The effect of relative humidity on eddy covariance latent heat flux measurements and its implication for partitioning into transpiration and evaporation

Zhang, Weijie; Jung, Martin; Migliavacca, Mirco; Poyatos, R.; Miralles, Diego G.; El‐Madany, Tarek S.; Galvagno, Marta; Carrara, Arnaud; Arriga, Nicola; Ibrom, Andreas; Mammarella, Ivan; Papale, Dario; Cleverly, Jamie; Liddell, Michael J.; Wohlfahrt, Georg; Markwitz, Christian; Mauder, Matthias; Paul‐Limoges, Eugénie; Schmidt, Marius; Wolf, Sebastian; Brümmer, Christian; Arain, M. Altaf; Fares, Silvano; Kato, Tomomichi; Ardö, Jonas; Oechel, Walter C.; Hanson, Chad; Korkiakoski, Mika; Biraud, S.; Steinbrecher, R.; Billesbach, D. P.; Montagnani, Leonardo; Woodgate, William; Shao, Changwei; Reichstein, Markus; Nelson, Jacob A.

doi:10.1016/j.agrformet.2022.109305

Cited by 16 publications

(10 citation statements)

References 61 publications

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“…Clearly, we can not assume that all systematic energy balance closure issues scale only with AE (Foken, 2008; Mauder et al., 2020), while our method accounts explicitly only for this part of the energy imbalance (embedding the relative humidity dependent correction proposed in Zhang et al. (2023)). Strictly speaking this implies that our method is incomplete and it indeed shows a weak under‐closure, which is more pronounced at high AE conditions.…”

Section: Discussionmentioning

confidence: 99%

“…For consistency, data were aggregated to hourly resolution because flux measurements are not available at the half-hourly scale for all sites. Hourly LE measurements (when the quality flags for Rn, H, LE, and net exchange of carbon dioxide flux being 0) were then corrected using the high relative humidity correction method (Zhang et al, 2023), where we also confirmed the small effect of G. To reconcile the complication and uncertainty of the diurnal cycle and the strong G (and storage flux) effects at the sub-daily scale, hourly data were averaged to the daily scale with discarding days when less than half the percentage of good quality within a day. In theory, the effect of G at a daily scale should be minimized and may be neglected, as the heat flux stored in the soil surface during the day should be largely released from the surface during the night, and we further diagnosed the G effect for sites with G measurements (comparing the energy balance closure when setting G to zero across the network, Figures S1 and S2 in Supporting Information S1) to gain more confidence, we then gap-fill G by setting all missing values to 0 to cover as many days and sites as possible.…”

Section: Datamentioning

confidence: 99%

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A New Post‐Hoc Method to Reduce the Energy Imbalance in Eddy Covariance Measurements

Zhang,

Nelson,

Miralles

et al. 2024

Geophysical Research Letters

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Latent and sensible heat flux observations are essential for understanding land–atmosphere interactions. Measurements from the eddy covariance technique are widely used but suffer from systematic energy imbalance problems, partly due to missing large eddies from sub‐mesoscale transport. Because available energy drives the development of large eddies, we propose an available energy based correction method for turbulent flux measurements. We apply our method to 172 flux tower sites and show that we can reduce the energy imbalance from −14.99 to −0.65 W m−2 on average, together with improved consistency between turbulent fluxes and available energy and associated increases in r2 at individual sites and across networks. Our results suggest that our method is conceptually and empirically preferable over the method implemented in the ONEFlux processing. This can contribute to the efforts in understanding and addressing the energy imbalance issue, which is crucial for the evaluation and calibration of land surface models.

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Section: Discussionmentioning

confidence: 99%

Section: Datamentioning

confidence: 99%

A New Post‐Hoc Method to Reduce the Energy Imbalance in Eddy Covariance Measurements

Zhang,

Nelson,

Miralles

et al. 2024

Geophysical Research Letters

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“…Additionally, high relative humidity can produce an underestimation of LE, especially with closed-path systems, as the cut-off frequency of the closed-path system for water vapour concentration measurements decreases exponentially with increasing relative humidity (Zhang et al, 2023a). Based on these assumptions, we implemented two corrections separately.…”

Section: Energy Balance Closure and Latent Heat Flux Underestimationsmentioning

confidence: 99%

“…2. The Zhang et al (2023a) correction, hereafter high relative humidity correction (HRHC), which adjusts LE considering the impact of high relative humidity.…”

Section: The Mauder Et Al (2013) Correction Hereafter Thementioning

confidence: 99%

Comparison of carbon and water fluxes and the drivers of ecosystem water use efficiency in a temperate rainforest and a peatland in southern South America

Perez-Quezada,

Trejo,

Lopatin

et al. 2024

Biogeosciences

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Abstract. The variability and drivers of carbon and water fluxes and their relationship to ecosystem water use efficiency (WUE) in natural ecosystems of southern South America are still poorly understood. For 8 years (2015–2022), we measured carbon dioxide net ecosystem exchange (NEE) and evapotranspiration (ET) using eddy covariance towers in a temperate rainforest and a peatland in southern Chile. NEE was partitioned into gross primary productivity (GPP) and ecosystem respiration (Reco), while ET was partitioned into evaporation (E) and transpiration (T) and used to estimate different expressions of ecosystem WUE. We then used the correlation between detrended time series and structural equation modelling to identify the main environmental drivers of WUE, GPP, ET, E and T. The results showed that the forest was a consistent carbon sink (−486 ± 23 g C m−2 yr−1), while the peatland was, on average, a small source (33 ± 21 g C m−2 yr−1). WUE is low in both ecosystems and likely explained by the high annual precipitation in this region (∼ 2100 mm). Only expressions of WUE that included atmospheric water demand showed seasonal variation. Variations in WUE were related more to changes in ET than to changes in GPP, while T remained relatively stable, accounting for around 47 % of ET for most of the study period. For both ecosystems, E increased with higher global radiation and higher surface conductance and when the water table was closer to the surface. Higher values for E were also found with increased wind speeds in the forest and higher air temperatures in the peatland. The absence of a close relationship between ET and GPP is likely related to the dominance of plant species that either do not have stomata (i.e. mosses in the peatland or epiphytes in the forest) or have poor stomatal control (i.e. anisohydric tree species in the forest). The observed increase in potential ET in the last 2 decades and the projected drought in this region suggests that WUE could increase in these ecosystems, particularly in the forest, where stomatal control may be more significant.

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“…These methods were reviewed by Kool et al [ 14 ] and classified into two categories: models and measurements. The commonly used models were the Shuttleworth–Wallace model [ 19 ] and its improved structures [ 20 , 21 , 22 , 23 , 24 , 25 ], the clumped model [ 26 , 27 ], the FAO-56 dual crop coefficient model [ 28 , 29 , 30 ] and other improved dual-source models [ 31 , 32 , 33 ], while measurements were mainly eddy covariance techniques [ 34 , 35 , 36 , 37 , 38 ] and Bowen ratio systems [ 39 , 40 , 41 ] (acquiring ET), stable isotopes [ 42 , 43 , 44 , 45 ] (acquiring E s or T ), sap flow meters [ 46 , 47 , 48 , 49 ] (acquiring T ), microlysimeters [ 50 , 51 , 52 ] (acquiring E s ) and water collection tanks [ 53 , 54 ] (acquiring E i ). Among these methods, the modeling approach has the advantage of its applicability over a wide range of time scales and can be applied to the spatial scale of an entire ecosystem [ 55 , 56 ], but these models always require complex parameterizations and still require validation.…”

Section: Introductionmentioning

confidence: 99%

Environmental Controls on Evapotranspiration and Its Components in a Qinghai Spruce Forest in the Qilian Mountains

Gao,

Guo,

Feng

et al. 2024

Plants

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Qinghai spruce forests, found in the Qilian mountains, are a typical type of water conservation forest and play an important role in regulating the regional water balance and quantifying the changes and controlling factors for evapotranspiration (ET) and its components, namely, transpiration (T), evaporation (Es) and canopy interceptions (Ei), of the Qinghai spruce, which may provide rich information for improving water resource management. In this study, we partitioned ET based on the assumption that total ET equals the sum of T, Es and Ei, and then we analyzed the environmental controls on ET, T and Es. The results show that, during the main growing seasons of the Qinghai spruce (from May to September) in the Qilian mountains, the total ET values were 353.7 and 325.1 mm in 2019 and 2020, respectively. The monthly dynamics in the daily variations in T/ET and Es/ET showed that T/ET increased until July and gradually decreased afterwards, while Es/ET showed opposite trends and was mainly controlled by the amount of precipitation. Among all the ET components, T always occupied the largest part, while the contribution of Es to ET was minimal. Meanwhile, Ei must be considered when partitioning ET, as it accounts for a certain percentage (greater than one-third) of the total ET values. Combining Pearson’s correlation analysis and the boosted regression trees method, we concluded that net radiation (Rn), soil temperature (Ts) and soil water content (SWC) were the main controlling factors for ET. T was mainly determined by the radiation and soil hydrothermic factors (Rn, photosynthetic active radiation (PAR) and TS30), while Es was mostly controlled by the vapor pressure deficit (VPD), atmospheric precipitation (Pa), throughfall (Pt) and air temperature (Ta). Our study may provide further theoretical support to improve our understanding of the responses of ET and its components to surrounding environments.

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The effect of relative humidity on eddy covariance latent heat flux measurements and its implication for partitioning into transpiration and evaporation

Cited by 16 publications

References 61 publications

A New Post‐Hoc Method to Reduce the Energy Imbalance in Eddy Covariance Measurements

A New Post‐Hoc Method to Reduce the Energy Imbalance in Eddy Covariance Measurements

Comparison of carbon and water fluxes and the drivers of ecosystem water use efficiency in a temperate rainforest and a peatland in southern South America

Environmental Controls on Evapotranspiration and Its Components in a Qinghai Spruce Forest in the Qilian Mountains

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