the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Aug 2024
Gap-Filling for Daily Evapotranspiration Observations with full-factorial method at Global Flux Sites
Abstract. Evapotranspiration (ET) plays a crucial role in the regional water-energy cycle, illustrating intricate interactions among climate, vegetation and soil. Eddy covariance (EC) technology is a primary method for measuring ET. However, incomplete data due to adverse weather conditions and equipment malfunctions are common. This study utilizes the full-factorial method to address the ET gaps at 339 sites across multiple global flux networks, and the filled ET data are compared with three ET products: the Land component of the Fifth Generation of European Reanalysis (EAR5-Land), Global Land Evaporation Amsterdam Model (GLEAM), and Breathing Earth System Simulator (BESS). Results indicate high consistency between filled ET data and three ET products at 264 out of 339 sites. The absolute average mean error (|MAE|) and root mean system error (|RMSE|) are 0.32 mm/d and 0.92 mm/d, respectively. Among the remaining 75 sites, 49 sites exhibit better agreement between filled ET and measured ET data than ET products in terms of seasonal variations and numerical ranges. Further verification is needed for the reliability of filled ET data at the other 26 sites due to a scarcity of measured ET data. Overall, the gap-filled ET data from 313 sites (2210 site-years) demonstrate high-quality. These sites exhibit a strong correlation between available energy and turbulent fluxes, with R2, MAE, and RMSE for different surface types ranging from 0.84 to 0.94, 21.49 to 28.67 W/m2, and 28.37 to 36.91 W/m2, respectively. And the average energy balance closure rate is 0.73, indicating a relatively high degree of closure in the energy balance. These 313 sites with high-quality filled ET data can be utilized for ET model validation, ET product verification, water demand assessment, and other related tasks. The filled ET dataset can be publicly accessed at https://doi.org/10.57760/sciencedb.11651 (Wang & Jiang, 2024).
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RC1: 'Comment on essd-2024-291', Anonymous Referee #1, 04 Oct 2024
The study employs the full-factorial gap-filling method, introduced by Jiang et al. (2022), to address evapotranspiration (ET) data gaps from eddy covariance sites. The authors successfully generated high quality gap-filled ET data for 313 sites covering 2210 site-years. Additionally, the paper reports the energy balance closure at these sites and demonstrates consistency with existing spatially explicit ET products. The analysis is comprehensive, and the presentation is clear and easy to follow. Overall, the paper represents a significant advancement in filling ET data gaps. Although I have selected "major revisions", the paper is very close to being published. Before it can be recommended for publication in its current form, the following concerns should be addressed:
L106: More clarification is needed on what "obs" refers to. Does it relate to the nearest time period from the gap? Is there a threshold for how far back/ahead the "obs" data can be? It may be prudent to leave certain gaps unfilled if the distance to the nearest data point is too large. It's important to note that Equation 1 assumes that beta (not beta*) in both the gap and obs are similar. While this may hold within a day, the temporal variability of beta is unclear. The authors could consider deriving the autocorrelation length of beta (in days) for each site, and use that to guide the maximum allowable gap length for filling.
L122: The phrase "specific gaps were randomly created and tested within the measured ET values" may overlook the fact that gaps are often continuous rather than isolated. The fully random method used here might not represent real-world conditions where gaps vary in length and are often continuous. Is the current methodology potentially limiting?
L121 (Equation 10): The modification of the gap-filling method to leverage the median to address outliers, particularly when (Rn - G)_obs approaches zero, may not fully satisfy the assumption in Equations 1 and 10 that beta in gap and obs are similar. This assumption is likely violated when "all measured ET values" are used in equation 10. A more straightforward approach might involve adding a small positive quantity to the denominator when (Rn - G)_obs is close to zero to avoid anomalous data. This reviewer wonders why such an alternative was not considered.-- While the paper's exploration of how the energy balance ratio (EBR) varies between sites with poor and better consistency between filled ET and ET products is appreciated, it would be beneficial to statistically assess whether the EBR performance differs significantly across the 313 sites and other locations.
-- Further discussion is needed on how the gap-filling process could be improved. Possible suggestions may include modifying Equation 10, using nearby ET data from adjacent time periods only, or incorporating dynamic vegetation indices when evaluating surface conductance, etc.Minor comments:
L41: The phrase "parameterized issues" should be revised to "parameterization issues."
L46: The word "comparative" can be omitted.
L45-48: The sentence "Consequently... balance" can be rephrased for clarity and improved readability.
L68: "Exiting methods" should likely be "existing methods."
L80-81: The statement "has demonstrated superior logicality and reasonableness in its underlying mechanisms" is vague, as it is unclear how "logicality" and "reasonableness" are quantified. These terms were not demonstrated clearly in Jiang et al. This sentence should be rephrased.Citation: https://doi.org/10.5194/essd-2024-291-RC1 -
RC2: 'Comment on essd-2024-291', Anonymous Referee #2, 13 Oct 2024
The comment was uploaded in the form of a supplement: https://essd.copernicus.org/preprints/essd-2024-291/essd-2024-291-RC2-supplement.pdf
Status: closed
-
RC1: 'Comment on essd-2024-291', Anonymous Referee #1, 04 Oct 2024
The study employs the full-factorial gap-filling method, introduced by Jiang et al. (2022), to address evapotranspiration (ET) data gaps from eddy covariance sites. The authors successfully generated high quality gap-filled ET data for 313 sites covering 2210 site-years. Additionally, the paper reports the energy balance closure at these sites and demonstrates consistency with existing spatially explicit ET products. The analysis is comprehensive, and the presentation is clear and easy to follow. Overall, the paper represents a significant advancement in filling ET data gaps. Although I have selected "major revisions", the paper is very close to being published. Before it can be recommended for publication in its current form, the following concerns should be addressed:
L106: More clarification is needed on what "obs" refers to. Does it relate to the nearest time period from the gap? Is there a threshold for how far back/ahead the "obs" data can be? It may be prudent to leave certain gaps unfilled if the distance to the nearest data point is too large. It's important to note that Equation 1 assumes that beta (not beta*) in both the gap and obs are similar. While this may hold within a day, the temporal variability of beta is unclear. The authors could consider deriving the autocorrelation length of beta (in days) for each site, and use that to guide the maximum allowable gap length for filling.
L122: The phrase "specific gaps were randomly created and tested within the measured ET values" may overlook the fact that gaps are often continuous rather than isolated. The fully random method used here might not represent real-world conditions where gaps vary in length and are often continuous. Is the current methodology potentially limiting?
L121 (Equation 10): The modification of the gap-filling method to leverage the median to address outliers, particularly when (Rn - G)_obs approaches zero, may not fully satisfy the assumption in Equations 1 and 10 that beta in gap and obs are similar. This assumption is likely violated when "all measured ET values" are used in equation 10. A more straightforward approach might involve adding a small positive quantity to the denominator when (Rn - G)_obs is close to zero to avoid anomalous data. This reviewer wonders why such an alternative was not considered.-- While the paper's exploration of how the energy balance ratio (EBR) varies between sites with poor and better consistency between filled ET and ET products is appreciated, it would be beneficial to statistically assess whether the EBR performance differs significantly across the 313 sites and other locations.
-- Further discussion is needed on how the gap-filling process could be improved. Possible suggestions may include modifying Equation 10, using nearby ET data from adjacent time periods only, or incorporating dynamic vegetation indices when evaluating surface conductance, etc.Minor comments:
L41: The phrase "parameterized issues" should be revised to "parameterization issues."
L46: The word "comparative" can be omitted.
L45-48: The sentence "Consequently... balance" can be rephrased for clarity and improved readability.
L68: "Exiting methods" should likely be "existing methods."
L80-81: The statement "has demonstrated superior logicality and reasonableness in its underlying mechanisms" is vague, as it is unclear how "logicality" and "reasonableness" are quantified. These terms were not demonstrated clearly in Jiang et al. This sentence should be rephrased.Citation: https://doi.org/10.5194/essd-2024-291-RC1 -
RC2: 'Comment on essd-2024-291', Anonymous Referee #2, 13 Oct 2024
The comment was uploaded in the form of a supplement: https://essd.copernicus.org/preprints/essd-2024-291/essd-2024-291-RC2-supplement.pdf
Data sets
Gap-Filling for Daily Evapotranspiration Observations with full-factorial method at Global Flux Sites Xiaowei Wang and Yazhen Jiang https://doi.org/10.57760/sciencedb.11651
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