3.1.1. Water balance and water availability
The annual ETIa-WPR divided by the annual PCP (ETIa/PCP) during 2009-2018 for Africa is shown in Figure 3. The annual ETIa-WPR exceeds the annual PCP (ETIa/PCP >1) on 55% occasions for all basins over the ten years study period. The highest number of exceedances occur in 2014 and 2016 (64%), and the lowest number of exceedances occur in 2018 (27%). The majority of these exceedances, 66%, are by less than 10%. The average ETIa-WPR to PCP ratio for the continent of Africa is 0.93. The lowest ratio is in 2010, 0.87, and the highest is in 2015, 0.97. These ratios are significantly higher than the suggested average, 0.65, of evapotranspiration to precipitation ratio over the global terrestrial surfaces (McDonald, 1961). This ratio is expected to be lower in dry regions or parts of the continent . Except for Lake Chad Basin, basins in the Central, North and West of Africa have ETIa-WPR less than PCP. Most of the exceedances (ETIa>PCP) occur in the South of Africa and on the Horn of Africa.
The basins have the highest ETIa-WPR/PCP ratio in 2015, particularly in Southern Africa. All basins south of Zambezi Basin show a significant decrease in PCP from 2014 to 2015, including a 246, 98 and 238 mm/year drop in Limpopo, Orange and the South Interior respectively. In the same timeframe, the largest ETIa-WPR change is in Limpopo, with a 17mm/year increase, followed by the South Atlantic Coast with a 35mm/year decrease. The decrease in PCP is due to the drought in this region during this period as a result of the El Nino climatic event (USAID, 2016). However, ETIa-WPR does not seem to respond appropriately to these extreme drops in PCP.
The average (av.), minimum (min) and maximum (max) annual ETIa-WPR and PCP values for the 2009-2018 period are shown in Table 5. Where literature values were available, annual estimates of ETIa-WPR and PCP are compared with historical estimates on annual ETa and PCP, with ETa from MODIS Global Evapotranspiration Project (ETa-MOD16) and with the ETa-WB. In most cases, the ETIa-WPR is larger than the ETa values in literature, from the water balance and from MOD16. The PCP falls within the range of literature for all but three basins. The average PCP in the database is higher than that in literature for the Congo. The PCP is less than that found in literature in the Limpopo and Orange Basin, which is also likely due to the drought in this region which occurred after the estimates as reported in the literature. It is also important to note that the Congo River Basin, Central West Coast and west coast basins have vast areas of low-quality NDVI and LST layers for much of the year. They are making the annual mean ETIa-WPR values derived from remote sensing much less reliable in these basins.
The ETIa-WPR and ETa-MOD16 are plotted against the ETa-WB in Figure 4. The relationship between both the ETIa-WPR and ETa-MOD16 products show strong linear relationships with ETa-WB. While the ETa-WPR product has a better R2, the ETa-MOD16 has a lower bias. The ETIa-WPR shows a slightly positive bias, which is increasing with increasing ETa-WB. The absolute difference between the ETIa-WPR and the ETa-WB is typically increasing with increasing ETa-WB. The relative differences between ETIa-WPR and ETa-WB are lower at high ETa values. The absolute difference and relative difference between ETIa-WPR and ETa-MOD16 were greater at lower ETa-MOD16. The absolute relative difference, between ETIa-WPR and ETa-WB typically decreased with increasing PCP. The long term ETIa-WPR is larger than the ETa-WB on 13 out of 22 basins. The Q represented from 4.4% (South Interior) up to 47.0% (Central West Coast), with a median of 18.6%, of the long term PCP. The Q is greater in basins with greater ETIa-WPR and PCP. In basins where the long term average Q is less than 150mm/year (18 basins), the relative difference between ETa estimates ranged from -20% to +70%. When the long term average Q is greater than 200mm/year the relative difference ranged from -12% to +20%.
The long term (2009-2018) ETIa-WPR is estimated to be 634.0mm/year, which is 18.2% larger than the long term ETa-WB is estimated to be 518.7mm/year. This is compared to long term ETa estimates, shown in Figure 5, from ETIa-WPR V1.0 (2009-2017), GLEAM (1980-2013), MOD16 (2000-2013), SSEBop (2003-2017), WECENN (2007-2015) and MTE (1983-2012) which have relative differences of -6.2%, -13.2%, -9.1%, 0.5% and 4.3% respectively (Weerasinghe et al. , 2019).