Quantitative assessment of the scale conversion from instantaneous to daily GPP under various sky conditions based on MODIS local overpassing time

Figures & data

Table 1. Eddy covariance tower sites from the FLUXNET 2015 Tier one dataset used in this study. 39 tower sites were selected where the incoming and diffuse incoming photosynthetic photon flux density data exist.

ID	Site name	IGBP	Lat	Long	Period	Reference
AT-Neu	Neustift	GRA	47.1167	11.3175	2002–2012	Wohlfahrt et al. (2008)
BE-Bra	Brasschaat	MF	51.3076	4.5198	2002–2014
CA-Qfo	Quebec - Eastern Boreal, Mature Black Spruce	ENF	49.6925	−74.342	2003–2010
CH-Oe1	Oensingen grassland	GRA	47.2858	7.7319	2002–2008	Ammann et al. (2009)
CH-Oe2	Oensingen crop	CRO	47.2863	7.7343	2004–2014	Dietiker et al. (2010)
CZ-BK1	Bily Kriz forest	ENF	49.5021	18.5369	2004–2008	Acosta et al. (2013)
CZ-BK2	Bily Kriz grassland	GRA	49.4944	18.5429	2004–2006
DE-Geb	Gebesee	CRO	51.1001	10.9143	2002–2014	Anthoni et al. (2004)
DE-Hai	Hainich	DBF	51.0792	10.453	2002–2012	Knohl et al. (2003)
DE-Tha	Tharandt	ENF	50.9624	13.5652	2002–2014	Grünwald and Bernhofer (2007)
DK-Sor	Soroe	DBF	55.4859	11.6446	2002–2014	Pilegaard et al. (2011)
ES-Ln2	Lanjaron-Salvage logging	OSH	36.9695	−3.4758	2009
FI-Hyy	Hyytiala	ENF	61.8474	24.2948	2002–2014	Suni et al. (2003)
FI-Lom	Lompolojankka	WET	67.9972	24.2092	2007–2009	Aurela et al. (2015))
FR-Gri	Grignon	CRO	48.8442	1.9519	2004–2013	Aurela et al. (2015)
FR-LBr	Le Bray	ENF	44.7171	−0.7693	2002–2008	Berbigier et al. (2001)
FR-Pue	Puechabon	EBF	43.7413	3.5957	2002–2014	Rambal et al. (2004)
GF-Guy	Guyaflux (French Guiana)	EBF	5.2788	−52.925	2004–2014	Bonal et al. (2008)
IT-BCi	Borgo Cioffi	CRO	40.5238	14.9574	2004–2014	Vitale et al. (2016)
IT-Col	Collelongo	DBF	41.8494	13.5881	2002–2014	Valentini et al. (1996)
IT-MBo	Monte Bondone	GRA	46.0147	11.0458	2003–2013	Marcolla et al. (2011)
IT-PT1	Parco Ticino forest	DBF	45.2009	9.061	2002–2004	Migliavacca et al. (2009)
IT-Ren	Renon	ENF	46.5869	11.4337	2002–2013	Montagnani et al. (2009)
IT-Ro2	Roccarespampani 2	DBF	42.3903	11.9209	2002–2012	Tedeschi et al. (2006)
IT-SR2	San Rossore 2	ENF	43.732	10.291	2013–2014
IT-SRo	San Rossore	ENF	43.7279	10.2844	2002–2012	Chiesi et al. (2005)
NL-Loo	Loobos	ENF	52.1666	5.7436	2002–2013	Moors (2012)
RU-Ha1	Hakasia steppe	GRA	54.7252	90.0022	2002–2004	Marchesini et al. (2007)
US-Me1	Metolius - Eyerly burn	ENF	44.5794	−121.5	2004–2005
US-Me2	Metolius mature ponderosa pine	ENF	44.4523	−121.56	2002–2014	Thomas et al. (2009)
US-MMS	Morgan Monroe State Forest	DBF	39.3232	−86.413	2002–2014	Dragoni et al. (2007)
US-Ne1	Mead - irrigated continuous maize site	CRO	41.1651	−96.477	2002–2013	Suyker et al. (2005)
US-Ne2	Mead - irrigated maize-soybean rotation site	CRO	41.1649	−96.47	2002–2013
US-Ne3	Mead - rainfed maize-soybean rotation site	CRO	41.1797	−96.44	2002–2013
US-Syv	Sylvania Wilderness Area	MF	46.242	−89.348	2002–2014	Desai et al. (2008)
US-UMB	Univ. of Mich. Biological Station	DBF	45.5598	−84.714	2002–2014	Curtis et al. (2002)
US-UMd	UMBS Disturbance	DBF	45.5625	−84.698	2007–2014
US-Var	Vaira Ranch- Ione	GRA	38.4133	−120.95	2002–2014	Ryu et al. (2008)
US-WCr	Willow Creek	DBF	45.8059	−90.08	2002–2014	Cook et al. (2004)

Figure 1. Flow diagram for key variables in this study. Different time scales are distinguished with box colors indicated at the bottom left. The box with dotted lines indicates the corrected temporal upscaling scheme.

Figure 2. Histograms of the local overpass time for the terra-MODIS and aqua-MODIS. Histogram colors denote the cloudiness of the sky (i.e. clear, cloudy, and ambiguous) at its local overpass time. The cloudiness of the sky was distinguished based on the MODIS cloud mask product (i.e. MOD06 and MYD06).

Figure 3. Site-level seasonal correlations between GPP_inst,diff with three environmental variables (PAR_inst,diff—yellow, Ta_inst,diff—red, VPD_inst,diff—blue). The correlation was calculated when the number of samples is larger than 10.

Figure 4. Site-level seasonal correlations between GPP_inst,diff with three environmental variables (PAR_inst,diff—yellow, Ta_inst,diff—red, VPD_inst,diff—blue) under clear and cloudy conditions. The correlation was calculated when the number of samples is larger than 10.

Figure 5. The station-based comparison of slope (refer to Section 3.2) between GPP_daily,ref and the upscaled GPP (i.e. blue: GPP_{daily,cos(SZA)}, purple: GPP_daily,ESI, yellow: GPP_daily,PAR) are presented according to vegetation types: (a) Deciduous broadleaf forests (DBF), (b) Evergreen broadleaf forests (EBF), (c) Evergreen needleleaf forests (ENF), (d) Mixed forests (MF), (e) Grasslands (GRA), and (f) Croplands (CRO). The comparison performed when the number of samples are larger than 10. The x-axis represents the local overpassing time of MODIS divided into one-hour intervals. The y-axis denotes the slope. The red dotted line shows where the slope is 1.

Figure 6. The station-based comparison of R² (referred to section 3.2) between GPP_daily,ref and the upscaled GPP (i.e. blue: GPP_{daily,cos(SZA)}, purple: GPP_daily,ESI, yellow: GPP_daily,PAR) are presented according to vegetation types: (a) deciduous broadleaf forests (DBF), (b) evergreen broadleaf forests (EBF), (c) evergreen needleleaf forests (ENF), (d) mixed forests (MF), (e) grasslands (GRA), and (f) croplands (CRO). The comparison performed when the number of samples are larger than 10. The x-axis represents the local overpassing time of MODIS divided into one-hour intervals. The y-axis denotes the R². The red dotted line shows where the R² is 1.

Figure 7. The comparison between the clearness index and the difference between GPP_daily,ref and the three temporal upscaling approaches: (a) cos(SZA)-based, (b) ESI-based, (c) PAR-based. The red represents high point density while the blue indicates low point density.

Figure 8. The relationships between GPP_inst,diff and PAR_inst,diff under (a) clear and (b) cloudy sky conditions. Colors indicate the diffuse ratio calculated from the eddy flux data.

Figure 9. Density plots between the upscaled daily GPP and reference data: (a) GPP_{cos (SZA),daily}, (b) corrected GPP_{cos (SZA),daily}, (c) GPP_ESI,daily, and (d) corrected GPP_ESI,daily.

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Data availability statement

The FLUXNET2015 dataset is available at https://fluxnet.org/data/fluxnet2015-dataset/. The MODIS cloud products (i.e. MOD06_L2 and MYD06_L2) used in this study can be accessed through the NASA LAADS DAAC (https://ladsweb.modaps.eosdis.nasa.gov/search/).