This study evaluated the accuracy of seven upscaling methods in simulating daily latent heat flux (LE) from instantaneous values
using observations from 148 global sites under all sky conditions and at different times during the day. Daily atmospheric transmissivity (τ)
was used to represent the sky conditions. The results showed that all seven
methods could accurately simulate daily LE from instantaneous values. The
mean and median of Nash–Sutcliffe efficiency were 0.80 and 0.85,
respectively, and the corresponding determination coefficients were 0.87 and
0.90, respectively. The sine and Gaussian function methods simulated mean
values with relatively higher accuracy, with relative errors generally
within ±10 %. The evaporative fraction (EF) methods, which use
potential evapotranspiration and incoming shortwave radiation, performed
relatively better than the other methods in simulating daily series.
Overall, the EF method using potential evapotranspiration had the highest
accuracy. However, the sine function and the EF method using
extraterrestrial solar irradiance are recommended in upscaling applications
because of the relatively minimal data requirements of these methods and
their comparable or relatively higher accuracy. The intra-day distribution
of the LE showed greater consistency with the Gaussian function than the
sine function. However, the accuracy of simulated daily LE series using the
Gaussian function method did not improve significantly compared with the
sine function method. The simulation accuracy showed a minor difference when using the same type of method, for example, the same type of mathematical
function or EF method. In any upscaling scheme, the simulation accuracy from
multi-time values was significantly higher than that from a single-time value. Therefore, when multi-time data are available, multi-time values
should be used in evapotranspiration upscaling. The upscaling methods show
the ability to accurately simulate daily LE from instantaneous values from
09:00 to 15:00, particularly for instantaneous values between 11:00 and 14:00. However, outside of this time range the upscaling methods performed poorly.
These methods can simulate daily LE series with high accuracy at τ>0.6; when τ<0.6, simulation accuracy is
significantly affected by sky conditions and is generally positively related to daily atmospheric transmissivity. Although every upscaling scheme
can accurately simulate daily LE from instantaneous values at most sites,
this ability is lost at tropical rainforest and tropical monsoon sites.
Introduction
Evapotranspiration (ET) is a critical and unique bridge connecting the
hydrologic cycle, surface energy balance, and carbon cycle (Jasechko et al.,
2013; Lian et al., 2018). Approximately 60 % of precipitation on the
global land surface returns to the atmosphere via ET (Oki and Kanae, 2006).
More than half of the solar energy absorbed by land surfaces is currently
used in the process of ET (Trenberth et al., 2009). Accurate simulations of
ET represent the core of hydrologic processes, crop growth, and ecosystem
water efficiency simulations (Ponce-Campos et al., 2013). These simulations
are important for agriculture, ecology, and water resource management.
However, field ET observations are expensive and labor-intensive (Jaksa et
al., 2013) and cannot meet the required level of spatial accuracy. In recent decades, remote sensing ET retrieval based on the combination of
satellite remote sensing data and the land surface energy model has become
an increasingly important area of research, as it can represent the spatial
heterogeneity of terrestrial ET at regional or global scales (Jung et al.,
2010; Miralles et al., 2011; Mu et al., 2011; Zhang et al., 2019).
However, the remote sensing technique can only detect the instantaneous ET
rate at the time of satellite overpasses. Additionally, instantaneous ET
data are not useable for practical applications such as ecohydrological modeling and water resource management. For practical
purposes, we are concerned with ET over a period of time; temporal upscaling
of instantaneous ET over a period of time is necessary for remote sensing ET maps. Temporal upscaling has become one of the key issues and future
research directions in the context of ET estimation from remote sensing data
(Kalma et al., 2008; Li et al., 2009; Liu et al., 2020). A critical temporal
upscaling step is upscaling from instantaneous to daily ET values (Chen and
Liu, 2020).
Temporal upscaling methods have been reviewed thoroughly by several studies
(Kalma et al., 2008; Li et al., 2009; Chen and Liu, 2020) and may be divided into three categories: the sine function method, the constant
evaporative fraction (EF) method, and the constant ratio between the actual ET and potential ET (PET). Jackson (1983) assumed that diurnal solar
irradiance and ET may be described by a sine function and developed this function to calculate daily ET from instantaneous values. Sugita and
Brutsaert (1991) found that the evaporative fraction (EF) usually varies
little during the daytime; the EF was defined as the ratio of the latent
heat flux (LE =ρλE, where ρ and λ are the
density of water and the latent heat of vaporization, respectively) to the
available energy flux (Rn-G) at the surface. It may be assumed that EF is constant during daylight hours in order to upscale instantaneous ET to daily
values. Investigations on the environmental factors that contribute to EF
variability showed that EF is almost independent of major forcing factors,
including air temperature, wind velocity, and incoming solar radiation
(Crago, 1996; Gentine et al., 2007). However, cloudy weather and proximity
to surface discontinuities or fronts may cause significant EF variability.
The diurnal shape of EF is dependent on atmospheric forcing and surface
conditions (Gentine et al., 2007); the EF is generally constant in the
morning and increases sharply in the afternoon (Lhomme and Elguero, 1999;
Gentine et al., 2007; Delogu et al., 2012). Hoedjes et al. (2008) found that
although the EF method could accurately simulate daily ET under dry
conditions, it significantly underestimated daily ET in wet conditions. They
incorporated a daily scaling factor into EF for wet conditions by
parameterizing the diurnal shape of EF as a function of incoming solar
radiation and relative humidity; this was found to improve the accuracy of
the simulation (Hoedjes et al., 2008; Delogu et al., 2012).
In addition to Rn-G, Brutsaert and Sugita (1992) used field measurements to validate effective EF ratios with net radiation (Rn) and incoming shortwave radiation (Rs). All these EF scaling factor approaches require
surface energy flux data. An alternative approach with lower data
requirements (Ryu et al., 2012; Van Niel et al., 2012) assumes a constant EF
ratio for the LE to extraterrestrial solar irradiance (Re). Similarly to the sine function method, this temporal upscaling scheme requires only latitude,
longitude, and time as data inputs. EF methods based on variables such as
Rn-G, Rn, Rs, and Re are abbreviated as EF(Rn-G), EF(Rn), EF(Rs), and EF(Re), respectively. Another temporal upscaling approach is maintaining a
constant ratio between the actual ET and the PET (Kalma et al., 2008). Allen
et al. (2007) proposed a constant ETrF in which PET was calculated using the reference ET during the daytime for temporal upscaling. Tang and Li (2017a, b) developed a decoupling factor using the Priestley–Taylor
equation for PET. This decoupling factor method provides a theoretical framework for temporal upscaling (Chen and Liu, 2020). However, the ETrF
approach requires additional weather measurements including air temperature,
humidity, atmospheric pressure, and wind speed that are only recorded when
the satellite overpasses.
All these methods are only used for upscaling daytime ET; as such, upscaling
methods may underestimate daily ET due to nocturnal transpiration, which is
the main cause of uncertainty in ET upscaling (Kalma et al., 2008;
Blatchford et al., 2019). There have been several evaluations of these
upscaling methods, which have found that the accuracy of the upscaling
methods varies between regions. Zhang and Lemeur (1995) evaluated the sine
function and EF(Rn-G) using an experiment in southwestern France, finding that both methods could accurately estimate daily ET from instantaneous
measurements; they recommended a sine function due to its lower data
requirements. The sine function and three EF methods, Rn, Rn-G, and Re, were evaluated for the upscaling of monthly ET at two sites in Australia (Van Niel et al., 2012). A monthly bias was used to correct the upscaling
methods; the results showed that the EF(Rn) was the preferable monthly
upscaling method, as it had the lowest root-mean-square deviation (RMSD)
before and after correction. The evaluation of EF and ETrF methods at four
sites in France and Morocco showed that the EF method outperforms the ETrF
method at sites experiencing a higher frequency of water stress periods
(Delogu et al., 2012). Cammalleri et al. (2014) evaluated four methods,
EF(Rn-G), EF(Rs), EF(Re), and ETrF, in upscaling daily ET at 12 AmeriFlux
stations. They found that the EF(Rs) method showed more robust overall
performance in terms of accuracy and site-to-site variability. In contrast,
Tang et al. (2013) evaluated four upscaling methods (EF(Rn-G), EF(Re),
EF(Rs), and ETrF) for daily LE simulations at a flux site in China. Their results showed that the ETrF method had the best performance among the four
methods, while the EF(Rs) method was the second best. In general, previous research has largely evaluated upscaling methods on a regional scale. Based on 126
FLUXNET global sites, Wandera et al. (2017) evaluated three EF methods – Rs,
Re, and Rn-G – finding that the EF(Rs) method yielded relatively better accuracy in daily ET simulations. However, they only used EF methods for global evaluation.
The FLUXNET dataset provides a good opportunity to evaluate upscaling
methods at the global scale (Pastorello et al., 2020) and has been widely used to evaluate ET estimation from remote sensing data (Fisher et al.,
2008; Ershadi et al., 2014; Carter and Liang, 2018; Knox et al., 2019;
Pastorello et al., 2020). The new FLUXNET-CH4 community product was released
in 2020; this data series has been expanded to 2018. In 2020, the FLUXNET
dataset published the observation height and vegetation height of each site,
enabling the calculation of PET using the Penman–Monteith equation. This calculation is more consistent with the actual observational land surface
than the reference ET. This study uses the FLUXNET dataset to
comprehensively evaluate the ability of various upscaling schemes to
accurately simulate daily LE at global flux measurement sites.
This study addresses four key objectives: (1) evaluating the accuracy of
seven upscaling methods (the sine function, EF, and ETrF methods) in
simulating daily LE from instantaneous values; (2) investigating the
performance of upscaling methods under all sky conditions and calibrating the optimal threshold of sky conditions required to accurately simulate
daily LE; (3) evaluating the simulation accuracy of upscaling methods at
different times during the day; (4) investigating the spatial distribution of simulation accuracy at global flux observation sites.
Materials and methodsObservation data
This study used the FLUXNET eddy covariance observations that cover all
continents; this includes the FLUXNET2015 (Pastorello et al., 2020) and
FLUXNET-CH4 community products (Knox et al., 2019). FLUXNET2015 contains 212
observation sites from 1991 to 2014, while the FLUXNET-CH4 community product
contains 81 sites from 2006 to 2018. The longest observational record was 25 years, while the shortest record was less than 1 year. Half-hourly data series on LE, Rs, Rn, and ground heat flux (G) were used for the upscaling schemes,
while the observed air temperature, wind speed, atmospheric pressure, vapor
pressure deficit, crop heights, observation height of wind, and humidity
data were used in the Penman–Monteith equation. All missing values were eliminated; for example, if there were missing values on a certain day, all
data on that day were discarded. As such, only days with fully available
half-hourly data were used in the analysis. Then, only sites with a data
series longer than 360 d were used. These eliminations ultimately meant a
total of 122 FLUXNET2015 sites and 42 FLUXNET-CH4 sites were used in the
analysis due to the lack of observations (Table S1). There were 16
sites belonging to both FLUXNET2015 and FLUXNET-CH4, and flux observation
data from four sites in Australia were obtained from the TERN OzFlux
dataset; the latter dataset was a long and continuous series up to 2019 (Beringer et al., 2016). LE was corrected using the energy balance closure
correction factor.
Location of eddy covariance observations.
Methods of temporal upscaling of instantaneous <inline-formula><mml:math id="M50" display="inline"><mml:mi mathvariant="italic">λ</mml:mi></mml:math></inline-formula>ET to daily values
A Gaussian function was used in this study in addition to the widely used
sine function. The distribution of λET (LE) during the daytime was
more in line with the Gaussian function (this is shown in Sect. 3.1). In total, seven temporal upscaling methods for upscaling instantaneous LE to
daily values were evaluated; this includes the sine function, Gaussian
function, four EFs – EF(Rn-G), EF(Rn), EF(Rs), and EF(Re) – and the ETrF methods. In general, the relationship between instantaneous LE and LE over time may be expressed as follows:
LET=∫t=0TLEtdt,
where LET and LEt are the LE over a period of time and
instantaneous LE, respectively.
The sine (Jackson, 1983) and Gaussian function upscaling methods assume that
the daytime LE obeys the sine and Gaussian functions, respectively:
2SINEt=sinπt-t0tn-t0,3GAUSSIANt=1σ2πe-12(t-μσ)2,
where t0 and tn are the sunrise and sunset times, respectively;
μ is the solar noon time, equal to (t0+tn)/2; and σ is a shape parameter of the Gaussian function. Sunrise and sunset times
were calculated using the National Oceanic and Atmospheric Administration
(NOAA) solar calculations
(https://www.esrl.noaa.gov/gmd/grad/solcalc/calcdetails.html, last access: 3 August 2021). Subsequently,
the sine and Gaussian function upscaling methods may be described as
follows:
4LEd=1n∑t=t0tnSINEtSINEtLEi,5LEd=1n∑t=t0tnGAUSSIANtGAUSSIANtLEi,
where LEd is the simulated daily LE during the daytime, and LEi is the instantaneous LE used in the simulation. The LEt was calculated
using Eqs. (2) and (3) for the sine and Gaussian functions,
respectively.
The EF and ETrF methods assume a constant ratio between LE and the upscaling
variable; this may be described as follows:
LEdVd=LEiVi,
where LEi and LEd are the instantaneous and daytime LE,
respectively, and Vi and Vd are the instantaneous and daily upscaling variables, respectively.
The four EF methods involve the upscaling variables, Rn, Rn-G, Rs and Re; the former three variables are measured by FLUXNET. Re, which is also known as the top-of-atmosphere solar irradiance, is calculated by the following equation (Ryu et al., 2012):
Re=Ssc×[1+0.033cos2πDOYYdmax]cosβ,
where Ssc is the solar constant (1360 W m-2); DOY is
the day of the year; Ydmax is the maximum number of days (365 or 366)
for the specified year; β is the specific time-of-day solar zenith angle calculated using the NOAA solar calculations.
The ETrF method involves the upscaling variable, PET, herein referred to as
EF(PET); PET is calculated using the Penman–Monteith equation (Penman, 1948; Monteith, 1981; Allen et al., 1998):
ρλE=ΔRn-G+ρacp(es-ea)raΔ+γ1+rsra,
where ρ is the density of water; λ is the latent heat of
vaporization, which is the unit conversion coefficient between ET and LE;
Δ is the slope of the saturation vapor pressure–temperature relationship; (Rn-G) is the available energy flux; ρa is the mean air density at constant pressure; cp is the specific heat of the air;
(es-ea) represents the vapor pressure deficit of the air;
γ is the psychometric constant; and rs and ra are the
surface and aerodynamic resistances, respectively. The calculation of
Δ, ρa, cp, γ, rs, and ra follows
the method specified in Allen et al. (1998), in which additional
observations of air temperature wind velocity, atmospheric pressure, vegetation height, and observation heights of wind and humidity are
required.
The daily LE, derived from Eq. (6), may also be simulated as follows:
LEd=VdViLEi.
For the EF(Rn), EF(Rn-G), and EF(Rs) methods, V is the observed Rn, Rn-G, and Rs, respectively. For EF(Re) and EF(PET), V is calculated from Eqs. (7) and (8), respectively. When the absolute value of Vi is extremely
low, the observed or calculated Vi in Eq. (9) may generate an
anomaly in the Vd/Vi ratio. This will produce an abnormally high
simulated LEd; as such, abnormal Vd/Vi ratios (i.e., > 10) were discarded in the simulation.
Sky conditions
The daily atmospheric transmissivity coefficient (τ), calculated as
the ratio of incoming shortwave radiation to extraterrestrial radiation, was used to represent the sky conditions; this is indicative of daily
atmospheric transmissivity. The hypothesis is that during clear-sky
conditions, shortwave incoming radiation is strongly correlated with
extraterrestrial radiation, although it deviates in cloudy conditions. The daily τ is calculated as follows (Baigorria et al., 2004; Wandera et
al., 2017):
τ=RsdRed,
where Rsd and Red are the observed daily incoming shortwave
radiation and calculated top-of-atmosphere solar irradiance (in
MJ m-2 d-1, converted from W m-2) during the daytime, respectively.
Evaluation criteria
The accuracy of the seven upscaling methods was evaluated using homogeneous
datasets across a range of temporal scales and variable sky conditions. The
criteria used to evaluate these methods included the relative error (RE),
root-mean-square error (RMSE), Nash–Sutcliffe efficiency (NSE), and
determination coefficient (R2). The RE and RMSE represented bias
deviation from observed values, while NSE and R2 are indicative of the
goodness of fit of the simulated and observed data series. The best fit value was 1.0, while the goodness of fit deteriorated with increasing deviation from 1.0. The evaluation criteria were calculated as follows.
11RE=1n∑i=1nXmi-XoiXoi,12RMSE=1n∑i=1n[(Xmi-Xm‾)-(Xoi-Xo‾)]2,13NSE=1-∑i=1nXmi-Xoi2∑i=1nXoi-Xo‾2,14R2=1-∑i=1nXmi-fi2∑i=1nXmi-Xm‾2,
where Xmi and Xoi are the ith values of the modeled and
observed LE time series, respectively; n is the length of a time series;
Xm‾ and Xo‾ are the means of the modeled and observed LE, respectively; and f(i) is a linear fitted function between the
observed and modeled daily LE series.
ResultsIntra-day distribution of observed LE and its influencing variables
The intra-day distribution characteristics of each flux variable were
analyzed based on the field observation data. Figure 2 shows the
intra-day distribution of half-hourly LE, Rn, Rs, Re, and PET, derived from
the mean of 148 FLUXNET sites. LE was stable and showed little variance from
20:00 to 06:00. During this period, LE accounted for only 5.4 % of the total daily LE, while it showed unimodal distribution from 06:00 to 19:00. Factors that directly or indirectly affected LE, including Rn, Rs, Re, and PET,
exhibited a similar intra-day distribution to that of LE. Among them, the
intra-day distribution of PET demonstrated the best agreement with the
measured LE (Fig. 2a). However, the intra-day distributions of Rn,
Rs, and Re showed an overall deviation from that of the measured LE. The
distribution of Rn and Rs was generally half an hour earlier than the
measured LE, while that of Re was 1 h earlier. The intra-day distribution of the observed LE from 06:00 to 19:00 was compared with the sine and Gaussian functions (Fig. 2b). The results showed that daytime
LE was more consistent with the latter than the sine function, which is
commonly used to upscale instantaneous ET to daily values in remote sensing
applications. The Gaussian function matched LE perfectly at any time during
the day. The sine function slightly underestimated LE during the afternoon and tended to overestimate LE from 06:00 to 10:00 and from 15:00 to 17:00.
Intra-day distribution of normalized LE, Rn, Rs, Re, and PET and the values from the sine and Gaussian functions.
Accuracy of seven upscaling methods in simulating daily LE series
Figure 3 presents the results from evaluating the daily LE
simulations using the seven remote sensing ET upscaling methods, which
include the sine and Gaussian functions, EF(Rn), EF(Rn-G), EF(Rs), EF(Re),
and EF(PET). The performance of each upscaling scheme while simulating the
mean value shows that daily LE simulated by most schemes was lower than the
observed values, where the underestimation was generally less than 20 %.
Among them, the sine and Gaussian function methods demonstrated a relatively
better performance for the mean values, where the RE was generally within
±10 %. The Gaussian and sine functions also performed the best in
simulating the mean daily LE at 10:30 and 13:30, respectively. The mean
values of daily LE simulated by the EF(PET) method were also relatively
closer to measured values. The EF(Rn) method exhibited the poorest
performance for mean daily LE simulation of all upscaling schemes. The
simulated RE using this method generally ranged from 0 % to -40 %, with the
mean RE of all sites being approximately -20 %. In general, there was only
a small difference between upscaling simulations using the single-time value and those using multi-time values. However, the mean of simulated daily LE
by the upscaling schemes at 13:30 was significantly higher than that at
10:30. The mean REs of all upscaling schemes for the former and latter time points were -2.3 % and -9.7 %, and the corresponding median REs
were -1.8 % and -9.2 %, respectively. As such, the mean daily LE
upscaled from 13:30 was closer to the measured value than that from 10:30;
the performance of upscaling methods was better at 13:30 than at 10:30.
Simulation accuracy of daily LE using seven remote sensing
ET upscaling methods, including the sine and Gaussian functions, EF(Rn),
EF(Rn-G), EF(Rs), EF(Re), and EF(PET). Note: S and M represent simulations by a single and average of multi-time values, respectively. For example, S10:30
is simulated by the ratio of a daytime value to a single-time value at 10:30, while M10:30 is simulated by the ratio of a daytime value to the
average of three-time values at 10:00, 10:30, and 11:00.
The RMSE evaluation showed that the RMSE of each upscaling scheme at each
site ranged from 5 to 30 W m-2, where the mean of all
simulated RMSEs was 13.5 W m-2. In the RMSE evaluation, there was only a small difference between the upscaling simulations at 10:30 and
13:30, as opposed to the RE evaluation results. However, the simulation
accuracy of multi-time values was slightly higher than the single-time value. The mean RMSEs of all upscaling schemes for the former and latter
were 15.0 and 11.7 W m-2, while the corresponding median
values were 13.8 and 10.5 W m-2, respectively.
Figure 3 also presents the evaluations based on the NSE and R2
data series criteria, which evaluate the goodness of fit of the simulated and observed data series. In general, all upscaling schemes could accurately
simulate daily LE series. The median NSE and R2 were generally higher
than 0.70 and 0.80 for all sites under each upscaling scheme, respectively.
This means that the daily LE series simulated by each upscaling scheme was
relatively consistent with observed values and was strongly correlated with the measured data series. Similarly to the RMSE evaluation, simulations using multi-time values were more accurate than those using a single-time value. For example, when a single-time value is used in upscaling schemes, the
simulated NSE of each site mainly fell between 0.60 and 0.80. In contrast,
when multi-time values were used for simulations, the NSE of each site
increased to between 0.70 and 0.90, where the median exceeded 0.80. In
single-time value simulations, the median of the simulated R2 of all
sites was approximately 0.80; when multi-time values were used, the R2
improved to a value exceeding 0.90. There was a minor difference between the 10:30 and 13:30 upscaling schemes based on the NSE and R2 evaluation
criteria. This is because the upscaling scheme assumes that the intra-day
distribution of the upscaling variable is similar to that of the observed
LE. Therefore, the upscaling scheme can successfully simulate daily LE at
any time during the day. However, there was significant variability in the
accuracy of upscaling methods when simulated in the evening or nighttime.
A comparative analysis of the different upscaling methods was also
performed. The daily LE data series simulated by the EF(PET) and EF(Rs)
methods showed a relatively greater level of consistency with observed
values than those simulated by the other five methods. For example, in
simulations of multi-time values, the mean and median NSE simulated by the
two methods at each site were 0.83 and 0.89, while the corresponding values
simulated by the other five methods were 0.77 and 0.84, respectively. The
RMSE evaluation results were similar to those for NSE. For example, in
simulations of multi-time values, the mean and median RMSEs simulated by the two methods were 9.8 and 8.9 W m-2, respectively, while the
corresponding values for the other five methods were 11.0 and 10.3 W m-2. In terms of the evaluation results of the correlation index R2, in general, there was little difference between the
performance of the seven methods. The mean R2 at each site was 0.87,
and the corresponding median was 0.90.
Based on this comprehensive evaluation, while the EF(PET) method was the
most optimal of all seven methods, it also had the greatest input data
requirements. The sine function, Gaussian function, and EF(Re) methods,
which required the least input data, also produced relatively accurate
simulations. Among them, the Gaussian function method demonstrated the best
performance for the mean value simulation. The EF(Re) method was similar to
the PET method as per the RMSE, NSE, and R2, with a larger RE range.
Spatial distribution of the accuracy of the sine function and EF(<inline-formula><mml:math id="M191" display="inline"><mml:mrow><mml:msub><mml:mi>R</mml:mi><mml:mi mathvariant="normal">e</mml:mi></mml:msub></mml:mrow></mml:math></inline-formula>) methods
In general, all upscaling methods demonstrated an ability to accurately
simulate daily LE data series at most sites, particularly for simulations
using multi-time values. The spatial distribution of the accuracy of the
sine function and EF(Re) methods simulated using multi-time values was
evaluated using NSE and R2 (Fig. 4). The NSE of the sine
function (134/148) and EF(Re) (133/148) methods was higher than 0.60 at 90 % of sites worldwide. There were 86 and 90 sites that had an NSE
exceeding 0.80 for the sine function and EF(Re) methods, respectively. In
terms of the correlation evaluation criterion, R2, the number of sites
in which the R2 exceeded 0.80 was 117 and 121 for the two methods,
respectively.
Spatial distribution of NSE and R2
simulated by the sine function and EF(Re) methods with multi-time values (a
and b are simulated by the sine function method, and c and d are simulated by the EF(Re) method; a and c are NSE, and b and d are R2).
Notably, in tropical rainforest (e.g., BR-Sa3, GH-Ank, ID-Pag) and tropical monsoon (PH-RiF) climatic conditions, the two methods demonstrated a poor
ability to simulate daily LE. This was particularly the case for tropical
rainforest climate regions, where the NSE is even lower than 0; this may be
due to irregular changes in the LE in these regions. For example, there is
little seasonal variation in LE in tropical rainforest climate regions, and
the fluctuation of daily LE data series is relatively small. This results in
poor agreement between simulated daily LE and measured values (Fig. 5). However, the SD-Dem site, also located near the Equator, was characterized by seasonal variation in LE due to the tropical grassland
climate in this region. As such, the simulated daily LE at this site
demonstrated greater consistency with measured values. Although the
performance of upscaling methods was poor in agreement with the daily LE
data, there was an apparent correlation between simulated daily LE and the
measured data. For example, the R2 was higher than 0.30 and 0.40 at the
GH-Ank and ID-Pag sites, respectively, while it was greater than 0.50 at the
PH-RiF site.
Observed and simulated daily LE series by the sine
function upscaling method in tropical regions. Note: (a) to (c) represent BR-Sa3, GH-Ank, and ID-Pag, located in the tropical rainforest climate region; (d) and (e) are the sites of PH-RiF and SD-Dem, located in the tropical monsoon
and grassland climate regions, respectively.
The spatial distribution of the accuracy of the sine function and EF(Re)
methods simulated by multi-time values was also evaluated using the RE and
RMSE criteria (Fig. 6). The simulated RE at all sites ranged from
-33.7 % to 24.2 %, while the RMSE was lower than 40.4 W m-2. Most sites tended to underestimate daily LE using the two
upscaling methods; this underestimation was generally less than 20 %. In
East Asia, central Australia, northeastern Africa, central and northwestern
North America, and southern South America, the upscaling methods
underestimated daily LE by 10 %–20 %. In the Gulf of Guinea in Africa and the northeastern region of South America, both methods generally
overestimated daily LE by less than 10 %. Both methods tended to
underestimate the daily LE in the remaining regions by less than 10 %. The
simulated RMSE of the upscaling methods exceeded 30 W m-2 at
three tropical rainforest sites and a site in southeastern Australia. The remaining sites had RMSE values below 30 W m-2. There were
89 % (132/148) of sites with a simulated RMSE lower than 20 W m-2.
Spatial distribution of RE and RMSE simulated by the sine
function and EF(Re) methods using multi-time values (a and b are simulated
by the sine function method, and c and d are simulated by the EF(Re) method; a and c are RMSE, and b and d are RE).
Accuracy of upscaling schemes in simulating daily LE under all sky conditions
In this study, the simulation accuracy of upscaling methods under a
different daily atmospheric transmissivity coefficient (τ) was
evaluated using observed data from sites with a daily time series length
greater than 1000. First, all data from these sites were constructed into a
data series. Then, the accuracy of daily LE simulations using the sine
function and EF(PET) upscaling methods under differing daily atmospheric
transmissivity coefficients was evaluated; the results are presented in
Fig. 7. In general, the simulation accuracy was positively
correlated with the daily atmospheric transmissivity coefficient,
particularly when τ<0.6. The overall RE, RMSE, NSE, and
R2 were 6.0 % and 9.1 %, 14.3 and 11.8 W m-2, 0.81
and 0.86, and 0.83 and 0.88 for the sine function method using the single-time value at 10:30 and the EF(PET) method using the multi-time values at
13:30, respectively. The simulation accuracy under sky conditions where
τ<0.6 was significantly lower than the overall accuracy. For example, when τ<0.2, the two methods underestimated the daily
LE by 36.7 % and 25.0 %, respectively. Although the simulation accuracy
was not as high as that under large atmospheric transmissivity, the
simulated NSE exceeded 0.50, even when τ<0.2. When
0.4 <τ<0.5, the simulated NSE had improved to exceed
0.70, and the corresponding R2 was greater than 0.75. This indicates
that remote sensing ET upscaling methods can achieve satisfactory simulation
accuracy even when 0.4 <τ< 0.5. The simulation
accuracy of the two methods was relatively stable when τ>0.6, particularly for the EF(PET) method of multi-time values at 13:30 in
which the corresponding NSE stabilized around 0.85, and R2 was stable
around 0.87. The RE also became relatively stable when τ>0.6; this is consistent with the R2 results, as shown in Fig. 8a and b. This indicates that the daily LE simulated by the
sine function and EF(PET) upscaling methods was closer to the measured
values, and the simulation accuracy of these methods was high and more
reliable when τ>0.6. The accuracy evaluation results of
the other upscaling methods were similar (not shown).
Overall evaluation of the sine function method simulated
by a single-time value of 10:30 and the EF(PET) method simulated by multi-time values at 13:30 for all sites under differing daily atmospheric
transmissivity coefficients.
Overall, under sky conditions where τ> 0.6, the upscaling
schemes could simulate the daily LE series with high accuracy. However, when
τ<0.6, this simulation accuracy was significantly affected by
sky conditions and was generally positively correlated with the daily atmospheric transmissivity coefficient. Although not as accurate as when
atmospheric transmissivity is high (τ> 0.6), the upscaling
schemes still demonstrated an ability to accurately simulate daily LE even
when atmospheric transmissivity was relatively low (i.e., 0.4 <τ< 0.5).
In addition to the overall evaluation of the data series constructed across
all sites, the performances of different sites under all sky conditions were evaluated. Figure 8a and b show the R2 when using
the sine function and EF(PET) methods under differing daily atmospheric
transmissivity coefficients. In general, the R2 of the upscaling
methods increased with the atmospheric transmissivity coefficient; when
τ> 0.6, the simulation accuracy was stable. Based on the
evaluation of the EF(PET) method with multi-time values at 13:30, when τ<0.3, the R2 of each site was mainly between 0.4 and 0.7.
When τ increased to between 0.3 and 0.4, the R2 had increased to
between 0.6 and 0.8 at most sites. When τ> 0.6, the R2
of each site generally increased to greater than 0.8. Figure 8c
and d show the simulated daily LE against observed values under
different levels of daily atmospheric transmissivity; the simulation
systematically underestimates the simulations with observed values under low levels of τ (i.e., τ< 0.2). For the instant simulation
in upscaling schemes, it is important to note that simulations may be
upscaled from instantaneous variables when the sky is cloudy, while the
remainder of the daytime is clear. However, the opposite may also occur in
which the sky conditions are clear during instant simulation and cloudy at
other times.
Box plots showing the R2 of the (a) sine function method simulated by a single time at 10:30 and the (b) EF(PET) method simulated by multi-time values at 13:30 for each site. Scatter plots between observed and simulated daily LE by the (c) sine
function and (d) EF(PET) methods under differing daily atmospheric transmissivity coefficients.
RE and NSE of simulated daily LE using the sine function
and EF(Re) methods at different times of the day.
Accuracy of upscaling schemes in simulating daily LE from different times of day
Remote sensing ET upscaling was conducted based on the monitoring value of
the satellite overpass time; the overpass times of different satellites may
vary in different regions. Therefore, the simulation accuracy of the
temporal upscaling methods was also evaluated at different times of the day.
Figure 9 presents the RE and NSE of the simulations using the sine
function and EF(Re) methods at different times of the day; these two methods
required minimal input data. In general, the simulation accuracy of the sine
function method had initially increased and then decreased during the
daytime. Before 09:00, the mean RE for all sites increased linearly from
-65.8 % to -14.9 %, and the RE varied significantly at each site. For
example, the RE at each site ranged from -80 % to 30 % when the
simulation was upscaled at 08:00. From 09:00 to 16:30, the mean RE for all
sites was also increasing, although the magnitude of this increase was
significantly reduced, increasing from -14.9 % to 13.0 %. During this
period, the performance of the upscaling method was relatively stable,
particularly during 11:00–14:00, where the RE at each site was mainly
distributed from -20 % to 20 %. However, from 17:00, the mean RE showed
a sharper decrease, and the performance of the upscaling method became
extremely unstable at each site; this meant the RE varied significantly at
each site. The daily LE was overestimated by more than 90 % at some sites,
while it was also underestimated by more than -90 % at other sites. With
respect to the EF(Re) method, the RE generally showed an increasing trend
during the daytime. There were three distinct stages. (1) From 06:00 to 09:00, the mean RE at all sites increases linearly from -57.6 % to -20.5 %. (2) From 09:00 to 14:30, the mean RE was also increasing, although at a lower
rate from -20.5 % to 10.5 %. (3) After 15:00, the mean RE exhibits a sharp linear increase from 17.5 % to 60.3 % at 17:00 and then always
exceeds 60 % thereafter.
According to the NSE evaluation criterion, the accuracy of the sine function
and EF(Re) upscaling methods in simulating daily LE data series also showed
significant variability at different times of the day. The intra-day
variation of NSE based on two methods at different times of the day may also be divided into three distinct stages: (1) a general linear increase
before 10:00, (2) a period of relative stability from 10:00 to 13:30, and (3) a general linear decrease after 14:00. During (1), the mean NSE of all sites increased from below -0.60 to 0.60 and 0.61 for the sine function and
EF(Re) methods, respectively, while the median NSE increased from below
-0.80 to 0.73 for both methods. The two methods showed the highest simulation accuracy for 10:00–13:30. In each single time point, the mean
and median NSEs of all sites based on the sine function method were 0.65 and
0.74, while the corresponding values using the EF(Re) method were 0.66 and
0.76, respectively. In addition, most sites had an NSE higher than 0.5 at single time points of 09:00, 09:30, 14:00, and 14:30. This indicates that the two methods also produce a certain accuracy in simulating daily LE at a
single time point, as the mean NSE of all sites was approximately 0.50 and the corresponding median exceeded 0.60. However, the simulation accuracy of
the two methods was relatively poor in the remaining periods, with a mean
and median NSE lower than 0.50, particularly before 08:00 and after 16:00; in these periods, the NSE for most sites was lower than 0.20 or even lower than 0. In other words, the two methods lose the ability to upscale
instantaneous LE to daily data series during these periods.
Overall, the accuracy of the sine function and EF(Re) upscaling methods in
simulating daily LE exhibits significant variability during the daytime. The simulation accuracy of both methods was relatively high from 09:00 to 15:00,
with the mean REs at all sites within ±20 % and the mean and median NSEs being higher than 0.50 and 0.60, respectively. In particular, from 11:00 to 13:30, the simulation accuracy of the two methods was relatively
high and stable at each site. The RE of each site was within ± 20 %,
and the mean and median NSEs were 0.65 and 0.74, respectively. However, the
two methods lose the ability to accurately simulate daily LE data during
other times of the day, exhibiting poor simulation accuracy. Evaluation of
the simulation accuracy for the other upscaling methods (not shown) at
different times of the day was generally consistent with those of the sine function and EF(Re) methods, supporting the conclusions of this study.
Variability of simulation accuracy among different upscaling schemes and
sites
Based on data from 122 sites from FLUXNET2015, the standard deviation of the
NSE was used to evaluate the variability of simulation accuracy among the
different upscaling schemes and sites (Fig. 10). For remote sensing
ET upscaling, the variability of simulation accuracy among different
upscaling schemes is typically lower than the variability among different
sites. At the same site, the mean standard deviation of data series composed
of NSE by each upscaling scheme (the length of each series is 28, equal to
the number of upscaling schemes) was 0.096. The standard deviation of the
NSE by each scheme was lower than 0.20 at most sites (119/122). There were
63 % of sites (77/122) with a standard deviation of less than 0.10; these were the results for all upscaling schemes examined in this study. For the
seven methods within each type of upscaling scheme (e.g., S10:30, S13:30,
M10:30, or M13:30 shown in Fig. 10a), the variability of
simulation accuracy among different methods was even lower, whereby the
standard deviation of NSE in each scheme was less than 0.10, at more than
75 % of sites. For the simulation of multi-time values at 10:30, the
standard deviation of NSE among the sine and Gaussian functions and EF(Rn), EF(Rn-G), EF(Rs), EF(Re), and EF(PET) methods averaged only 0.052, and the
number of sites with a standard deviation of less than 0.10 was up to 112 (92 %). This indicates that the variability of simulation accuracy among
different upscaling schemes was relatively small for upscaling instantaneous
remote sensing ET to daily values. In addition, the variability of the
simulation accuracy when using multi-time values was lower than that using
the single-time value.
Standard deviation of NSE based on (a) different methods and (b) different sites. Note: in the left-hand-side figure, S and M represent different methods by the single- and multi-time values, respectively; “All” contains all upscaling schemes used in this study. In the right-hand-side figure, the site-to-site standard deviations of simulated NSE for every upscaling scheme
are shown as columns of a box plot. The five columns represent all sites where the average NSE for all upscaling schemes is higher than 0.5, 0.6, 0.7, and 0.8, and the corresponding numbers of sites are 122, 118, 111, 97, and 67,
respectively.
The variability of simulation accuracy among different sites was evaluated
through the site-to-site standard deviation of NSE, as shown in Fig. 10b. In each upscaling scheme, the site-to-site standard deviation of data
series composed of NSE for every site (where the length of each series is
122, equal to the number of sites) ranged from 0.21 to 0.28, while the mean and median NSE of all upscaling schemes was 0.25. In each case, the
variability of simulation accuracy among different sites was greater than
that among upscaling schemes as the site-to-site standard deviation was
always larger than the standard deviation among upscaling schemes. This
higher site-to-site standard deviation is mainly due to the extremely low
NSEs at several individual sites (as shown in Fig. 4). The
site-to-site standard deviation significantly reduces if we exclude the four sites with an NSE lower than 0.5. For example, when only considering
sites with an NSE greater than 0.5 (118), the site-to-site standard
deviation is mainly distributed between 0.10 and 0.15, with mean and median
values of 0.12. The site-to-site standard deviation falls below 0.09 in each
upscaling scheme, when only 66 sites with an NSE greater than 0.8 were used to calculate the site-to-site standard deviation. The corresponding mean and
median values of the standard deviation were 0.06. Overall, the variability of simulation accuracy among different sites was mainly affected by a limited
number of sites with an extremely low NSE. Indeed, the large variations in simulation accuracy among different upscaling schemes with a standard
deviation of NSE exceeding 0.20 (Fig. 10a) occurred at these four sites.
Discussion
In the temporal upscaling of instantaneous remote sensing ET to daily
values, the current methods focus only on daytime ET. In other words, the
upscaling methods only result in an ET during the daytime and do not include nocturnal ET. As for the difference in upscaled daytime LE and daily
values, typically, a correction coefficient corrects this deviation. For example, Gentine et al. (2007) introduced a constant correction factor of
1.1 into the EF upscaling method; this reduced systematic underestimation and improved the performance of the method in terms of accuracy and bias for
daily ET estimates (Ryu et al., 2012; Van Niel et al., 2012). In addition,
time-dependent correction factors may further improve EF performance (Van
Niel et al., 2011); this was also validated by the results of this study.
The observation of LE at 148 global sites from FLUXNET shows that the
percentage of nocturnal LE to daytime LE ranges from -2.8 % to 19.6 %,
with an average of 7.8 %. The correction coefficient was calculated
according to the half-hourly observed LE data series at each site; this
coefficient is equal to 1 plus the ratio of nocturnal LE to daytime LE. The
results show that the simulation accuracy with the correction coefficient
was slightly higher than that without the correction coefficient. As such,
when LE observation data become available, the correction coefficient should be used to correct the simulation of daily LE in the remote sensing
ET upscaling schemes. However, hourly LE observation data are seldom
available in the actual application of remote sensing ET upscaling; as such,
it is necessary to consider the simulation accuracy of upscaling schemes
without hourly LE data support. Therefore, the evaluation results presented
in this study were simulations without any correction coefficients. In
addition, note that even in the absence of LE observational data, a
correction coefficient of 1.08 on the average global sites may be used to
correct daily LE simulated by these upscaling methods.
The evaluation results show that the simulation accuracy of these different
methods varied based on the evaluation index used. The comprehensive
evaluation results show that the simulation accuracy using the EF(PET)
method was the best among all seven upscaling methods. Previous studies
often used reference evapotranspiration as PET in EF(PET) upscaling schemes
(Trezza, 2002; Colaizzi et al., 2006; Allen et al., 2007; Cammalleri et al.,
2014). The reference crop is defined as a hypothetical crop with an assumed
height of 0.12 m, a surface resistance of 70 s m-1 and an albedo of 0.23 (Allen et al., 1998). However, PET is related to
differences in the aerodynamic properties between the reference surface and
the actual landscape around the flux measurement site. In this study, PET
was calculated by considering the parameters of the (bulk) surface and the
aerodynamic resistance for water vapor flow based on the actual vegetation
conditions at each observation site. This is more consistent with the actual
situation at each site than the reference ET. However, the greatest
disadvantage of this method is that it requires the input of multiple
observational datasets, such as air temperature, humidity, wind speed,
atmospheric pressure, crop height, and observation height. It should be
noted that FLUXNET includes both raw and corrected LE data. There was little difference between the evaluation results of the corrected data and
those of the raw data.
The sine function and EF(Re) methods may be more suitable for regional
remote sensing applications due to their relatively simpler inputs and
comparable or higher accuracies when compared to other methods. This is
consistent with the conclusions of other studies (Zhang and Lemeur, 1995;
Liu and Hiyama, 2007; Van Niel et al., 2012; Ryu et al., 2012). Compared
with the sine function, the intra-day distribution of LE was more consistent
with the Gaussian function. However, in terms of the overall performance of
upscaling methods, the simulation accuracy of the Gaussian function for
daily LE did not show significant improvement. This may be mainly caused by
the complementary effect between the underestimation of the sine function
method around 12:00 and the overestimation of the method in the morning and
afternoon. This results in an upscaled LE in the daytime by the sine
function, which is similar to that of the Gaussian function.
The upscaling variable originally used by the EF method was Rn-G (Sugita and
Brutsaert, 1991); in general, G is negligible in the daily energy balance
(Price, 1982; Li et al., 2009; Cui et al., 2020). However, for the
application of the EF method to upscale instantaneous ET to a daily scale,
the instantaneous value of G is required. As Rn is also recommended in the
EF upscaling method (Brutsaert and Sugita, 1992), the EF(Rn) method has been
validated at several sites. For example, Van Niel et al. (2012) showed that
EF(Rn) underestimated monthly ET by -16 % at two sites in Australia; the
magnitude of underestimation was lower than that simulated by the EF(Rn-G)
method (-34 %). In this study, the performance of the EF(Rn) and EF(Rn-G) methods in upscaling LE at 148 global sites with a long data series
(including seasonal variations) was compared. The results showed that there
was little difference between the simulation accuracies of the EF(Rn) and EF(Rn-G) methods; this may be good news for remote sensing ET applications.
Compared to Rn, G is very difficult to detect using remote sensing (Kalma et
al., 2008; Li et al., 2009), as it is usually calculated from the empirical
relationship between Rn and land surface parameters (Bastiaanssen et al.,
1998; Su et al., 2002; Li et al., 2019). In addition, due to the combined
errors in Rn and G, the available energy (Rn-G) error estimated by remote
sensing methods can reach ±10 %–20 % (Bisht et al., 2005; Kalma et
al., 2008). However, if LE is only upscaled for the winter, ignoring the
effect of G may produce large errors in the simulation (Cammalleri et al.,
2014).
In any upscaling scheme, the simulation accuracy of multi-time values is
clearly higher than that of a single-time value, which may be due to better stability in the Vd/Vi ratio (Eq. 8) offered by the former
than the latter. It is recommended that multi-time values are used in remote
sensing ET upscaling when multi-time data are available. For example, if
meteorological observations are selected as upscaling variables, an
upscaling scheme based on multi-time values is recommended for simulations.
The spatial distribution of the simulation accuracy of each upscaling scheme
showed that most sites could accurately upscale instantaneous LE to daily
values. However, sites located in tropical rainforests and tropical monsoon
regions performed poorly in accurately simulating daily LE, with an NSE
lower than 0.20. This is consistent with the results reported by Ryu et al. (2012), who assumed that the poor performance in tropical rainforest regions was mainly due to irregular cloudiness. In addition, remote sensing
products only sense the top of the canopy and thus ignore the energy storage
within the canopy. Especially for forest this can be significant (Jiménez-Rodríguez et al., 2020; Coenders-Gerrits et al., 2020).
This partially explains the poor performance in tropical rainforest regions. The performance of the upscaling schemes under all sky conditions
was evaluated using various daily atmospheric transmissivities. High
atmospheric transmissivity represents a clear-sky condition with little cloudiness. However, the simulation accuracy of these tropical rainforests
and tropical monsoon regions under conditions of high atmospheric
transmissivities was also low. There was little seasonal variation in LE in
tropical rainforest climate regions, and the fluctuation range of daily LE
data was relatively small. This may be one of the causes of the poor simulation accuracy of daily LE in these regions. Although the performance
of upscaling schemes was in poor agreement with the daily LE series, it
indeed showed a rough correlation between the simulated and measured daily
LEs at these tropical rainforest and tropical monsoon sites.
Delogu et al. (2012) used four European flux sites to evaluate the
performance of the EF(Rn-G) and EF(PET) methods at different times from
10:00 to 14:00, finding that the simulation accuracy at 11:00–13:00 was
slightly higher than outside of this time range. Based on the 126 FLUXNET
sites from 1999 to 2006, Wandera et al. (2017) evaluated the EF(Rs) method
at different times between 10:30 and 14:00 and found that there was only slight variance in the accuracy of daily LE simulations during this period.
However, the performance of upscaling methods during other daytime periods
has seldom been investigated. In this study, the performance of seven
upscaling methods at different times during the day (06:00–19:00) was
evaluated; the simulation accuracy of upscaling methods was observed to vary
significantly during the day. The upscaling methods were only able to
simulate daily LE with relatively high accuracy between 09:00 and 15:00. All the methods lost their ability to accurately simulate daily LE outside of these
hours. The upscaling methods exhibited the highest simulation accuracy from
11:00 to 14:00. This is consistent with previous results (Delogu et al., 2012; Wandera et al., 2017). Overall, in upscaling instantaneous ET to daily
values in remote sensing applications, instantaneous values between 11:00
and 14:00 are recommended for simulations. However, if the simulation is
upscaled from a time outside of 09:00–15:00, simulation accuracy cannot be
guaranteed.
The performance of remote sensing ET upscaling schemes may vary
significantly under different sky conditions. Wandera et al. (2017) analyzed
the performance of the EF(Rs) method for four different classes of daily
atmospheric transmissivity, including 0.25 ≥τ≥ 0, 0.50 ≥τ≥ 0.25, 0.75 ≥τ≥ 0.50, and 1 ≥τ≥ 0.75, where the first class represented a high degree of cloudiness and the fourth class represented clear skies. They found a relatively better
simulation accuracy for the atmospheric transmissivity class above 0.75. In
this study, a more refined classification of daily atmospheric
transmissivity and a greater number of upscaling methods were evaluated. The
results showed that the upscaling methods can simulate daily LE series with
high accuracy at τ> 0.6. When τ<0.6, the
simulation accuracy of each upscaling method was significantly affected by
sky conditions; accuracy was observed to be generally positively related to
daily atmospheric transmissivity. However, it was also found that the
upscaling methods could accurately simulate daily LE series even when the
atmospheric transmissivity was relatively low (i.e., 0.4 <τ<0.5).
Remote sensing-derived ET includes many other uncertainties, such as the
uncertainty in the ET model and remote sensing data, which are indirectly
related to the upscaling scheme. Although this study evaluated the accuracy
of upscaling schemes in terms of simulating daily LE, in the application of
remote sensing retrieval of ET, the uncertainties of remote sensing data and
the ET retrieval model need to be considered.
Conclusion
The accuracy of seven upscaling methods in simulating daily LE from
instantaneous values was evaluated using observations from 148 flux sites
under all sky conditions and at different times during the day. The
simulation accuracies of different methods varied based on the evaluation
index that was used. All the methods could accurately simulate daily LE from instantaneous values, whereby the mean and median NSEs were 0.80 and 0.85
and the corresponding R2 was 0.87 and 0.90, respectively. The sine and
Gaussian function methods showed relatively higher accuracy in simulations
of mean values, with REs generally within ±10 %. The EF(PET) and
EF(Rs) methods showed relatively better performance in simulating daily
series, where the mean and median NSEs at each site were 0.83 and 0.89,
respectively. This comprehensive evaluation demonstrates that the EF(PET)
method generally had the highest accuracy. However, the sine function and
EF(Re) methods may be more suitable for remote sensing upscaling applications due to their relatively minimal data requirements and
comparable or higher accuracy. The intra-day distribution of the LE was more
consistent with the Gaussian function than the sine function; however, the
accuracy of the former method in simulating daily LE did not improve
significantly compared with latter. This may be due to the complementary
effect between the underestimation of the sine function method around 12:00
and the overestimation of the method in the morning and afternoon. The
simulation accuracy showed little difference using the same type of method, for example, the type of mathematical function method or EF method. In any
upscaling scheme, the accuracy of simulation from multi-time values was
significantly higher than that from a single-time value. Therefore, multi-time values should be used in ET upscaling when multi-time data are
available. The upscaling methods show the ability to accurately simulate
daily LE from instantaneous values from 09:00 to 15:00, particularly for instantaneous values between 11:00 and 14:00. However, the performance of
upscaling methods was poor outside of this time range. The upscaling methods
could simulate daily LE with a high accuracy at τ> 0.6; when τ<0.6, the simulation accuracy was significantly affected by
sky conditions, being generally positively related to daily atmospheric
transmissivity. The spatial distribution of simulation accuracy shows that
every upscaling scheme has the ability to accurately simulate daily LE from
instantaneous values at most sites; however, this ability is lost at
tropical rainforest and tropical monsoon sites.
Data availability
The FLUXNET and TERN OzFlux datasets can be
downloaded freely from https://fluxnet.org/data/download-data/ (FLUXNET, 2021) and http://www.ozflux.org.au/ (Australian Terrestrial Ecosystem Research Network, 2021), respectively.
The supplement related to this article is available online at: https://doi.org/10.5194/hess-25-4417-2021-supplement.
Competing interests
The author declares that there is no conflict of interest.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We are grateful to FLUXNET (https://fluxnet.org/, last access: 3 August 2021) and the OzFlux (http://www.ozflux.org.au/, last access: 3 August 2021) for providing us with FLUXNET and TERN OzFlux datasets. We would also like to thank Dario Papale, Gilberto Pastorello, and Jason Beringer for their kind assistance with data access.
Financial support
This research has been supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (grant nos. XDA23090302 and XDA2006020202) and the National Natural Science Foundation of China (grant nos. 41571027 and 41661144030).
Review statement
This paper was edited by Niko Wanders and reviewed by Miriam Coenders-Gerrits and one anonymous referee.
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