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.
- 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%.