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.