio ◽

Teodoro Georgiadis ◽

Marianna Nardino

Keyword(s):

Life Cycle Assessment ◽

Life Cycle ◽

Greenhouse Gases ◽

Eddy Covariance ◽

Large Scale ◽

Cropping Systems ◽

Substantial Reduction ◽

Co2 Absorption ◽

Positive Contribution ◽

Efficiency Ratio

While a substantial reduction of GHG (greenhouse gases) is urged, large-scale mitigation implies a detailed and holistic knowledge on the role of specific cropping systems, including the effect of management choices and local factors on the final balance between emissions and removals, this last typical of cropping systems. Here, a conventionally managed irrigated kiwifruit orchard has been studied to assess its greenhouse gases emissions and removals to determine its potential action as a C sink or, alternately, as a C source. The paper integrates two independent approaches. Biological CO2 fluxes have been monitored during 2012 using the micrometeorological Eddy covariance technique, while life cycle assessment quantified emissions derived from the energy and material used. In a climatic-standard year, total GHG emitted as consequence of the management were 4.25 t CO2-eq−1 ha−1 yr−1 while the net uptake measured during the active vegetation phase was as high as 4.9 t CO2 ha−1 yr−1. This led to a positive contribution of the crop to CO2 absorption, with a 1.15 efficiency ratio (sink-source factor defined as t CO2 stored/t CO2 emitted). The mitigating activity, however, completely reversed under extremely unfavorable climatic conditions, such as those recorded in 2003, when the efficiency ratio became 0.91, demonstrating that the occurrence of hotter and drier conditions are able to compromise the capability of Actinidia to offset the GHG emissions, also under appropriate irrigation.

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