2.3.2 Spatial and temporal consistency
The temporal and spatial trends were observed over the African continent
in space and time by observing mean ETIa-WPR, SMC and NDVI for all
climate zones during the study period on a dekadal basis. The
Koppen-Geiger classification (Figure 2) is used to consider the mean
dekadal values for the main climatic zones in Africa (Kottek, Grieser,
Beck, Rudolf, & Rubel, 2006). A sample size of 30,000 stratified random
pixels is used to represent the continental. This corresponds to less
than 0.01% of the total image, however, is considered suitable to
represent seasonal trends for the major climate zones. The arid or
desert class – B – dominates Africa (57.2%), followed by the tropical
class - A (31%) and then warm temperate - C (11.8%). The largest
sample count corresponds to the largest climatic zones, with a linear
1:1 line representing area to count. The data is further disaggregated
based on the northern and southern hemispheres to account for opposite
seasonal patterns.
2.4 Direct validation
The ETIa-WPR is compared to the in-situ ETa from eddy covariance (EC)
fluxes (ETa-EC) at a dekadal scale using 14 locations (13 across Africa
and 1 in the Spain extension area) (Figure 2). The country, station
code, vegetation, climate zones and available data for comparison – for
both WaPOR and the local site, are shown in Table 3. The majority of EC
sites are in shrubland or savannas. Egypt stations (EG), the NG-WAM
station and GH-ANK station which are located in an irrigated area,
agricultural land and forested areas respectively.
The SA-SKU, SNDHR, GH-ANK, SD-DEM, CG-TCH, ZM-MON and ES-SCL EC sites
were obtained from the global Fluxes Database Cluster Dataset (FLUXNET).
The FLUXNET 2015 (https://fluxnet.fluxdata.org/) dataset consist
of open-source high-quality data products collected from multiple
regional networks. The NE-WAM, NE-WAF and BN-NAL sites were obtained
from the African Monsoon Multidisciplinary Analysis—Coupling the
Tropical Atmosphere and the Hydrological Cycle (AMMA-CATCH) project,
aiming at establishing long term observations on the climate and the
environment over Western Africa. KWSTI is operated by the International
Institute for Geo-Information Science and Earth Observation at the
University of Twente (ITC-UTWENTE) in partnership with Water Resources
Management Authority (WRMA), the Kenya Wildlife Services (KWS) and
Egerton University. The EG-ZAN, EG-SAA and EG-SAB sites were operated
through the University of Tsukuba, in partnership with Cairo University,
National Water Research Center, Delta Barrage, Qalubia, Egypt and the
Agriculture Research Center, Giza, Egypt in the Nile Delta. These
irrigated sites in the Nile Delta, were under rotation with three major
summer crops – rice, maize and cotton – and four major winter crops –
wheat, berseem, fava beans and sugar beet.
ETIa-WPR for L1 (250m) were spatially averaged over a 3x3 pixel window
surrounding the EC station, based on the assumption that the window
represents the measurement footprint of the EC station. The ETa-EC data
was derived from LE flux and then aggregated temporally to dekadal
averages to match the temporal resolution of the ETIa-WPR products.
Intermediate products, including WaPOR NDVI, SMC and the NDVI and LST
quality layers were analysed along with the ETa trends to identify
possible sources of error. Reworking the LE flux data to daily values
was done (accounting for NaN, non‐removed spikes, early morning (dawn)
and evening (day‐night inversions), dew spiking, etc.) which are not
necessarily removed by the standard Eddy Covariance pre‐processing
software’s (converting very high frequency sonic 30‐sec and gas analyzer
measurements to 30‐minute interval fluxes).
2.5 Level consistency
L3 and L2 ETIa-WPR were compared to the L1 data for the period of
2009-2018 on a dekadal basis. A bilinear resampling method was used to
spatially aggregate the high-resolution L3 and L2 layers to the
resolution of the coarse L1 layer. A random stratified sample of 30,000
points over the entire L2 extent is used for the comparison of the L1
and L2. The L1 and L3 were compared over the entire L3 extent of the
Awash, Zankalon, ODN and Koga L3 irrigation areas for all pixels. Table
4 shows the description of each L3 irrigated area. The EC station at
Zankalon is located in a L3 area. Therefore, as part of the level
consistency, all three levels were also compared to the ETa-EC at this
station. The method described in section 2.4 was used to extract the L3
and L3 ETIa-WPR at the station.