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).