2.3.1. Water balance and water availability
The basin-scale performance of ETIa-WPR is analysed for 22 major hydrological basins of Africa (Lehner & Grill, 2013) through three approaches (Figure 2). First, the ETIa-WPR was compared to the PCP on an annual basis to analyse the water consumed through ETIa to the water available from PCP.
Second, the basin-scale water balance approach compared the long term ETIa-WPR product to the long term ETa derived from the water balance (ETa-WB). In many studies, the long term water balance (>1 year) for large basins assume a negligible change in storage (Hobbins, Ramírez, & Brown, 2001; Wang & Alimohammadi, 2012; Zhang et al. , 2012). The long term water balance, taken from 2009-2018 in this case, is therefore defined using equation 2.
  1. ETa-WB (mm/yr) = PCP (m/yr) – Q (mm/yr)
Where PCP is the long term precipitation and Q is the long term basin run-off or streamflow, and the ETa-WB is the long term ETa derived from the water balance. The PCP product found in the WaPOR portal was obtained from the Climate Hazards Group Infrared Precipitation with Stations (CHIRPS) dataset (Funk et al. , 2015). The long term Q was obtained from the Global Streamflow Characteristics Dataset (GSCD) (Beck, De Roo, Van Dijk, 2015). The GSCD consists of global streamflow maps, including percentile and mean Q, providing information about runoff behaviour for the entire land surface including ungauged regions.
Third, the ETIa-WPR and PCP annual values were compared to the ETa from MODIS Global Evapotranspiration Project (ETa-MOD16) for the period 2000-2013 (Mu Heinsch, Zhao, & Running, 2007; Mu, Zhao, & Running, 2013) and to values from the literature for basins where data is available. The ETa-MOD16 product is also based on the PM equation and considers the surface energy partitioning process and environmental constraints on ETa. The algorithm uses both ground-based meteorological observations and remote sensing observations from MODIS. Basins were excluded in the ETa-MOD16 comparison missing data on an annual level exceeded 20%.