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Evaluation of Global Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) Products at 500 m Spatial Resolution

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The fraction of absorbed photosynthetically active radiation (FAPAR) is a key biophysical variable directly associated with the photosynthetic activity of plants. Several global FAPAR products with different spatial resolutions have been generated from remote sensing data, and much work has focused on validating them. However, those studies have primarily evaluated global FAPAR products at a spatial resolution of 1 km or more, whereas few studies have evaluated the global 500 m resolution FAPAR product distributed in recent years. Furthermore, there are a few FAPAR products, including black-sky, white-sky and blue-sky FAPAR datasets, and almost no studies have evaluated these products. In this article, three global FAPAR products at 500 m resolution, namely MODIS (only black-sky FAPAR), MUSES and EBR (black-sky, white-sky and blue-sky FAPAR) were compared to evaluate their temporal and spatial discrepancies and direct validation was conducted to compare these FAPAR products with the FAPAR values derived from the high-resolution reference maps from the Validation of Land European Remote Sensing Instrument (VALERI) and Implementing Multi-Scale Agricultural Indicators Exploiting Sentinels (IMAGINES) projects. The results showed that the MUSES FAPAR product exhibited the best spatial integrity, whereas the MODIS and EBR FAPAR products had many missing pixels in the equatorial rainforest regions and at high latitudes in the Northern Hemisphere. The MODIS, MUSES and EBR FAPAR products were generally consistent in their spatial patterns. However, a relatively large discrepancy among these FAPAR products was present in the equatorial rainforest regions and the middle and high latitude regions where the main vegetation type was forest. The differences between the black-sky and white-sky FAPAR datasets at the global scale were significant. In January, the MUSES and EBR black-sky FAPAR values were larger than their white-sky FAPAR values in the region north of 30° north latitude but they were smaller than their white-sky FAPAR values in the region south of 30° north latitude. In July, the MUSES and EBR black-sky FAPAR values were lower than their white-sky FAPAR values in the region north of 30° south latitude and they were larger than their white-sky FAPAR values in the region south of 30° south latitude. The temporal profiles of the MUSES FAPAR product were continuous and smooth, whereas those of the MODIS and EBR FAPAR products showed many fluctuations, particularly during the growing seasons. Direct validation indicated that the MUSES FAPAR product had the best accuracy (R2 = 0.6932, RMSE = 0.1495) compared to the MODIS FAPAR product (R2 = 0.6202, RMSE = 0.1710) and the EBR FAPAR product (R2 = 0.5746, RMSE = 0.1912).
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Citation: Zheng, Y.; Xiao, Z.; Li, J.;
Yang, H.; Song, J. Evaluation of
Global Fraction of Absorbed
Photosynthetically Active Radiation
(FAPAR) Products at 500 m Spatial
Resolution. Remote Sens. 2022,14,
3304. https://doi.org/10.3390/
rs14143304
Academic Editor: Jose Moreno
Received: 11 May 2022
Accepted: 6 July 2022
Published: 8 July 2022
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4.0/).
remote sensing
Article
Evaluation of Global Fraction of Absorbed Photosynthetically
Active Radiation (FAPAR) Products at 500 m Spatial Resolution
Yajie Zheng , Zhiqiang Xiao * , Juan Li, Hua Yang and Jinling Song
State Key Laboratory of Remote Sensing Science, Faculty of Geographical Science, Beijing Normal University,
Beijing 100875, China; 202021051060@mail.bnu.edu.cn (Y.Z.); 201921051077@mail.bnu.edu.cn (J.L.);
yh_crs@bnu.edu.cn (H.Y.); songjl@bnu.edu.cn (J.S.)
*Correspondence: zhqxiao@bnu.edu.cn
Abstract:
The fraction of absorbed photosynthetically active radiation (FAPAR) is a key biophysical
variable directly associated with the photosynthetic activity of plants. Several global FAPAR products
with different spatial resolutions have been generated from remote sensing data, and much work
has focused on validating them. However, those studies have primarily evaluated global FAPAR
products at a spatial resolution of 1 km or more, whereas few studies have evaluated the global
500 m resolution FAPAR product distributed in recent years. Furthermore, there are a few FAPAR
products, including black-sky, white-sky and blue-sky FAPAR datasets, and almost no studies have
evaluated these products. In this article, three global FAPAR products at 500 m resolution, namely
MODIS (only black-sky FAPAR), MUSES and EBR (black-sky, white-sky and blue-sky FAPAR) were
compared to evaluate their temporal and spatial discrepancies and direct validation was conducted
to compare these FAPAR products with the FAPAR values derived from the high-resolution reference
maps from the Validation of Land European Remote Sensing Instrument (VALERI) and Implementing
Multi-Scale Agricultural Indicators Exploiting Sentinels (IMAGINES) projects. The results showed
that the MUSES FAPAR product exhibited the best spatial integrity, whereas the MODIS and EBR
FAPAR products had many missing pixels in the equatorial rainforest regions and at high latitudes in
the Northern Hemisphere. The MODIS, MUSES and EBR FAPAR products were generally consistent
in their spatial patterns. However, a relatively large discrepancy among these FAPAR products was
present in the equatorial rainforest regions and the middle and high latitude regions where the main
vegetation type was forest. The differences between the black-sky and white-sky FAPAR datasets at
the global scale were significant. In January, the MUSES and EBR black-sky FAPAR values were larger
than their white-sky FAPAR values in the region north of 30
north latitude but they were smaller
than their white-sky FAPAR values in the region south of 30
north latitude. In July, the MUSES and
EBR black-sky FAPAR values were lower than their white-sky FAPAR values in the region north of
30
south latitude and they were larger than their white-sky FAPAR values in the region south of 30
south latitude. The temporal profiles of the MUSES FAPAR product were continuous and smooth,
whereas those of the MODIS and EBR FAPAR products showed many fluctuations, particularly
during the growing seasons. Direct validation indicated that the MUSES FAPAR product had the
best accuracy (R
2
= 0.6932, RMSE = 0.1495) compared to the MODIS FAPAR product (R
2
= 0.6202,
RMSE = 0.1710) and the EBR FAPAR product (R2= 0.5746, RMSE = 0.1912).
Keywords: FAPAR; evaluation; MODIS; MUSES; EBR
1. Introduction
The fraction of absorbed photosynthetically active radiation (FAPAR) is defined as
the fraction of solar radiation absorbed by living vegetation in the 400–700 nm spectral
range [
1
]. Depending on the relative contributions of direct and diffuse irradiances, FAPAR
estimates may be related to just direct solar radiation (black-sky FAPAR), diffuse radiation
(white-sky FAPAR), or it may include both direct and diffuse radiation (blue-sky FAPAR).
Remote Sens. 2022,14, 3304. https://doi.org/10.3390/rs14143304 https://www.mdpi.com/journal/remotesensing
Remote Sens. 2022,14, 3304 2 of 19
FAPAR is a key biophysical variable that is directly related to the productivity of living
vegetation. It is considered one of the Essential Climate Variables (ECVs), playing a key role
in the energy balance of ecosystems [
2
]. Additionally, FAPAR can be used as a critical input
variable in many ecological and climate models [
3
6
] or as an additional constraint during
assimilation [
7
]. Furthermore, the long time series of FAPAR products can be applied to
monitor vegetation state, to detect drought events [8], in phenology [9,10] and so on.
The values of FAPAR can be derived from ground measurement and remote sensing
data. The FAPAR values derived from ground measurements have some limitations, such
as their short time scale and small spatial coverage. Remote sensing is the only feasible way
to estimate the FAPAR values on a large scale over long periods of time. Many algorithms
have been developed to retrieve FAPAR values from remote sensing data, and several global
FAPAR products have been generated from remote sensing data acquired by the Moderate
Resolution Imaging Spectroradiometer (MODIS) [
11
], the Medium Resolution Imaging
Spectrometer (MERIS) [
12
], the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) [
13
], the
SPOT/VEGETATION [
14
,
15
], the Multi-angle Imaging SpectroRadiometer (MISR) [
16
] and
the Earth Polychromatic Imaging Camera (EPIC) [
17
,
18
]. The spatial resolutions of these
products are approximately 1 km or above. However, many applications, including crop
yield estimations and ecological environment monitoring, require high-quality FAPAR
products with higher spatial resolution. Therefore, new versions of these products have
been produced in recent years, and their spatial resolution has been improved, such as
the most recent (Collection 6) MODIS FAPAR product and the MUltiscale Satellite remotE
Sensing (MUSES) FAPAR product. The MODIS and MUSES FAPAR products have achieved
a spatial resolution of 500 m. Other research groups have published new global FAPAR
products, such as the energy balance residual method-based (EBR) FAPAR product [
19
].
The EBR FAPAR product has also achieved a spatial resolution of 500 m. Furthermore,
the MUSES and EBR FAPAR products provide black-sky, white-sky and blue-sky FAPAR
datasets simultaneously.
Many studies have analyzed the discrepancies among the existing FAPAR products
and evaluated their accuracy [
20
23
]. Xiao et al. [
20
] reported that the global land surface
satellite (GLASS), MODIS, Geoland2/BioPar version 1 (GEOV1) and SeaWiFS FAPAR
products exhibited similar spatial distribution patterns, but some discrepancies existed in
equatorial forest regions and areas around 50–60
N latitude. Meanwhile, Xiao et al. [
20
]
also reported that GLASS FAPAR values were more accurate than other products when
compared with the FAPAR values derived from the ground measurements of the Validation
of Land European Remote sensing Instrument (VALERI) project (http://w3.avignon.inra.fr/
valeri/; accessed on 10 May 2022). A comparison among GEOV1, MODIS and CYCLOPES
FAPAR products at 1 km spatial resolution was conducted by Camacho et al. [
21
] and it was
demonstrated that the GEOV1 FAPAR product exhibited reasonable spatial distribution and
good seasonality profiles, and it showed good performance for bare areas and dense forests.
Tao et al. [
22
] compared five global FAPAR products: MODIS, MISR, MERIS, SeaWiFS and
GEOV1. Their results showed that MODIS, MISR and GEOV1 were in great agreement
with each other, as well as MERIS and SeaWiFS, but the difference between the two groups
could be up to 0.1.
The studies described above focused on the evaluation of global FAPAR products. Other
evaluations focused on FAPAR products in local areas [
24
28
]. For example, D’Odorico
et al. [
24
] focused on comparisons of the JRC-TIP (Joint Research Centre Two-stream Inver-
sion Package) FAPAR product derived from the MODIS [
29
], the European Space Agency
(ESA) JRC FAPAR product obtained using the MEdium Resolution Imaging Spectrometer
(MERIS) Global Vegetation Index (MGVI) [
12
] and the MODIS FAPAR product [
30
] over Eu-
rope, and they demonstrated that these FAPAR products had consistent spatial distributions
overall but there were large differences in magnitude (as large as 0.1). Martínez et al. [
26
]
assessed four FAPAR products derived from MODIS, SEVIRI and MERIS (TOAVEG and
MGVI algorithms) over the Iberian Peninsula and found that the differences among these
FAPAR products over this area were mainly in terms of temporal variations and absolute
Remote Sens. 2022,14, 3304 3 of 19
values. Additionally, some studies evaluated FAPAR products for different vegetation
types [
31
,
32
]. Serbin et al. [
32
] evaluated the performance of the MODIS FAPAR product
across forests with different ages in northern Manitoba, Canada and found that the MODIS
FAPAR product overestimated FAPAR values for the youngest forests but underestimated
FAPAR values for the oldest forests.
Existing studies have focused on evaluating FAPAR products at spatial resolutions
of 1 km or more, whereas few studies have evaluated the latest global FAPAR products
at a spatial resolution of 500 m. Furthermore, these existing studies evaluated FAPAR
products without distinguishing the black-sky, white-sky and blue-sky FAPAR datasets.
Currently, almost no research has examined the black-sky, white-sky or blue-sky FAPAR
datasets separately and described the differences among them at the global scale to enable
effective application.
In this study, the black-sky, white-sky and blue-sky FAPAR datasets from MUSES and
EBR products were compared to assess their temporal and spatial differences. The black-
sky dataset from the latest MODIS FAPAR product was also examined for comparison.
Furthermore, the three black-sky FAPAR datasets were compared with the FAPAR values
derived from ground measurements.
2. Materials and Methods
2.1. Data
2.1.1. MUSES FAPAR Product
Xiao et al. [
20
] developed a method to estimate blue-sky FAPAR values using the
transmittance of photosynthetically active radiation (PAR) through the entire canopy,
which can be calculated from the corresponding canopy leaf area index (LAI) values and
other information. The method is efficient and was used to generate the GLASS FAPAR
product at the spatial resolution of 0.05
and 1 km from the GLASS LAI product to ensure
physical consistency between LAI and FAPAR retrievals.
Similar to the estimate of the blue-sky FAPAR values, the black-sky and white-sky
FAPAR values can be calculated according to the transmittance of direct and diffuse PAR
through the entire canopy, respectively. Therefore, the method developed by Xiao et al. [
20
]
was refined to generate a new version of the global FAPAR product (denoted by MUSES for
clarification in this study). The MUSES FAPAR product includes three datasets: black-sky
FAPAR, white-sky FAPAR and blue-sky FAPAR. The MUSES FAPAR product has a spatial
resolution of 500 m and a temporal resolution of 8 days. It is provided in a sinusoidal
projection and spans 2000 to 2019. The performance of the MUSES FAPAR product was
evaluated in this study.
2.1.2. EBR FAPAR Product
Liu et al. [
19
] developed an algorithm to generate global black-sky, white-sky and
blue-sky FAPAR products. Firstly, based on a non-linear spectral mixture model (NSM),
the snow-free soil albedo was derived using the surface visible (VIS) albedo (MCD43A3),
LAI (MCD15A2H) and clumping index (CI) [
33
] products. Secondly, the black-sky and
white-sky FAPAR were retrieved based on the energy balance residual (EBR) principle with
the help of data including MODIS surface VIS albedo (MCD43A3), LAI (MCD15A2H) and
CI products, as well as the above snow-free soil albedo data. The EBR FAPAR product is
provided in a sinusoidal projection at a spatial resolution of 500 m and a temporal resolution
of 8 days.
2.1.3. MODIS FAPAR Product
Since the MODIS FAPAR product was produced in 2000, the MODIS science team has
been updating the product. The latest version (Collection 6) of the MODIS FAPAR product
was released to the public in August of 2015 [
34
]. The Collection 6 MODIS FAPAR product
is provided in a sinusoidal projection. It has two datasets at a spatial resolution of 500 m:
MCD15A2H and MCD15A3H. The MCD15A2H FAPAR product has a temporal resolution
Remote Sens. 2022,14, 3304 4 of 19
of 8 days, whereas the MCD15A3H FAPAR product has a temporal resolution of 4 days. In
this study, the MCD15A2H FAPAR product was used for evaluation.
The MODIS FAPAR retrieval algorithm consists of a main algorithm and a backup
algorithm [
30
]. The main algorithm is based on look-up tables simulated through a three-
dimensional radiative transfer model. The backup algorithm estimates the FAPAR values
on the basis of biome-specific FAPAR-NDVI relationships. When the main algorithm fails,
the backup algorithm is used to estimate FAPAR values. The quality of the FAPAR values
derived by the backup algorithm is poor due to residual clouds and poor atmosphere
correction [
35
]. Consequently, only the MODIS FAPAR values retrieved by the main
algorithm were used in the performance evaluation, except the spatial integrity comparison
for the MODIS FAPAR product in our study. Because the inversion algorithm considers
only direct solar radiation, the MODIS FAPAR product corresponds to the instantaneous
black-sky FAPAR at the time of the Terra overpass (10:30 AM).
2.1.4. High-Resolution FAPAR Reference Maps
To validate the biophysical parameter products derived from remote sensing data, such
as LAI and FAPAR products, the VALERI project during 2001–2005 (http://w3.avignon.inra.
fr/valeri; accessed on 10 May 2022) and the Implementing Multi-Scale Agricultural Indica-
tors Exploiting Sentinels (IMAGINES) project during 2013–2016 (http://fp7-imagines.eu/;
accessed on 10 May 2022) conducted field experiments to collect digital hemispherical
photos (DHP) at several sites with different biome types. The DHPs were processed with
CanEye software to derive FAPAR ground measurements. Then, an empirical transfer
function between high spatial resolution reflectance data and FAPAR ground measurements
was constructed to derive high-resolution FAPAR reference maps for each site.
Over the VALERI sites, the FAPAR values correspond to the instantaneous black-sky
FAPAR at 10:00 AM. Over the IMAGINES sites, only part of the sites’ FAPAR corresponded
to the instantaneous black-sky values at 10:00 AM, whereas the FAPAR values over other
sites corresponded to the daily integrated black-sky FAPAR. Considering that instantaneous
FAPAR at the time of the satellite overpass (around 10:00 AM) is a good approximation of
daily integrated black-sky FAPAR [
21
,
36
], the instantaneous and daily integrated black-sky
FAPAR values of high-resolution reference maps at the VALERI and IMAGINES sites were
chosen to evaluate the MODIS, MUSES and EBR black-sky FAPAR products. Consequently,
58 high-resolution FAPAR reference maps over 36 VALERI and IMAGINES sites were used
in this study.
For the VALERI sites, the high resolution FAPAR reference maps are over a 3
×
3 km
region, and the high resolution FAPAR reference maps at all VALERI sites except for the
Fundulea and Gnangara sites have a spatial resolution of 20 m. The spatial resolution of
the FAPAR reference map for the Fundulea site is 10 m, whereas the spatial resolution
of the FAPAR reference map for the Gnangara site is 30 m. For the IMAGINES sites, the
high resolution FAPAR reference maps are over a 5
×
5 km region, and the high resolution
FAPAR reference maps at all IMAGINES sites except for the SouthWest_1 (DOY = 191,
207), Mayo_Alfalfa and Mayo_Shurb sites have a spatial resolution of 30 m. The high
resolution FAPAR reference maps for the SouthWest_1 (DOY = 191, 207), Mayo_Alfalfa and
Mayo_Shurb sites have a spatial resolution of 10 m.
These high-resolution FAPAR reference maps were aggregated to the same spatial
resolution as that of the FAPAR products, approximatively 500 m. A summary with main
characteristics of the selected sites and their mean FAPAR values of a 500
×
500 m region
can be found in Table 1.
Remote Sens. 2022,14, 3304 5 of 19
Table 1.
Main characteristics of the selected sites and their mean FAPAR values of a 500
×
500 m
region (DOY, day of year).
Site Name Country Lat () Lon ()Biome Type DOY/Year Mean FAPAR
Les_Alpilles * France 43.810 4.715 Broadleaf crops 204/2002 0.350
Barrax * Spain 39.057 2.104 Broadleaf crops 194/2003 0.083
Camerons * Australia 32.598 116.254 Savannas 63/2004 0.455
Concepcion * Chile 37.467 73.470 Deciduous broadleaf forests 9/2003 0.801
Counami * French 5.347 53.238 Evergreen broadleaf forests 286/2002 0.889
Fundulea * Romania 44.406 26.583 Grasses/cereal crops 151/2003 0.347
Gilching * Germany 48.082 11.320 Grasses/cereal crops 199/2002 0.714
Gnangara * Australia 31.534 115.882 Savanna 61/2004 0.258
Haouz * Morocco 31.659 7.600 Shrubs 71/2003 0.295
Laprida * Argentina 36.990 60.553 Broadleaf crops 292/2002 0.608
Larose * Canada 45.380 75.217 Savanna 219/2003 0.871
Plan-de-Dieu * France 44.199 4.948 Broadleaf crops 189/2004 0.245
Sonian * Belgium 50.768 4.411 Deciduous broadleaf forests 174/2004 0.921
Sud_Ouest * France 43.506 1.238 Broadleaf crops 189/2002 0.634
Turco * Bolivia 18.239 68.193 Shrubs 240/2002 0.025
105/2003 0.050
Zhangbei * China 41.279 114.688 Grasses/cereal crops 221/2002 0.594
Pshenichne #Ukraine 50.077 30.232 Grasses/cereal crops 134/2013 0.218
166/2013 0.721
196/2013 0.871
SouthWest_1 #France 43.551 1.089 Grasses/cereal crops 173/2013 0.774
191/2013 0.135
207/2013 0.201
230/2013 0.224
247/2013 0.277
SouthWest_2 #France 43.447 1.145 Grasses/cereal crops 173/2013 0.662
191/2013 0.306
207/2013 0.434
230/2013 0.409
247/2013 0.368
Mayo_Alfalfa #Argentina 37.907 67.746 Grasses/cereal crops 40/2014 0.376
Mayo_Shurb #Argentina 37.939 67.789 Shrubs 40/2014 0.186
Rosasco #Italy 45.253 8.562 Grasses/cereal crops 184/2014 0.840
LaReina #Spain 37.819 4.862 Grasses/cereal crops 140/2014 0.076
140/2014 0.577
Barrax #Spain 39.054 2.101 Broadleaf crops 149/2014 0.674
Albufera #Spain 39.274 0.316 needleleaf forests 158/2014 0.186
175/2014 0.441
196/2014 0.648
219/2014 0.724
234/2014 0.816
Pshenichne #Ukraine 50.077 30.232 Grasses/cereal crops 163/2014 0.562
212/2014 0.885
Capitanata #Italy 41.464 15.487 Grasses/cereal crops 77/2014 0.802
Barrax #Spain 39.054 2.101 Broadleaf crops 145/2015 0.489
203/2015 0.354
Pshenichne #Ukraine 50.077 30.232 Grasses/cereal crops 174/2015 0.623
188/2015 0.735
204/2015 0.785
Peyrousse #France 43.666 0.220 Grasses/cereal crops 174/2015 0.195
Urgons #France 43.640 0.434 Broadleaf crops 174/2015 0.585
Creón#France 43.994 0.047 Evergreen broadleaf forests 175/2015 0.641
Condom #France 43.974 0.336 Grasses/cereal crops 176/2015 0.354
Savenès#France 43.824 1.175 Grasses/cereal crops 176/2015 0.262
Collelongo #Italy 41.850 13.590 Deciduous broadleaf forests 189/2015 0.893
266/2015 0.896
Capitanata #Italy 41.464 15.487 Grasses/cereal crops 113/2015 0.907
UpperTana #Kenya 0.772 36.974 Grasses/cereal crops 68/2016 0.544
* VALERI; #IMAGINES.
Remote Sens. 2022,14, 3304 6 of 19
2.2. Methods
2.2.1. Spatial and Temporal Consistency Analysis
For comparison of the spatial consistencies, the MODIS, MUSES and EBR FAPAR
products were aggregated to the monthly products through computing the average of the
high quality FAPAR values in a month. The average was calculated only if there were 3
or 4 high quality FAPAR values in the month. Then, global maps of the MODIS, MUSES
and EBR FAPAR products in January and July of 2016 were produced in this study. For
comparison of the differences between the black-sky and white-sky FAPAR datasets of
the MUSES and EBR products, their global difference maps in January and July of 2016
were also constructed. The histograms of the three FAPAR products in July of 2016 in the
northern and southern hemispheres were calculated to analyze the distribution of each
product. Additionally, the histogram of these FAPAR products for each vegetation type
according to the MODIS land cover type product (MCD12Q1) in the year 2016 were also
calculated. Only the pixels where all the products provided FAPAR values were used to
create the histograms.
For evaluation of the temporal consistencies, the time series curves of the three FAPAR
products from 2002 to 2017 at seven sites (Table 1) with different biome types were com-
pared. The black-sky FAPAR profiles were also compared with the mean values derived
from the high-resolution FAPAR reference maps to analyze the precision of each product in
the time series.
2.2.2. Direct Validation
Direct validation refers to comparing the MODIS, MUSES and EBR FAPAR values
with FAPAR values derived from the high-resolution FAPAR reference maps. Because
of the different spatial resolutions between them, the method proposed by Morisette
et al. [
37
] was adopted to validate the MODIS, MUSES and EBR FAPAR products in this
study. The high-resolution FAPAR reference maps were re-projected onto the sinusoidal
projection, which was used by the FAPAR products. Meanwhile, the high-resolution FAPAR
reference maps were aggregated to the same spatial resolution as the FAPAR products by
the spatial-average method. Additionally, owing to the differences in time between the
FAPAR products and ground measurements, the FAPAR products adjacent to the time
of ground measurements were processed by the linear interpolation method to obtain
the FAPAR values with the same time as the ground measurements in this study. The
performances of the three black-sky FAPAR products were quantified with coefficient of
determination (R2) and root mean square error (RMSE).
3. Results
3.1. Spatial Consistency
The global maps of the MUSES, EBR and MODIS FAPAR products in January and July
of 2016 are shown in Figures 1and 2, respectively. Areas masked in gray correspond to
pixels with missing FAPAR values. Because of clouds, the MODIS and EBR FAPAR products
have many pixels with missing FAPAR values. In January, there are some pixels with
missing values in rainforest regions and high latitude regions of the northern hemisphere
for the MODIS and EBR FAPAR products, but the EBR FAPAR product has more pixels with
missing values. The rate of pixels with missing FAPAR values for the EBR FAPAR product
is nearly 50% in these regions. In July, the pixels with missing values for the MODIS and
EBR FAPAR products are concentrated in rainforest areas. However, there are few pixels
with missing values for the MUSES FAPAR products, because their retrieval algorithm
used the spatially and temporally complete LAI product [20].
Remote Sens. 2022,14, 3304 7 of 19
Remote Sens. 2022, 14, x FOR PEER REVIEW 7 of 20
3. Results
3.1. Spatial Consistency
The global maps of the MUSES, EBR and MODIS FAPAR products in January and
July of 2016 are shown in Figures 1 and 2, respectively. Areas masked in gray correspond
to pixels with missing FAPAR values. Because of clouds, the MODIS and EBR FAPAR
products have many pixels with missing FAPAR values. In January, there are some pixels
with missing values in rainforest regions and high latitude regions of the northern hemi-
sphere for the MODIS and EBR FAPAR products, but the EBR FAPAR product has more
pixels with missing values. The rate of pixels with missing FAPAR values for the EBR
FAPAR product is nearly 50% in these regions. In July, the pixels with missing values for
the MODIS and EBR FAPAR products are concentrated in rainforest areas. However,
there are few pixels with missing values for the MUSES FAPAR products, because their
retrieval algorithm used the spatially and temporally complete LAI product [20].
(a) MUSES, black-sky
(b) MUSES, white-sky
(c) MUSES, blue-sky
(d) EBR, black-sky
(e) EBR, white-sky
(f) EBR, blue-sky
(g) MODIS, black-sky
Figure 1. Global spatial maps of MUSES, EBR and MODIS FAPAR products in January of 2016.
(a)
MUSES, black-sky. (b) MUSES, white-sky. (c) MUSES, blue-sky. (d) EBR, black-sky. (e) EBR, white-
sky. (f) EBR, blue-sky. (g) MODIS, black-sky.
(a) MUSES, black-sky
(b) MUSES, white-sky
(c) MUSES, blue-sky
(d) EBR, black-sky
(e) EBR, white-sky
(f) EBR, blue-sky
Figure 1.
Global spatial maps of MUSES, EBR and MODIS FAPAR products in January of 2016.
(
a
) MUSES, black-sky. (
b
) MUSES, white-sky. (
c
) MUSES, blue-sky. (
d
) EBR, black-sky. (
e
) EBR,
white-sky. (f) EBR, blue-sky. (g) MODIS, black-sky.
Remote Sens. 2022, 14, x FOR PEER REVIEW 7 of 20
3. Results
3.1. Spatial Consistency
The global maps of the MUSES, EBR and MODIS FAPAR products in January and
July of 2016 are shown in Figures 1 and 2, respectively. Areas masked in gray correspond
to pixels with missing FAPAR values. Because of clouds, the MODIS and EBR FAPAR
products have many pixels with missing FAPAR values. In January, there are some pixels
with missing values in rainforest regions and high latitude regions of the northern hemi-
sphere for the MODIS and EBR FAPAR products, but the EBR FAPAR product has more
pixels with missing values. The rate of pixels with missing FAPAR values for the EBR
FAPAR product is nearly 50% in these regions. In July, the pixels with missing values for
the MODIS and EBR FAPAR products are concentrated in rainforest areas. However,
there are few pixels with missing values for the MUSES FAPAR products, because their
retrieval algorithm used the spatially and temporally complete LAI product [20].
(a) MUSES, black-sky
(b) MUSES, white-sky
(c) MUSES, blue-sky
(d) EBR, black-sky
(e) EBR, white-sky
(f) EBR, blue-sky
(g) MODIS, black-sky
Figure 1. Global spatial maps of MUSES, EBR and MODIS FAPAR products in January of 2016.
(a)
MUSES, black-sky. (b) MUSES, white-sky. (c) MUSES, blue-sky. (d) EBR, black-sky. (e) EBR, white-
sky. (f) EBR, blue-sky. (g) MODIS, black-sky.
(a) MUSES, black-sky
(b) MUSES, white-sky
(c) MUSES, blue-sky
(d) EBR, black-sky
(e) EBR, white-sky
(f) EBR, blue-sky
Remote Sens. 2022, 14, x FOR PEER REVIEW 8 of 20
(g) MODIS, black-sky
Figure 2. Global spatial maps of MUSES, EBR, MODIS FAPAR products in July of 2016.
(a) MUSES,
black-sky
.
(b) MUSES, white-sky
.
(c) MUSES, blue-sky
.
(d) EBR, black-sky
.
(e) EBR, white-sky
.
(f)
EBR, blue-sky
.
(g) MODIS, black-sky
.
Figures 1 and 2 demonstrate similar spatial patterns among the MUSES, EBR and
MODIS FAPAR products. In January, higher FAPAR values are found in the equatorial
forest areas, whereas lower FAPAR values are found in the middle and high latitudes of
the northern hemisphere. In July, higher values are distributed in equatorial forest regions
and in the regions around 5060°N, whereas lower FAPAR values are distributed in
sparsely vegetated areas.
However, discrepancies among these FAPAR products are evident in some areas. In
January, the MODIS black-sky FAPAR product exhibits significantly higher values (ap-
proximately 0.9) than the MUSES and EBR black-sky FAPAR products in the rainforest
region near the equator, and the EBR black-sky FAPAR values (approximately 0.75) are
slightly lower than the MUSES black-sky FAPAR values (approximately 0.8) in this region
(Figure 1). In the regions around 60°N, the MUSES FAPAR product has the largest black-
sky FAPAR values, followed by the EBR FAPAR product, and the MODIS FAPAR prod-
uct has the smallest black-sky FAPAR values (between 0 and 0.1). In Australia, the MUSES
black-sky FAPAR values (approximately 0.1) are lower than the MODIS and EBR black-
sky FAPAR values (approximately 0.2). For white-sky FAPAR products in Figure 1, the
MUSES white-sky FAPAR values (approximately 0.9) are higher than the EBR white-sky
FAPAR values (approximately 0.8) in tropical rainforests near the equator. A similar dis-
tribution is observed in the MUSES and EBR blue-sky FAPAR products.
In July, the MODIS black-sky FAPAR values are generally the largest, followed by
the MUSES black-sky FAPAR values, and the EBR black-sky FAPAR values are the small-
est over the regions around 5060°N (Figure 2). The MODIS and MUSES black-sky
FAPAR values (approximately 0.8) are larger than the EBR black-sky FAPAR values (ap-
proximately 0.7) in tropical rainforests near the equator. However, in Australia, the
MODIS and EBR black-sky FAPAR values (approximately 0.3) are larger than the MUSES
black-sky FAPAR values (approximately 0.1). For the white-sky and blue-sky FAPAR
products, the MUSES white-sky and blue-sky FAPAR values are significantly higher than
the corresponding EBR white-sky and blue-sky FAPAR values in tropical rainforests near
the equator and in middle and high latitudes of the northern hemisphere.
For comparison of the differences in the black-sky and white-sky products, the global
maps of differences between the black-sky and white-sky FAPAR values of the MUSES
and EBR products are shown in Figure 3. Discrepancies between the black-sky and white-
sky FAPAR values for the MUSES and EBR products are evident. In January, the MUSES
black-sky FAPAR values are larger than the MUSES white-sky FAPAR values in the re-
gion north of 3north latitude (Figure 3a). The higher the latitude, the larger the differ-
ences in the FAPAR values. In high latitude regions, the differences could be as much as
0.5. However, the MUSES black-sky FAPAR values are smaller than the MUSES white-
sky FAPAR values in the region south of 30° north latitude. The differences are generally
as much as 0.25, except in the equatorial rainforest region and Australia, where the differ-
ences are generally 0.1. A similar distribution of the differences between the EBR black-
sky and white-sky FAPAR values is observed in Figure 3c. However, the differences for
the EBR FAPAR product (approximately 0.1 for most pixels) are lower than those for the
MUSES FAPAR product. In July, the MUSES and EBR black-sky FAPAR values are lower
Figure 2.
Global spatial maps of MUSES, EBR, MODIS FAPAR products in July of 2016. (
a
) MUSES,
black-sky. (
b
) MUSES, white-sky. (
c
) MUSES, blue-sky. (
d
) EBR, black-sky. (
e
) EBR, white-sky. (
f
) EBR,
blue-sky. (g) MODIS, black-sky.
Figures 1and 2demonstrate similar spatial patterns among the MUSES, EBR and
MODIS FAPAR products. In January, higher FAPAR values are found in the equatorial
forest areas, whereas lower FAPAR values are found in the middle and high latitudes of the
northern hemisphere. In July, higher values are distributed in equatorial forest regions and
Remote Sens. 2022,14, 3304 8 of 19
in the regions around 50–60
N, whereas lower FAPAR values are distributed in sparsely
vegetated areas.
However, discrepancies among these FAPAR products are evident in some areas.
In January, the MODIS black-sky FAPAR product exhibits significantly higher values
(approximately 0.9) than the MUSES and EBR black-sky FAPAR products in the rainforest
region near the equator, and the EBR black-sky FAPAR values (approximately 0.75) are
slightly lower than the MUSES black-sky FAPAR values (approximately 0.8) in this region
(Figure 1). In the regions around 60
N, the MUSES FAPAR product has the largest black-
sky FAPAR values, followed by the EBR FAPAR product, and the MODIS FAPAR product
has the smallest black-sky FAPAR values (between 0 and 0.1). In Australia, the MUSES
black-sky FAPAR values (approximately 0.1) are lower than the MODIS and EBR black-sky
FAPAR values (approximately 0.2). For white-sky FAPAR products in Figure 1, the MUSES
white-sky FAPAR values (approximately 0.9) are higher than the EBR white-sky FAPAR
values (approximately 0.8) in tropical rainforests near the equator. A similar distribution is
observed in the MUSES and EBR blue-sky FAPAR products.
In July, the MODIS black-sky FAPAR values are generally the largest, followed by the
MUSES black-sky FAPAR values, and the EBR black-sky FAPAR values are the smallest
over the regions around 50–60
N (Figure 2). The MODIS and MUSES black-sky FAPAR
values (approximately 0.8) are larger than the EBR black-sky FAPAR values (approximately
0.7) in tropical rainforests near the equator. However, in Australia, the MODIS and EBR
black-sky FAPAR values (approximately 0.3) are larger than the MUSES black-sky FAPAR
values (approximately 0.1). For the white-sky and blue-sky FAPAR products, the MUSES
white-sky and blue-sky FAPAR values are significantly higher than the corresponding EBR
white-sky and blue-sky FAPAR values in tropical rainforests near the equator and in middle
and high latitudes of the northern hemisphere.
For comparison of the differences in the black-sky and white-sky products, the global
maps of differences between the black-sky and white-sky FAPAR values of the MUSES and
EBR products are shown in Figure 3. Discrepancies between the black-sky and white-sky
FAPAR values for the MUSES and EBR products are evident. In January, the MUSES
black-sky FAPAR values are larger than the MUSES white-sky FAPAR values in the region
north of 30
north latitude (Figure 3a). The higher the latitude, the larger the differences
in the FAPAR values. In high latitude regions, the differences could be as much as 0.5.
However, the MUSES black-sky FAPAR values are smaller than the MUSES white-sky
FAPAR values in the region south of 30
north latitude. The differences are generally as
much as 0.25, except in the equatorial rainforest region and Australia, where the differences
are generally 0.1. A similar distribution of the differences between the EBR black-sky
and white-sky FAPAR values is observed in Figure 3c. However, the differences for
the EBR FAPAR product (approximately 0.1 for most pixels) are lower than those for
the MUSES FAPAR product. In July, the MUSES and EBR black-sky FAPAR values are
lower compared with their white-sky values in the region north of 30
south latitude and
are larger than their white-sky FAPAR values in the region south of 30
south latitude
(Figure 3b,d). However, the differences between the MUSES black-sky and white-sky
values (approximately 0.2) are slightly larger than those between the EBR black-sky and
white-sky values (approximately 0.1).
Figure 4shows histograms of the MODIS, MUSES and EBR FAPAR products in July
of 2016 in the northern and southern hemispheres. The histogram distributions of the
MODIS, MUSES and EBR black-sky FAPAR products in the northern hemisphere are
similar (Figure 4a). Most FAPAR values of three products are between 0.1 and 0.7. The
histogram distributions of the three products show two peaks in the southern hemisphere
(Figure 4d). The first peak position of the MODIS FAPAR product is around 0.3, whereas
those of the MUSES and EBR FAPAR products are around 0.1 and 0.2, respectively. The
second peak positions for the three products are all around 0.85. However, the frequency
of the MODIS FAPAR product at this peak is higher than those of the MUSES and EBR
FAPAR products. Similar histogram distributions of the MUSES and EBR white-sky FAPAR
Remote Sens. 2022,14, 3304 9 of 19
products in the northern hemisphere are shown in Figure 4b. However, the frequencies
of the MUSES white-sky FAPAR values between 0.1 and 0.6 are lower than those of the
EBR white-sky FAPAR values, and frequencies of the MUSES white-sky FAPAR values
between 0.7 and 0.8 are larger than those of the EBR white-sky FAPAR values. Figure 4c
shows that the histograms of the MUSES and EBR blue-sky FAPAR values in the northern
hemisphere are slightly different. The frequency of the MUSES blue-sky FAPAR product is
significantly higher than that of the EBR blue-sky FAPAR product when the FAPAR value
is 0.9. In contrast, the frequencies of the EBR blue-sky FAPAR values between 0.1 and 0.5
are higher than those of the MUSES blue-sky FAPAR values. In the southern hemisphere,
the white-sky and blue-sky FAPAR values of the MUSES and EBR products have histogram
distributions similar to the MUSES and EBR black-sky FAPAR values. The histograms of
the white-sky and blue-sky FAPAR values of the MUSES and EBR products have two peaks
in almost the same positions, although the frequencies of these FAPAR values are different.
Remote Sens. 2022, 14, x FOR PEER REVIEW 9 of 20
compared with their white-sky values in the region north of 30° south latitude and are
larger than their white-sky FAPAR values in the region south of 3south latitude (Figure
3b,d). However, the differences between the MUSES black-sky and white-sky values (ap-
proximately 0.2) are slightly larger than those between the EBR black-sky and white-sky
values (approximately 0.1).
(a) MUSES, January
(b) MUSES, July
(c) EBR, January
(d) EBR, July
Figure 3. Global maps of differences between black-sky and white-sky FAPAR values of MUSES
(top) and EBR (bottom) products in January (left) and July (right) of 2016.
(a) MUSES, January
.
(b)
MUSES, July
.
(c) EBR, January
.
(d) EBR, July
.
Figure 4 shows histograms of the MODIS, MUSES and EBR FAPAR products in July
of 2016 in the northern and southern hemispheres. The histogram distributions of the
MODIS, MUSES and EBR black-sky FAPAR products in the northern hemisphere are sim-
ilar (Figure 4a). Most FAPAR values of three products are between 0.1 and 0.7. The histo-
gram distributions of the three products show two peaks in the southern hemisphere (Fig-
ure 4d). The first peak position of the MODIS FAPAR product is around 0.3, whereas those
of the MUSES and EBR FAPAR products are around 0.1 and 0.2, respectively. The second
peak positions for the three products are all around 0.85. However, the frequency of the
MODIS FAPAR product at this peak is higher than those of the MUSES and EBR FAPAR
products. Similar histogram distributions of the MUSES and EBR white-sky FAPAR prod-
ucts in the northern hemisphere are shown in Figure 4b. However, the frequencies of the
MUSES white-sky FAPAR values between 0.1 and 0.6 are lower than those of the EBR
white-sky FAPAR values, and frequencies of the MUSES white-sky FAPAR values be-
tween 0.7 and 0.8 are larger than those of the EBR white-sky FAPAR values. Figure 4c
shows that the histograms of the MUSES and EBR blue-sky FAPAR values in the northern
hemisphere are slightly different. The frequency of the MUSES blue-sky FAPAR product
is significantly higher than that of the EBR blue-sky FAPAR product when the FAPAR
value is 0.9. In contrast, the frequencies of the EBR blue-sky FAPAR values between 0.1
and 0.5 are higher than those of the MUSES blue-sky FAPAR values. In the southern hem-
isphere, the white-sky and blue-sky FAPAR values of the MUSES and EBR products have
histogram distributions similar to the MUSES and EBR black-sky FAPAR values. The his-
tograms of the white-sky and blue-sky FAPAR values of the MUSES and EBR products
Figure 3.
Global maps of differences between black-sky and white-sky FAPAR values of MUSES (top)
and EBR (bottom) products in January (left) and July (right) of 2016. (
a
) MUSES, January. (
b
) MUSES,
July. (c) EBR, January. (d) EBR, July.
Figure 5shows frequency histograms of the MODIS, MUSES and EBR products for
different vegetation types in July 2016. The frequency histograms of the MODIS, MUSES
and EBR products for grasses/cereal crops are shown in Figure 5a. It is observed that the
histograms of the MODIS, MUSES and EBR black-sky FAPAR values show good agreement.
The histograms of the MODIS, MUSES and EBR black-sky FAPAR values have nearly
the same peak positions (approximately 0.1), but the frequency of the MUSES black-sky
FAPAR values at the peak position is slightly higher than those of the EBR and MODIS
black-sky values. Furthermore, the frequency distributions of the MUSES white-sky and
blue-sky FAPAR values also show good agreement with those of the EBR white-sky and
blue-sky values.
Remote Sens. 2022,14, 3304 10 of 19
Remote Sens. 2022, 14, x FOR PEER REVIEW 10 of 20
have two peaks in almost the same positions, although the frequencies of these FAPAR
values are different.
(a) NH, black-sky
(b) NH, white-sky
(c) NH, blue-sky
(d) SH, black-sky
(e) SH, white-sky
(f) SH, blue-sky
Figure 4. Histograms of the MODIS, MUSES and EBR FAPAR products in July of 2016 in the north-
ern hemisphere (NH) (top) and southern hemisphere (SH) (bottom).
(a) NH, black-sky
.
(b) NH,
white-sky
.
(c) NH, blue-sky
.
(d) SH, black-sky
.
(e) SH, white-sky
.
(f) SH, blue-sky
.
Figure 5 shows frequency histograms of the MODIS, MUSES and EBR products for
different vegetation types in July 2016. The frequency histograms of the MODIS, MUSES
and EBR products for grasses/cereal crops are shown in Figure 5a. It is observed that the
histograms of the MODIS, MUSES and EBR black-sky FAPAR values show good agree-
ment. The histograms of the MODIS, MUSES and EBR black-sky FAPAR values have
nearly the same peak positions (approximately 0.1), but the frequency of the MUSES
black-sky FAPAR values at the peak position is slightly higher than those of the EBR and
MODIS black-sky values. Furthermore, the frequency distributions of the MUSES white-
sky and blue-sky FAPAR values also show good agreement with those of the EBR white-
sky and blue-sky values.
Figure 4.
Histograms of the MODIS, MUSES and EBR FAPAR products in July of 2016 in the northern
hemisphere (NH) (top) and southern hemisphere (SH) (bottom). (
a
) NH, black-sky. (
b
) NH, white-sky.
(c) NH, blue-sky. (d) SH, black-sky. (e) SH, white-sky. (f) SH, blue-sky.
For broadleaf crops, most black-sky, white-sky and blue-sky FAPAR values are between
0.1 and 0.8. The histograms of the MUSES black-sky, white-sky and blue-sky FAPAR values
are bimodal, but the histograms of the black-sky, white-sky and blue-sky FAPAR values for
the MODIS and EBR products have only one peak.
For shrubs, all histograms of the black-sky, white-sky and blue-sky FAPAR values
for the MODIS, MUSES and EBR products have two peaks. The frequency of the MUSES
black-sky FAPAR values at the first peak position (around 0.1) is higher than those of
the EBR and MODIS black-sky FAPAR values. Similarly, the frequencies of the MUSES
white-sky and blue-sky FAPAR values at the first peak position are also higher than those of
the EBR white-sky and blue-sky FAPAR values. However, the frequencies of the black-sky
values for the MODIS, MUSES and EBR products and the frequencies of the white-sky and
blue-sky FAPAR values for the MUSES and EBR products at the second peak positions
(around 0.6) are similar.
For the savannas biome type, the frequency distributions of the black-sky, white-sky
and blue-sky FAPAR values for the MUSES, MODIS and EBR products have similar shapes
with one peak. The peak position of the MUSES black-sky FAPAR values (around 0.6) is
smaller than that of the MODIS black-sky FAPAR values (around 0.7) but larger than that of
the EBR black-sky FAPAR values (around 0.5). The peak positions of the MUSES white-sky
and blue-sky FAPAR values are larger than those of the EBR white-sky and blue-sky FAPAR
values. Moreover, the frequencies of the MUSES white-sky and blue-sky FAPAR values at
the peak position are higher than those of the corresponding EBR white-sky and blue-sky
FAPAR values. Therefore, the MUSES white-sky and blue-sky FAPAR values are generally
larger than the corresponding EBR white-sky and blue-sky FAPAR values for the savannas
biome type.
For evergreen broadleaf forests, the MUSES, MODIS and EBR FAPAR products have
similar frequency distribution histograms with narrow peaks. The MUSES black-sky, white-
sky and blue-sky FAPAR values have the same peak positions (approximately 0.9) as the
corresponding EBR black-sky, white-sky and blue-sky FAPAR values. However, the peak
Remote Sens. 2022,14, 3304 11 of 19
position of the MODIS black-sky FAPAR values (approximately 0.9) is higher than those of
the MUSES and EBR black-sky values (approximately 0.85). The frequencies of the MODIS
and MUSES black-sky FAPAR values at the peak positions are significantly higher than
those of the EBR black-sky FAPAR values, and the frequencies of the MUSES white-sky
and blue-sky FAPAR values at the peak positions are also significantly higher than the
corresponding frequencies of the EBR white-sky and blue-sky FAPAR values. Thus, the
MODIS and MUSES black-sky FAPAR values are generally larger than the EBR black-sky
FAPAR values, and the MUSES white-sky and blue-sky FAPAR values are usually larger
than the corresponding EBR white-sky and blue-sky FAPAR values.
Remote Sens. 2022, 14, x FOR PEER REVIEW 11 of 20
(a) Grasses/cereal crops
(b) Broadleaf crops
(c) Shrubs
(d) Savannas
(e) Evergreen broadleaf forests
(f) Deciduous broadleaf forests
(g) Evergreen needleleaf forests
(h) Deciduous needleleaf forests
Figure 5. Histogram of black-sky (left), white-sky (middle) and blue-sky (right) FAPAR values of
the MODIS, MUSES and EBR products in July of 2016 for different biome types.
(a) Grasses/cereal
crops
.
(b) Broadleaf crops
.
(c) Shrubs
.
(d) Savannas
.
(e) Evergreen broadleaf forests
.
(f) Deciduous
broadleaf forests
.
(g) Evergreen needleleaf forests
.
(h) Deciduous needleleaf forests
.
For broadleaf crops, most black-sky, white-sky and blue-sky FAPAR values are be-
tween 0.1 and 0.8. The histograms of the MUSES black-sky, white-sky and blue-sky
FAPAR values are bimodal, but the histograms of the black-sky, white-sky and blue-sky
FAPAR values for the MODIS and EBR products have only one peak.
For shrubs, all histograms of the black-sky, white-sky and blue-sky FAPAR values
for the MODIS, MUSES and EBR products have two peaks. The frequency of the MUSES
black-sky FAPAR values at the first peak position (around 0.1) is higher than those of the
EBR and MODIS black-sky FAPAR values. Similarly, the frequencies of the MUSES white-
sky and blue-sky FAPAR values at the first peak position are also higher than those of the
EBR white-sky and blue-sky FAPAR values. However, the frequencies of the black-sky
values for the MODIS, MUSES and EBR products and the frequencies of the white-sky
and blue-sky FAPAR values for the MUSES and EBR products at the second peak posi-
tions (around 0.6) are similar.
For the savannas biome type, the frequency distributions of the black-sky, white-sky
and blue-sky FAPAR values for the MUSES, MODIS and EBR products have similar
shapes with one peak. The peak position of the MUSES black-sky FAPAR values (around
0.6) is smaller than that of the MODIS black-sky FAPAR values (around 0.7) but larger
Figure 5.
Histogram of black-sky (left), white-sky (middle) and blue-sky (right) FAPAR values of
the MODIS, MUSES and EBR products in July of 2016 for different biome types. (
a
) Grasses/cereal
crops. (
b
) Broadleaf crops. (
c
) Shrubs. (
d
) Savannas. (
e
) Evergreen broadleaf forests. (
f
) Deciduous
broadleaf forests. (g) Evergreen needleleaf forests. (h) Deciduous needleleaf forests.
For the deciduous broadleaf forest biome type, the frequency distributions of the
MUSES and MODIS black-sky FAPAR values are highly consistent, with a narrow peak
around 0.85. The histogram of the EBR black-sky FAPAR values exhibits a single-peaked
distribution with the peak position around 0.8, but frequency of the EBR black-sky FAPAR
values at the peak position is smaller than those of the MODIS and MUSES black-sky
FAPAR values. The histograms of the MUSES and EBR white-sky and blue-sky FAPAR
values also show unimodal distributions, but the peak positions of the MUSES white-sky
and blue-sky FAPAR values are higher than the corresponding peak positions of the EBR
Remote Sens. 2022,14, 3304 12 of 19
white-sky and blue-sky values, and the frequencies of the MUSES white-sky and blue-sky
values at the peak positions are significantly larger than the corresponding frequencies
of the EBR white-sky and blue-sky FAPAR values. Therefore, for deciduous broadleaf
forests, the MODIS and MUSES black-sky FAPAR values are generally larger than the
EBR black-sky FAPAR values, and the MUSES white-sky and blue-sky FAPAR values are
generally larger than the corresponding EBR white-sky and blue-sky FAPAR values.
For evergreen needleleaf forests, the MODIS, MUSES and EBR FAPAR values are be-
tween 0.2 and 0.9 and show unimodal frequency distributions. The frequency distribution
of the MODIS black-sky FAPAR values is consistent with that of the MUSES black-sky
FAPAR values. However, the peak position of the EBR black-sky FAPAR values (approx-
imately 0.6) is smaller than those of the MODIS and MUSES black-sky FAPAR values
(approximately 0.8). In addition, the peak positions of the EBR white-sky and blue-sky
FAPAR values are also smaller than the corresponding peak positions of MUSES white-sky
and blue-sky FAPAR values. Therefore, the MODIS and MUSES black-sky FAPAR values
are generally larger than the EBR black-sky FAPAR values, and the MUSES white-sky and
blue-sky FAPAR values are generally larger than the corresponding EBR white-sky and
blue-sky FAPAR values.
For deciduous needleleaf forests, only one peak is found in the frequency distribution
histograms of the MODIS, MUSES and EBR FAPAR values. The MUSES black-sky, white-
sky and blue-sky FAPAR values have the same peak positions as the corresponding EBR
black-sky, white-sky and blue-sky FAPAR values. The peak position of the MODIS black-
sky FAPAR values (approximately 0.9) is larger than those of the EBR and MUSES black-sky
FAPAR values (approximately 0.8).
3.2. Temporal Consistency
Figure 6displays temporal profiles of the MUSES, MODIS and EBR FAPAR products
from 2002 to 2017 over seven sites with different biome types. The time series curves
of the black-sky, white-sky and blue-sky FAPAR values are shown in upper, middle and
lower panels, respectively, for each site in Figure 6. Among these sites, the MUSES FAPAR
product shows the best temporal continuity. The profiles of the MODIS and EBR FAPAR
products have missing FAPAR values in some sites, such as at Collelongo site. In addition,
the profiles of the MODIS and EBR FAPAR values show some fluctuations, particularly
during the growing seasons, whereas the temporal profiles of the MUSES FAPAR values are
smooth, because the temporally smooth LAI product was used in the retrieval algorithm of
the MUSES FAPAR products [9].
Figure 6a displays the time series curves of the MODIS, MUSES and EBR FAPAR
values at the Zhangbei site with the biome type of grasses/cereal crops. Across the black-
sky, white-sky or blue-sky FAPAR values, the MODIS, MUSES and EBR products achieve
excellent agreement and show similar temporal trajectories and seasonal cycles. However,
the MODIS black-sky FAPAR values and the EBR black-sky, white-sky and blue-sky FAPAR
values are slightly higher than the corresponding MUSES black-sky, white-sky and blue-sky
FAPAR values during the growing seasons. The MODIS and EBR black-sky FAPAR values
are close to the FAPAR values derived from the high-resolution FAPAR reference maps
in 2002.
Figure 6b shows the MODIS, MUSES and EBR FAPAR temporal trajectories at the
Urgons site with the broadleaf crop biome type. The MODIS and EBR FAPAR profiles
show dramatic fluctuations, whereas the MUSES FAPAR have continuous trajectories.
The MODIS and MUSES FAPAR values are generally larger than the EBR FAPAR values.
Compared with the blue-sky FAPAR values, the black-sky and white-sky FAPAR values
can better reflect seasonal changes of crops. It can also be observed that the MODIS and
EBR black-sky FAPAR values show earlier growing seasons than the MUSES black-sky
FAPAR values, whereas the MUSES and EBR white-sky FAPAR values show the same
growing seasons. In addition, the MODIS and MUSES black-sky FAPAR values show better
Remote Sens. 2022,14, 3304 13 of 19
agreement with the FAPAR value derived from the high-resolution FAPAR reference map
than the EBR black-sky FAPAR value.
Figure 6c shows the temporal profiles of the MODIS, MUSES and EBR FAPAR values
at the 25de_Mayo site where the vegetation type is shrubs. The MODIS, MUSES and EBR
FAPAR profiles show no clear seasonal changes. The MODIS and EBR FAPAR products
have some fluctuations, whereas the MUSES FAPAR profiles are smooth. The MODIS,
MUSES and EBR FAPAR values at this site are generally very small, between 0.1 and
0.4. Compared with the MODIS and EBR black-sky FAPAR values, the MUSES black-sky
FAPAR values are closer to the FAPAR value derived from the high-resolution FAPAR
reference map at this site.
Remote Sens. 2022, 14, x FOR PEER REVIEW 13 of 20
3.2. Temporal Consistency
Figure 6 displays temporal profiles of the MUSES, MODIS and EBR FAPAR products
from 2002 to 2017 over seven sites with different biome types. The time series curves of
the black-sky, white-sky and blue-sky FAPAR values are shown in upper, middle and
lower panels, respectively, for each site in Figure 6. Among these sites, the MUSES FAPAR
product shows the best temporal continuity. The profiles of the MODIS and EBR FAPAR
products have missing FAPAR values in some sites, such as at Collelongo site. In addition,
the profiles of the MODIS and EBR FAPAR values show some fluctuations, particularly
during the growing seasons, whereas the temporal profiles of the MUSES FAPAR values
are smooth, because the temporally smooth LAI product was used in the retrieval algo-
rithm of the MUSES FAPAR products [9].
(a) Zhangbei/Grasses and cereal crops
(b) Urgons/Broadleaf crops
(c) 25de_Mayo/Shrubs
Figure 6. Cont.
Remote Sens. 2022,14, 3304 14 of 19
Remote Sens. 2022, 14, x FOR PEER REVIEW 14 of 20
(d) Larose/Savannas
(e) Counami/Evergreen broadleaf forests
(f) Collelongo/Deciduous broadleaf forests
(g) Albufera/Needleleaf forests
Figure 6. Time series of the black-sky (1), white-sky (2) and blue-sky (3) FAPAR values from the
MODIS, MUSES and EBR products for (a) Zhangbei, (b) Urgons, (c) 25de_Mayo, (d) Larose, (e)
Counami, (f) Collelongo and (g) Albufera sites from 2002 to 2017.
Figure 6a displays the time series curves of the MODIS, MUSES and EBR FAPAR
values at the Zhangbei site with the biome type of grasses/cereal crops. Across the black-
sky, white-sky or blue-sky FAPAR values, the MODIS, MUSES and EBR products achieve
excellent agreement and show similar temporal trajectories and seasonal cycles. However,
Figure 6.
Time series of the black-sky (1), white-sky (2) and blue-sky (3) FAPAR values from
the MODIS, MUSES and EBR products for (
a
) Zhangbei, (
b
) Urgons, (
c
) 25de_Mayo, (
d
) Larose,
(e) Counami, (f) Collelongo and (g) Albufera sites from 2002 to 2017.
Remote Sens. 2022,14, 3304 15 of 19
For savannas, the temporal trajectories of the MODIS, MUSES and EBR FAPAR values
over the Larose site are displayed in Figure 6d. The MODIS, MUSES and EBR FAPAR
values show similar seasonal variations. However, the MODIS and EBR FAPAR values
show dramatic fluctuations during non-growing seasons. The MUSES FAPAR values are
higher than the MODIS and EBR FAPAR values, especially during non-growing seasons.
Compared with the MODIS and EBR black-sky FAPAR values, the MUSES black-sky
FAPAR values agree better with the FAPAR value derived from the high-resolution FAPAR
reference map at this site.
Figure 6e shows the time series curves of the MODIS, MUSES and EBR FAPAR values
at the Counami site. The vegetation type of this site is evergreen broadleaf forests. The
MUSES FAPAR values are between 0.7 and 0.8 and exhibit nearly flat profiles throughout
the years. However, the temporal profiles of the MODIS and EBR FAPAR values show
dramatic fluctuations that are inconsistent with the growth characteristics of the evergreen
broadleaf forests. The MODIS, MUSES and EBR black-sky values all agree well with the
FAPAR values derived from the high-resolution FAPAR reference maps at this site.
The temporal profiles of the MODIS, MUSES and EBR FAPAR values at the Collelongo
site with the biome type of deciduous broadleaf forests are shown in Figure 6f. Some
MODIS and EBR FAPAR values are missing at this site. The profiles of the MODIS and
EBR FAPAR values show many fluctuations during growing and non-growing seasons.
The MUSES FAPAR values are smaller than the MODIS and EBR FAPAR values during
the growing seasons but are larger than the MODIS and EBR FAPAR values during the
non-growing seasons. Compared with the MODIS and EBR black-sky FAPAR values, the
MODIS and EBR black-sky FAPAR values are closer to the FAPAR values derived from the
high-resolution FAPAR reference maps at this site.
Figure 6g shows temporal profiles of the MODIS, MUSES and EBR FAPAR values
at the Albufera site with the needleleaf forest biome type. In general, the three FAPAR
products show similar seasonal variations. The MUSES black-sky, white-sky and blue-sky
FAPAR values (around 0.6) are smaller than the corresponding black-sky, white-sky and
blue-sky FAPAR values of the MODIS and EBR products (around 0.9), particularly in 2010,
2011, 2016 and 2017. The MUSES, MODIS and EBR black-sky FAPAR values are close to
the FAPAR value derived from the high-resolution FAPAR reference map in 2014.
3.3. Direct Validation
Scatterplots of the MODIS, MUSES and EBR black-sky FAPAR values versus the
FAPAR values derived from the high-resolution FAPAR reference maps from the VALERI
and IMAGINES projects are shown in Figure 7. Compared with the FAPAR values derived
from the high-resolution FAPAR reference maps, the MODIS FAPAR values were generally
overestimated, whereas the EBR product were generally underestimated. The MUSES
black-sky FAPAR values were slightly overestimated for low FAPAR values but were
underestimated for high FAPAR values. Compared to the MODIS and EBR black-sky
FAPAR values, the scatters for the MUSES black-sky FAPAR values against the FAPAR
values derived from the high-resolution FAPAR reference maps are distributed more
closely around 1:1 line, which demonstrates that the MUSES black-sky FAPAR values
achieve better agreement with the FAPAR values derived from the high-resolution FAPAR
reference maps. Overall, the accuracy of the MUSES black-sky FAPAR product (R
2
= 0.6932
and
RMSE = 0.1495
) against the FAPAR values derived from the high-resolution FAPAR
reference maps outperforms those of the MODIS (R
2
= 0.6202 and RMSE = 0.1710) and EBR
(R2= 0.5746 and RMSE = 0.1912) black-sky FAPAR products.
Remote Sens. 2022,14, 3304 16 of 19
Remote Sens. 2022, 14, x FOR PEER REVIEW 16 of 20
2010, 2011, 2016 and 2017. The MUSES, MODIS and EBR black-sky FAPAR values are
close to the FAPAR value derived from the high-resolution FAPAR reference map in 2014.
3.3. Direct Validation
Scatterplots of the MODIS, MUSES and EBR black-sky FAPAR values versus the
FAPAR values derived from the high-resolution FAPAR reference maps from the VALERI
and IMAGINES projects are shown in Figure 7. Compared with the FAPAR values de-
rived from the high-resolution FAPAR reference maps, the MODIS FAPAR values were
generally overestimated, whereas the EBR product were generally underestimated. The
MUSES black-sky FAPAR values were slightly overestimated for low FAPAR values but
were underestimated for high FAPAR values. Compared to the MODIS and EBR black-
sky FAPAR values, the scatters for the MUSES black-sky FAPAR values against the
FAPAR values derived from the high-resolution FAPAR reference maps are distributed
more closely around 1:1 line, which demonstrates that the MUSES black-sky FAPAR val-
ues achieve better agreement with the FAPAR values derived from the high-resolution
FAPAR reference maps. Overall, the accuracy of the MUSES black-sky FAPAR product
(R
2
= 0.6932 and RMSE = 0.1495) against the FAPAR values derived from the high-resolu-
tion FAPAR reference maps outperforms those of the MODIS (R
2
= 0.6202 and RMSE =
0.1710) and EBR (R
2
= 0.5746 and RMSE = 0.1912) black-sky FAPAR products.
Figure 7. Scatterplots of the MODIS, MUSES and EBR black-sky FAPAR values versus the FAPAR
values derived from the high-resolution FAPAR reference maps.
Figure 7.
Scatterplots of the MODIS, MUSES and EBR black-sky FAPAR values versus the FAPAR
values derived from the high-resolution FAPAR reference maps.
4. Discussion
Although many studies have compared the existing FAPAR products, these studies
have focused on evaluating FAPAR products at spatial resolutions of 1 km or more, and few
studies have evaluated the latest global FAPAR products at a spatial resolution of 500 m.
Furthermore, these existing studies evaluated FAPAR products without distinguishing
the black-sky, white-sky and blue-sky FAPAR datasets. D’Odorico et al. [
24
] evaluated
the performance of JRC-TIP, ESA/JRC MGVI and MODIS FAPAR products over Europe
for the year 2011. The JRC-TIP FAPAR product is defined as the instantaneous FAPAR
under diffuse illumination by a green canopy, whereas the ESA/JRC MGVI and MODIS
FAPAR products correspond to the instantaneous FAPAR under direct illumination by
a green canopy. Similar comparisons among FAPAR products were reported by many
studies [
20
,
26
]. In this study, we evaluated the performance of the latest MODIS, MUSES
and EBR global FAPAR products at a spatial resolution of 500 m. The black-sky, white-sky
and blue-sky FAPAR datasets from the MODIS, MUSES and EBR products were separately
compared to evaluate their spatial and temporal discrepancies. The discrepancies of the
black-sky, white-sky or blue-sky FAPAR datasets among the MODIS, MUSES and EBR
products (Figures 1and 2) are partly explained by the different algorithm assumption used
in each product [24].
In this study, the differences between the black-sky and white-sky FAPAR datasets
were also evaluated at the global scale. It is found that the black-sky FAPAR values were
larger than the white-sky FAPAR values in the region north of 30
north latitude in January
(Figure 3a,c) and in the region south of 30
south latitude in July (Figure 3b,d), whereas
the black-sky FAPAR values were smaller than the white-sky FAPAR values in the region
south of 30
north latitude in January (Figure 3a,c) and in the region north of 30
south
latitude in July (Figure 3b,d). The spatial distribution of the differences between black-sky
Remote Sens. 2022,14, 3304 17 of 19
and white-sky FAPAR values in January and July was because the absorption of direct light
by canopy is significantly affected by solar altitude angle, whereas the absorption of diffuse
light is insensitive to the solar altitude angle [38].
In Figure 7, the MODIS, MUSES and EBR black-sky FAPAR values were compared
with the FAPAR values derived from the high-resolution FAPAR reference maps to evaluate
the accuracy of these FAPAR products. The results demonstrate that the MUSES black-sky
FAPAR product provides the greatest accuracy against the FAPAR values derived from the
high-resolution FAPAR reference maps compared to the MODIS and EBR black-sky FAPAR
products. The results also demonstrate that the MODIS FAPAR product provides better
accuracy than the EBR black-sky FAPAR product. However, Liu et al. [
19
] reported that
the EBR black-sky FAPAR product was more accurate than the MODIS FAPAR product.
The difference may be caused by the following reasons: (1) The spatial resolution of the
GEOV1 FAPAR product is 1 km and that of the MODIS and EBR FAPAR products is 500 m.
So, in the study of Liu et al., the MODIS, GEOV1 and EBR FAPAR products were validated
using the mean values for an area of 3 km
×
3 km. However, in our study, we used the
mean values for an area of 500 m
×
500 m. (2) The 27 high-resolution FAPAR reference
maps covering 22 VALERI sites were used to validate the GEOV1, MODIS and EBR FAPAR
products in the study of Liu et al. However, 58 high-resolution FAPAR reference maps
over 36 sites from the VALERI and IMAGINES projects were used to validate the MUSES,
MODIS and EBR FAPAR products in our study. Therefore, the sites used for validation are
not consistent in the two studies.
5. Conclusions
Three global 500 m resolution FAPAR products—MODIS (with only black-sky FA-
PAR), MUSES and EBR (black-sky, white-sky and blue-sky FAPAR) products—have been
produced in recent years. The performance of the three FAPAR products is evaluated in
this study. The methods include cross-comparison of the three FAPAR products to evaluate
the spatial and temporal consistency of these FAPAR products and direct validation among
the MODIS, MUSES and EBR black-sky FAPAR values and the FAPAR values derived
from the high-resolution reference maps. The EBR FAPAR product has the most missing
values, followed by the MODIS FAPAR product, and the MUSES FAPAR product has
the fewest missing values. The MODIS, MUSES and EBR FAPAR products are generally
consistent in their spatial patterns. However, a relatively large discrepancy among these
products is observed in the equatorial rainforest regions and the middle and high latitude
regions, where the main vegetation type is forest. The temporal profiles of the MUSES
FAPAR product are smooth, whereas those of the MODIS and EBR FAPAR products show
some fluctuations, particularly during the growing seasons. When compared with the
FAPAR values derived from the high-resolution reference maps, the MUSES black-sky
FAPAR product shows better accuracy than the MODIS and EBR black-sky FAPAR prod-
ucts. In summary, the MUSES FAPAR product shows the best performance among the
three global FAPAR products. However, the evaluation analyses of these global FAPAR
products were limited by the ground measurements from the VALERI and IMAGINES
projects. In recent years, more and more field measurement experiments are conducted
and have acquired multi-temporal ground measurement for FAPAR product validation. In
the near future, the authors will perform more extensive validation and analysis of these
global FAPAR products.
Author Contributions:
Conceptualization, Z.X. and Y.Z.; methodology, Z.X. and Y.Z.; software, Y.Z.
and J.L.; formal analysis, Y.Z., Z.X., H.Y. and J.S.; data curation, Y.Z. and J.L.; writing—original draft
preparation, Y.Z.; writing—review and editing, Z.X. and Y.Z. All authors have read and agreed to the
published version of the manuscript.
Funding:
This research was funded in part by the National Natural Science Foundation of China
under Grant 41771359 and in part by the Water Conservancy Science and Technology Project of
Jiangxi Province under Grant 202023ZDKT10.
Remote Sens. 2022,14, 3304 18 of 19
Acknowledgments:
The authors would like to thank the VALERI and IMAGINES projects for
providing high-resolution FAPAR reference maps.
Conflicts of Interest: The authors declare no conflict of interest.
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